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Please advise: L-arginine for increasing level of growth hormone

MetalPunk

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I use 5-8 iu of HGH but always search the way of increasing my results. I found the article with this suggestion. The question is will the level of the hormone increase if I use both HGH and L-arginine? Thanks for your answers.
 

thethinker48

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I use 5-8 iu of HGH but always search the way of increasing my results. I found the article with this suggestion. The question is will the level of the hormone increase if I use both HGH and L-arginine? Thanks for your answers.
Will beta alanine cause an increase in muscle growth in conjunction with trenbolone?

Sent from my Pixel XL using Tapatalk
 

little slice

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1540252668976.jpg


..
 

Voxide

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Temazepam is a drug with a high potential for misuse.

Benzodiazepines have been abused both orally and intravenously. Different benzodiazepines have different abuse potential; the more rapid the increase in the plasma level following ingestion, the greater the intoxicating effect and the more open to abuse the drug becomes. The speed of onset of action of a particular benzodiazepine correlates well with the ‘popularity’ of that drug for abuse. The two most common reasons for preference were that a benzodiazepine was ‘strong’ and that it gave a good ‘high’

A 1995 study found that temazepam is more rapidly absorbed and oxazepam is more slowly absorbed than most other benzodiazepines.

A 1985 study found that temazepam and triazolam maintained significantly higher rates of self-injection than a variety of other benzodiazepines
 

Elvia1023

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Will beta alanine cause an increase in muscle growth in conjunction with trenbolone?

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Yes it will. Trenbolone by 99.9999999999% and Beta Alanine by 0.0000000001%
 

thethinker48

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Yes it will. Trenbolone by 99.9999999999% and Beta Alanine by 0.0000000001%
A girl that I see frequently at my gym once told me that when she first saw me, she thought I had some facial tick disorder.

I kept biting my upper and lower lips because I was taking beta alanine in a pre workout lol

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aphextwin

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Yes

Don’t listen to these guys. All the tops pros use L-Arginine with Growth. Some guys are getting better gains with Arginine alone. You heading in the right direction.
 

Elvia1023

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A girl that I see frequently at my gym once told me that when she first saw me, she thought I had some facial tick disorder.

I kept biting my upper and lower lips because I was taking beta alanine in a pre workout lol

Sent from my Pixel XL using Tapatalk

I know a guy who has a bad facial tick. But it would be extra bad in the gym. Then I find out he often snorts lines of c as a preworkout :eek:
 

lookslikesausage

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I know a guy who has a bad facial tick. But it would be extra bad in the gym. Then I find out he often snorts lines of c as a preworkout :eek:

always wondered if you could have a decent workout after doing a line or so. Never tried it and probably never will so i'll never know. People i knew who used wouldn't be able to because they usually did line after line so once they started you damn well they weren't making it to the gym.
 

Voxide

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always wondered if you could have a decent workout after doing a line or so. Never tried it and probably never will so i'll never know. People i knew who used wouldn't be able to because they usually did line after line so once they started you damn well they weren't making it to the gym.

With the short duration I doubt it honestly. Plus it's terrible for your heart.

Overrated stimulant for me. If you offered a me a 30mg IR adderall vs a few lines of good blow I'd still take the adderall.
 

MetalPunk

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I have one likeminded person. Its heartening to see that)))

Ive found another info: When taken alone, arginine may boost HGH. Taking 15-20 mg grams per day, the HGH level increases by around 60% naturally

I didnt test this theory but looks interesting.
 

liquid_c

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Even if it did raise it, it would not be enough to make any tangible difference at all. There are no natural methods that work.
 

Fullblown

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I hardly think alanine would cause an increase. Ha
 

MetalPunk

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Thank you guys for your honest answers. I understood that I need some additional researches on the issue.
 

weltweite

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Any natural growth hormone stimulator would need to be taken before administering exogenous growth hormone.

Put simple - Elevated levels of growth hormone (at the dosages you described) would prevent growth hormone release from your pituitary.

Hydrolyzed collagen on an empty stomach in the morning has been shown to boost GH levels significantly.

It is important to have your own natural release of growth hormone because it contains all of the isoforms you need. Synthetic GH is only one of the isoforms... Do other isoforms matter? Yes.
 

jeroendebleser

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Thank you guys for your honest answers. I understood that I need some additional researches on the issue.

No, you need to stop thinking something OTC is going to give you much in terms of results. There's stuff that actually works like the Synthetek products but the majority of OTC stuff is just there for your piece of mind. ('I'm covering all my bases' :cool:)
 

weltweite

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Are you asking for a source of information on this topic because you are skeptical that the various isoforms of GH that your body produces... are important? Or have a use?

The body rarely does things "willy nilly".
If multiple isoforms are produced, intuition should tell you there is a reason.

I remember when I took molecular genetics, a professor was saying that certain non-coding gene regions seemed to not have purpose... but she quickly said, "the body is efficient and generally there is purpose for everything... we just don't know yet".

Either way here is some info:

"Two isoforms are equally anabolic: a 22kda form and a 20kda form

The 22kda form is the one used in synthetic GH and is 191 amino acids long.

The 20kda form is released naturally w/ 22kda. 20kda is identical however 15 amino acids in the part of the chain that interacts w/ the prolactin receptor are missing.

In rats 20kda produces less edema and less diabetogenic effect. In addition it doesn't interact w/ prolactin binding protein and has other characteristics a bit unique. In humans it is less clear if the side effects are less w/ 20kda then 22kda."

DatBtrue said:
"Natural GH is a blend of isoforms. Two of those forms are equally anabolic. One makes up the majority of natural GH release, weighs 22kda and is 191 amino acids long. The other weighs 20kda and has the 15 amino acids that interact with the prolactin receptor removed. The pharmaceutical companies chose to use the 22kda form for their drug. Noone makes a 20kda form.

In addition nature stacks these forms so you get 22kda:22kda; 22kda:20kda; 20kda:20kda stacks. Noone exactly knows why. There are also some naturally occurring fragments that are released.
What big pharma did was simply choose 22kda isoform of GH for their drug."
 

weltweite

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Growth hormone isoforms, Gerhard P. Baumann, Growth Hormone & IGF Research 19 (2009) 333–340

1. Introduction

Human growth hormone (hGH, or GH for the purposes of this article) is a heterogeneous protein that consists of several molecular isoforms. Heterogeneity arises at the levels of the GH gene, mRNA splicing, post-translational processing, and GH metabolism. The existence of multiple isoforms presents challenges for a full understanding of GH bioactivity, for the accurate measurement of GH in body fluids, and for assay standardization.

2. The GH gene cluster


The genetic locus encoding GH resides on human chromosome 17q24.2. It occupies 46.83 kilobases and contains 5 genes related to GH [1]. The genes in this cluster are thought to have arisen by gene duplication. Two encode 2 distinct GH variants and are named GH1 (or GH-N) and GH2 (or GH-V). Two others encode chorionic somatomammotropin (CS), also known as placental lactogen (PL), and are named CS1 (or CSA) and CS2 (or CSB). Their mature protein products are identical, namely CS. The fifth gene in the cluster, named CSL, codes for a CS-like-protein that is expressed at low levels and may not be translated efficiently. The function – or non-function – of this gene/pseudogene has been debated for years, but remains unclear. Each of the 5 genes in the cluster is composed of 5 exons and 4 introns. The GH1 gene is expressed in pituitary somatotrophs, whereas GH2, CS1 and CS2 are expressed in the placenta.
1.jpg

Fig. 1. Primary structure of human GH and its isoforms. The main chain represents 22K-GH (GH-N). The sequence indicated by the bold line from residue 32–46 is deleted in 20K-GH. The black dot at the amino terminus denotes the acyl group in N-acylated GH. The two asterisks denote the deamidated residues in desamido-GH forms. The amino acid designations next to the main chain denote the residues that are changed in GH-V (placental GH). The tree structure at residue 140 indicates the glycosylation site in glycosylated GH-V. Reproduced with permission fromBaumann G, Growth hormone heterogeneity: genes, isohormones, variants, and binding proteins, Endocrine Reviews 12 (1991) 424–449; Copyright 1991 The Endocrine Society.

3. GH gene cluster products

The main product of the GH1 gene is a 191 amino acid, 22,129 Da single chain protein with two disulfide bridges (Fig. 1). It is the prototype pituitary GH and is known as 22K-GH. Another

GH isoform, also derived from the GH1 gene by alternative mRNA splicing, has a structure analogous to 22K-GH, except for the deletion of internal residues 32–46. It has 176 amino acids and a molecular mass of 20,274 Da. It arises from the use of an alternative splice site in exon 3 [2] and is expressed at 5–10% of the expression level of 22K-GH. A third isoform, arising from skipping of exon 3 and lacking residues 32–71, has been proposed as an additional GH variant (17.5K-GH, molecular mass 17,483 Da) [3]. While this form exists under pathological conditions where the GH1 gene is mutated (genetic GH deficiency type II); it has not been shown to be expressed in significant amounts under normal conditions.

The GH2 gene product, GH2 or GH-V, is a 191 amino acid, 22,321 Da, single chain protein with two disulfide bridges, similar in structure to 22K-GH (Fig. 1). Its sequence differs from that of 22K-GH at 13 amino acid positions, and it is a more basic protein. Unlike 22K-GH it contains a consensus sequence for N-glycosylation at position 140. The GH2 gene transcript appears less prone to alternative splicing in exon 3 than the GH1 gene transcript, and the 20K form of GH-V has not been described [4,5], although one study showed small amounts of the GH-V-20K transcript in some, but not all placentas [6]. (Another GH2 transcript with a retained intron 4 is expressed in placenta [7], encoding a protein that diverges from GH in its carboxy-portion; it is retained in the placenta and does not act as a hormone.)

CS, while showing 85% structural homology with GH, is biologically quite distinct. It has very low affinity for the GH receptor (GHR) and therefore, does not act as a GH analogue at physiologically prevailing levels. Modern immunoassays for GH readily differentiate between GH and CS. The present review of GH isoforms will not further discuss CS.

4. GH isoforms and their properties

The 22K-GH form, first isolated and characterized by Li [8] and Niall [9], is the principal and most abundant GH form in the pituitary. The 20K-GH, first described by Lewis et al. [10] is the second most abundant form and accounts for about 5% of pituitary GH. It has a tendency to dimerize and was first isolated from a GH dimer fraction. Additional isoforms of GH are present in pituitary extracts, some are native and some are artifacts of extraction. Posttranslational modifications of 22K-GH include an amino-acylated form and two deamidated forms [11]. The deamidation occurs at positions 137 and 152 [12]. It is entirely possible that similarly modifed forms of 20K-GH exist, but they have not been identified. A glycosylated form of 22K-GH has been isolated [13]. Proteolytically modified forms of GH (cleaved or two-chain GH) have also been described in pituitary extracts, but they are considered extraction artifacts as they have not been identified in fresh pituitary extracts or in the circulation. GH is prone to proteolytic attack in the region between residues 134 and 145 (see [14] for review). Similar cleaved GH forms have been found in the process of producing recombinant GH [15,16]; they are removed during the purification process.

The tertiary structure of 22K-GH is a 4-helical twisted bundle [17]. This overall structure is likely preserved in the other isoforms, although no direct structural evidence is available. The C-terminal end of helix 1 and part of the loop between helices 1 and 2, including minihelix 1, are missing in 20K-GH. The conformation of this region in 20K-GH is only incompletely known; it affects receptor binding site 1 (see below).

In addition to the primary monomeric isoforms, GH exists as a series of oligomers composed of these isoforms. Both homologous and heterologous oligomers occur, they may be non-covalently associated or disulfide-linked [18–20]. A small fraction of these oligomers is non-dissociable; the type of linkage is not known or possibly involves a non-reducible disulfide bond [21]. As mentioned, 20K-GH is enriched in the dimeric fraction because of its propensity to dimerize.

GH-V is primarily if not exclusively expressed in the syncytiotrophoblast of the placenta; it is also known under the name “placental GH.” During pregnancy, it is released into the maternal (but not fetal) circulation, and it progressively supplants pituitary GH in the circulation as pregnancy advances. Towards term, most if not all GH in the maternal blood is GH-V. Two isoforms of GH-V are known: a non-glycosylated and a glycosylated form, the glycosylated residue is Asn140 [22,23]. Little is known about the oligomerization tendency of GH-V.

Fragments of GH can result as extraction artifacts in the process of preparation from pituitary glands or recombinant bacteria. They are also generated physiologically in the periphery as a result of GH catabolism. However, the latter have not been characterized in any detail. Fragments encompassing GH1-43 and GH44-191 have been postulated as pituitary products [24,25], but their existence as native species has not been unequivocally proven. A report of GH44- 191 immunoreactivity in serum is inconclusive as the physical nature of this immunoreactivity has not been positively identified [26].

5. Biological activities of GH isoforms

The principal biological activities of 22K-GH are listed in Table 1. The information on the other isoforms is limited because they are not available in pure form or in sufficient quantities for bioassay. Furthermore, there are species differences in activity or metabolism, such that activities obtained in classical rat assays or in cell systems in vitro are not necessarily applicable to in vivo conditions in man. Nevertheless, as far as is known, the bioactivities of 22K-GH are generally shared by the other isoforms, although depending on the species and assay system, there may be quantitative differences among them.
2.jpg

5.1. 20K-GH

The 20K-GH variant was initially reported to lack insulin-like effects and to have diminished diabetogenic activity [27,28]. Subsequent studies have yielded conflicting information in this regard, perhaps in part due to species differences [29–31]. The growth promoting activity of 20K-GH in vivo has been found to be similar to that of 22K-GH in the rat [10,32–34] despite diminished affinity of 20K-GH for the GHR (see below). In the obese mouse 20K-GH had mildly diminished growth-promoting and lipolytic activity compared to 22K-GH [35], but the two GH isoforms had equipotent lipolytic activity in chicken adipose tissue [36]. In various cell systems transfected with the human GHR, 20K-GH and 22K-GH were generally equipotent in eliciting biological responses [37–40]. In a 16 week trial in humans, 20K-GH exhibited metabolic activities qualitatively and quantitatively comparable to those of 22K-GH, including IGF-I and IGFBP3 generation, lipolytic activity, changes in body composition, and induction of insulin resistance [41]. Administration of 20K-GH suppresses secretion of endogenous 22K-GH via short-loop and long-loop feedback [42]. The converse (suppression of 20K-GH by administration of 22K-GH) is also true [43–46]. No human data on longitudinal growth in response to 20K-GH are available.

The 20K-GH form has lower affinity than 22K-GH for the both the human and the rat GHR [47–51]. Similarly, 20K-GH has low affinity for the human GH binding protein (GHBP) [52–55]. The region deleted in 20K-GH (end of helix 1, loop between helix 1 and 2, minihelix 1) involves receptor binding site 1, and Wada et al. have demonstrated that the affinity of Site 1 binding is reduced, whereas Site 2 binding is unaffected [55]. The apparent affinity of 20K-GH to the human GHR in cell preparations is greater than that seen with the soluble GHR (i.e. the GHBP) or with the GHR in membrane preparations [37,48]. Furthermore, the biological activity of 20KGH in vivo is equivalent to that of 22K-GH (see above). This was initially attributed to the slower clearance of 20K-GH [56]. However, the two isoforms are also equipotent in vitro in most assays (see above). The apparent discrepancy between full bioactivity despite low Site 1 binding affinity was postulated to be due to the strong interaction of two GHRs in their dimerization domain, thereby compensating for the weak Site 1 interaction [57]. The interaction of 20K-GH with the rat GHR is not known in the same detail; it may be significantly different from that with the human GHR. Taking all the above information together, it appears that in man the somatogenic activity of 20K-GH is qualitatively similar and quantitatively equivalent to that of 22K-GH.

In contrast, 20K-GH interacts poorly with the human prolactin receptor and is a weak lactogen compared to 22K-GH [33,38,40]. The low affinity for the prolactin receptor may be one reason for the poor binding of 20K-GH to human liver membranes, which contain both GHRs and prolactin receptors [50].

5.2. Acylated, deamidated and glycosylated GH

The deamidated and acylated forms have equivalent growth-promoting activity and diabetogenic activity in rodents [11,58,59] as well as in an in vitro cell proliferation assay [59], but information on other facets of the GH bioactivity spectrum is limited. One reason for the limited data is the unavailability of these isoforms in pure form. The same is true for glycosylated GH, whose bioactivity remains largely unknown.

5.3. GH-V

GH-V is similar to 22K-GH in its somatogenic activity, but has weak lactogenic properties [60,61]. Its metabolic activities are also similar [62]. In a transgenic mouse overexpressing GH-V, it causes gigantism, insulin resistance and carbohydrate intolerance [63,64]. The affinity of GH-V to the GHR is comparable to that of 22K-GH, whereas its affinity for the prolactin receptor is low [65]. GH-V binds to the GHBP with equal affinity as 22K-GH [66]. Little is known about potential differences in biological activities between non-glycosylated and glycosylated GH-V.

5.4. GH oligomers

The oligomeric forms extracted from pituitary have reduced growth-promoting activity [67], but they consist of a mixture of native oligomers and partially denatured/aggregated GH. A well characterized, naturally occurring disulfide dimer of GH has only 10% of the potency of monomeric GH in the rat weight gain assay [19]. A chemically crosslinked GH dimer shows enhanced somatogenic activity in an in vitro cartilage sulfation assay [68]. A biosynthetic, covalent GH dimer activates GH signaling similarly to GH monomer [69]. It is not clear whether the structures of the latter two GH dimers are representative of native forms.

Naturally occurring dimers/oligomers secreted by pituitary in vitro have been reported to have comparable receptor-binding activity as monomeric GH, in contrast to large molecular weight forms extracted from pituitary [70]. Dimeric fractions of pituitary GH (“Big GH”) has between 20% and 65% of binding potency in a GHR radioreceptor assay [71,72] Potency estimates obtained on dimeric/oligomeric of serum [72] suffer from the admixture of GHBP in the fractions containing these oligomers. GHBP inhibits binding of GH to the GHR, thereby rendering measurement of GH in unextracted serum unreliable.

Additional difficulties with assessing the biological activity of GH oligomers arise because few have been produced in pure form, and there is evidence of aggregation/disaggregation upon storage and perhaps during incubation with receptor preparations [73], [Baumann G, unpublished observations]. Nevertheless, taken together, the available data suggest that the bioactivity of naturally occurring GH oligomers compared to monomeric 22K-GH ranges from moderately reduced to full bioactivity. There appear to be no qualitative differences in the bioactivity spectrum.

6. Immunoreactivity of GH isoforms and immunoassays

There is substantial immunological cross-reactivity among GH isoforms with most antibodies, and especially with polyclonal antisera. The same is true for most monoclonal antibodies, although some may differ in their relative recognition of GH epitopes. A few antibodies without cross-reactivity between 22K-GH and 20K-GH have been generated, and accordingly immunoassays specific for 22K-GH [[74–76], Hybritech and Delphia commercial assays], 20K-GH [75,76] and GH-V [77,78] have been developed. It should be recognized that little is known about the recognition of oligomeric GH forms by these assays, and the extent of recognition of deamidated or acylated GH by the 22K-GH assay is not defined. Similarly, it is not known to what extent GH-V assays differentiate between glycosylated and non-glycosylated forms of GH-V.

Other immunoassays that are partially specific include a “non- 22K-GH assay” or “22K-GH exclusion assay” [79] and a “preferentially 22K-GH-recognizing assay” [80].

The heterogeneity of GH in the circulation is one important reason for the notorious difficulty in accurately measuring plasma GH and for the disparities of GH results obtained among assays and laboratories. Polyclonal assays used in the past minimized these differences, whereas modern, more specific monoclonal assays magnify them. These issues have been recently reviewed [81,82].

7. Metabolism

The metabolic clearance of both monomeric and dimeric 20KGH in the rat is slower than that of the corresponding 22K-GH isoforms [56,83]. The degradation of 20K-GH in vivo in the rat is also slower compared to 22K-GH [56,83]. In the guinea pig, the plasma half-lives of the two GH isoforms are similar [84]. In humans, the plasma half-life of endogenous 20K-GH is longer than that of 22K-GH [45,85]. Detailed pharmacokinetics of exogenous 20K-GH in humans are not currently available, with “half-lives” confined to measurements obtained after subcutaneous injection, reflecting superimposition of absorption and clearance [42].

The metabolic clearance rate of oligomeric GH forms is slower than that of monomeric GH in the rat as well as in humans [83,86]. In humans, plasma half-lives of 19, 26.5 and 45 min have been reported for monomeric, dimeric and oligomeric GH, respectively [86].

The metabolic clearance of endogenous GH-V, estimated following expulsion of the placenta or cesarean section, appears to be somewhat faster than that of 22K-GH [87–90].

8. Proportions of GH isoforms in the pituitary

Estimates of the average distribution among GH isoforms in the human pituitary are listed in Table 2. These figures are based on a composite of data from pituitary extracts [10,11,19,71,91] and GH isoforms released by pituitary cultures in vitro [92–95]. The proportion of extraction artifacs (e.g. aggregated, cleaved, or oxidized GH) varies depending on the extraction method, length and temperature of storage, freeze-thaw cycles, etc. The proportion of artifactual forms has progressively decreased as milder extraction methods came into use.
3.jpg

9. Regulation of secretion of GH isoforms

Pituitary GH secretion is pulsatile under hypothalamic control by GHRH, somatostatin, and probably ghrelin to a lesser degree. This secretion pattern affects all GH isoforms derived from the pituitary, with co-secretion of the contents of secretory granules [45,85,96,97]. No convincing evidence of specific regulation of individual isoform secretion has been presented.

The proportions of GH isoforms in blood change as a function of time after a secretory pulse, due to the different clearance rates of individual isoforms. Therefore, proportions prevailing in a random blood sample may not necessarily be representative of secreted proportions. It has been reported that patients with acromegaly or anorexia nervosa have slightly higher proportions of 20K-GH in their plasma than other subjects [97], although another study found no difference between acromegalic patients and controls [45]. In any case, the reported difference is small (9.1% vs. 6.3%) and may be explained by relative accumulation of 20K-GH due to its slower clearance in these chronic GH hypersecretion states. In contrast to pituitary GH isoforms, the placentally expressed GH-V is not under hypothalamic control. Its secretion pattern is chronic/tonic rather than pulsatile [98]. Its production starts at gestational weeks 5–8 and increases progressively until maternal blood levels reach a plateau about 4 weeks before term [78,87,99,100]. The factors controlling GH-V secretion, other than gestational progression itself, are largely unknown. GH-V does not cross to the fetal compartment and is undetectable in cord blood [99]. Placental GH-V progressively supplants pituitary GH during pregnancy through negative short-loop and IGF-I feedback [98,101].

10. Circulating GH isoforms

The mixture of GH isoforms in the circulation has been investigated using physico-chemical separation methods followed by immunoassay [71,102–107] and more recently by immunoassays specific for selected isoforms (namely 22K-GH, 20K-GH and GH-V) With the exception of the latter assays, GH isoform identifcation and quantification in serum remains a challenging and laborious task.

The spectrum of GH isoforms in the circulation generally reflects their relative abundance in the pituitary as they are co-secreted. However, three additional factors come into play: (1) the presence of GHBP in blood, (2) the clearance rate/half-life of different isoforms, and (3) the presence of circulating, immunoreactive GH fragments derived from peripheral GH metabolism. Plasma contains two GHBPs: a high and a low affinity GHBP (see [108] for review). The high affinity GHBP is the extracellular domain of the GHR. In humans, it is derived from the GHR by proteolytic cleavage at a membrane-proximate site by the metalloproteinase TACE (tumor necrosis factor alpha converting enzyme). In rodents, the GHBP, rather than being cleaved from the GHR, is generated as a splice variant of the ghr gene. The low affinity GHBP is a2-macroglobulin or a modified version thereof. Following secretion, GH rapidly associates with the high affinity GHBP. Up to a level of ~10 lg/L, 45–55% of 22K-GH and ~25% of 20K-GH is bound to this GHBP [109,110] (special techniques must be used for this measurement to minimize dissociation of the complex during analysis). Above 10 lg/L the bound fraction starts declining due to partial saturation of the GHBP. About 5–8% of circulating 22K-GH is bound to the low affinity GHBP, a2-macroglobulin. The 20K-GH variant binds preferentially to a2-macroglobulin because of its low avidity for the high af?nity GHBP. GH-V binds to GHBP in an identical manner as 22K-GH. The binding of the other isoforms of GH, including the oligomeric forms, to GHBPs is unknown. Binding to GHBPs is readily reversible and changes dynamically as GH levels fluctuate as a result of secretion and clearance [111].

The disappearance rate of GH isoforms following a secretory spike affects their relative proportions as a function of time lag from secretion. Thus, the proportions of 20K-GH and especially oligomeric forms tend to increase as time goes on. Indeed, in random blood samples the proportions of these forms are typically higher than their relative prevalence in the pituitary (reviewed in [14]).

Little is known about the precise nature of fragments generated by catabolism of GH in peripheral tissues. Nevertheless, there is evidence of immunoreactive fragments in the circulation [105]. Their precise nature has not been characterized, and their concentration is not known because of lack of a suitable assay standard. It is likely that different immunoassays recognize these fragments to varying degrees. As mentioned above, the postulated GH44-191 immunoreactivity [26] has not been positively identified as such, and the fact that its concentration in serum was reported to be higher than that of GH itself – as opposed to <1% of GH in the pituitary – raises the question of a non-specific immunoreactive serum component.

Isoform-specific immunoassays have recently been developed and applied to the measurement of 20K-GH, 22-K-GH and GH-V. It is not known to which degree these assays differentiate among monomeric and oligomeric forms; judging from the values and percentages reported, they probably recognize both. Using a 20KGH specific assay, the percentage of 20K-GH as part of total plasma GH has been investigated extensively and found to range between 3% and 15%, with a mean of approximately 5.2%, with no systematic or consistent differences between adults, children, genders, age, physiological state, health and disease (see above for comments on slightly higher mean values in one study of acromegaly and anorexia nervosa) [42,45,76,85,95,97,112,113]. The relatively wide range is due to a positively skewed distribution with a few high values. Variability also likely results from different sampling times relative to a secretory event, as well as perhaps minor variations in secreted proportions among subjects [95]. The 22K-GH specific assays, when employed, reported 22K-GH percentages ranging from 58% to 82%, with an approximate mean of 72% [44,85]. A partially specific assay that recognizes GH isoforms other than 22K-GH (“22K GH exclusion assay”) yields results that are roughly consistent with those of the specific assays [44,85,114,115]. Since this assay is defined only by what it does not measure, its use has been superseded by the more specific assays.

GH-V measurements apply only during pregnancy as serum levels are undetectable at any other time. GH-V proportions rise from 0% to 100% during the course of pregnancy as pituitary GH is progressively suppressed. The distribution between non-glycosylated and glycosylated GH-V is not known. In the third trimester towards term, GH-V levels reach a mean of 13–22 lg/L, depending on assay, with a wide range from <10–60 lg/L [78,100]. The distribution of pituitary GH isoforms in human serum 15– 30 min after secretion is summarized in Table 3. The averages shown are based on a best composite estimate from the literature cited in this article. As is evident, there is considerable complexity stemming from GH heterogeneity itself as well as from complex formation with GHBPs. This complexity is reflected in disparity of absolute serum GH values obtained by different assays.

11. Urinary GH isoforms

Table 3 Approximate mean distribution of pituitary GH isoforms in human blood 15–30 min after a secretory pulse.
GHs.jpg

GH is present in very small, but measurable quantities in urine. About 2/3 of the GH clearance is due to renal elimination, but <0.01% of the amount of GH filtered at the glomerulus appears in the final urine because of extensive reuptake and degradation in the proximal tubule (reviewed in [116]). This fact, and the interindividual and day-to-day variability of GH excretion render the determination of urinary GH unsuitable as a reliable tool for quantitation of GH production [117,118], and its determination for diagnostic purposes has been abandoned.

There is very limited information about GH isoforms in urine. One study showed that the composition of GH in human urine is similar to that of monomeric GH in blood, with 22K-GH predominating and minority components of 20K-GH and acidic GH also being present [116]. Oligomeric GH was not detectable in urine [116,119], as would be expected based on molecular size limits for glomerular filtration.

12. Summary

Human GH exists as a series of related proteins (isoforms) derived from several genetic and post-translational mechanisms. The biological significance of this heterogeneity remains largely unknown. The GH isoforms not only present challenges for accurate measurement of GH in blood, but also offer the opportunity to differentiate endogenous from exogenous GH.

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