Rapid enzymatic degradation of growth hormone-releasing hormone by plasma in vitro and in vivo to a biologically inactive product cleaved at the NH2 terminus

The effect of plasma on degradation of human growth hormone-releasing hormone (GRH) was examined in vitro and in vivo using high performance liquid chromatography (HPLC), radioimmunoassay (RIA), and bioassay. When GRH(1-44)-NH2 was incubated with human plasma, the t1/2 of total GRH immunoreactivity was 63 min (RIA). However, HPLC revealed a more rapid disappearance (t1/2, 17 min) of GRH(1-44)-NH2 that was associated with the appearance of a less hydrophobic but relatively stable peptide that was fully immunoreactive. Sequence analysis indicated its structure to be GRH(3-44)-NH2. Identity was also confirmed by co-elution of purified and synthetic peptides on HPLC. Biologic activity of GRH(3-44)-NH2 was less than 10(-3) that of GRH(1-44)-NH2. After intravenous injection of GRH(1-44)-NH2 in normal subjects, a plasma immunoreactive peak with HPLC retention comparable to GRH(3-44)-NH2 was detected within 1 min and the t1/2 of GRH(1-44)-NH2 (HPLC) was 6.8 min. The results provide evidence for GRH inactivation by a plasma dipeptidylaminopeptidase that could limit its effect on the pituitary.

Hypothalamic obesity: comparison of radio-frequency and electrolytic lesions in male and female rats

Obesity-inducing lesions of the ventromedial hypothalamus (VMH) have generally been produced by passing anodal electrolytic current through a stainless steel electrode, which in addition to tissue ablation leaves deposits of metallic ions that might chronically irritate surrounding tissue. Although a few early investigators reported that nonirritative lesions (i.e., those that do not leave deposits) resulted in little or no obesity, their results were largely ignored when others observed substantial weight gain in animals after using the same techniques. The results of the present experiment suggest that these discrepancies were the result of a sex difference (the former group of studies all used male rats, while the latter group all used females). Male rats in the present study gained less than 80% of the weight gained by female rats regardless of lesion type, while rats with nonirritative radio-frequency lesions gained about 60% of the weight gained by rats with electrolytic lesions regardless of sex. Thus, female rats with radio-frequency lesions displayed substantial (albeit suppressed) weight gain, whereas male rats with identical lesions gained only slightly more weight than sham-lesioned controls. It is concluded that obesity resulting from electrolytic lesions of the VMH is due in part to tissue ablation and in part to chronic irritation (possibly of parasympathetic efferents mediating insulin release).

Growth hormone-releasing hormone

The identification of GRH has been followed by an extraordinarily rapid rate of knowledge accumulation. Within a period of slightly more than 3 yr since the structure of the GRH was determined, nearly 500 papers have been published pertaining to the hormone. Extensive knowledge of its anatomy, chemistry, molecular biology, physiology, and pathology has been gathered and, in particular, studies in humans have proceeded faster than with any other of the hypophysiotropic hormones. New insights have been gained with respect to the pathogenesis of both GH deficiency and GH excess states, and the use of GRH and its analogs as diagnostic and therapeutic agents already represents a reality.

Growth hormone (GH)-releasing factor stimulates hypothalamic somatostatin release: an inhibitory feedback effect on GH secretion

GH-releasing factor (GRF) is a hypothalamic peptide that stimulates the secretion of pituitary GH. The possibility of feedback effects of GRF within the central nervous system was studied in conscious freely moving male rats with indwelling iv and intracerebroventricular (icv) cannulae. Animals were injected icv or iv with 10 ng-10 micrograms human (h) GRF(1-40)-OH (hGRF-40) or GRF(1-44)-NH2 (hGRF-44), and blood samples were obtained every 10-20 min from 1000-1400 h. GH secretion was pulsatile, with major secretory peaks at around 1200 h in most control animals. When 10 ng hGRF-40 were injected icv at 1100 h, immediately before the expected onset of the spontaneous GH secretory burst, GH secretion was suppressed during the following 2-h period. An iv injection of 10 ng hGRF-40 was without effect. In contrast, when 1 microgram hGRF-40 was injected icv or iv, plasma GH levels peaked at 20 and 10 min, respectively, and returned toward baseline shortly thereafter. The spontaneous GH secretory pulse after 1 microgram hGRF-40 (icv or iv) was suppressed in proportion to the magnitude of the GH secretory response to GRF (r = 0.78, p less than 0.01), and the prolongation of the interval between the injection of GRF and the subsequent spontaneous GH surge was directly related to the GH response to GRF (r = 0.85, p less than 0.001). The icv or iv injection of a larger dose of either hGRF-40 or hGRF-44 (10 micrograms) at 1100 h also resulted in marked and comparable increases in plasma GH levels, with peaks at 20 min (icv) and 10 min (iv) after injection. No changes in behavior or plasma glucose were observed up to 3 h after icv injection of any of the doses of hGRF-40 or of hGRF-44. The suppressive effect of centrally administered hGRF-40 (10 ng) on GH secretion was blocked by the iv administration of a specific antisomatostatin serum immediately before the injection of hGRF. These results demonstrate a dual action of GRF on spontaneous GH secretion and indicate the presence of an inhibitory feedback system within the central nervous system for the regulation of GH secretion which is mediated by hypothalamic somatostatin.

Decreased hypothalamic growth hormone-releasing hormone content and pituitary responsiveness in hypothyroidism

The effects of thyroidectomy (Tx) and thyroxine replacement (T4Rx) on pituitary growth hormone (GH) secretion and hypothalamic GH-releasing hormone (GRH) concentration were compared to define the mechanism of hypothyroid-associated GH deficiency. Thyroidectomized rats exhibited a complete loss of pulsatile GH secretion with extensive reduction in GRH responsiveness and pituitary GH content. Cultured pituitary cells from Tx rats exhibited reduced GRH sensitivity, maximal GH responsiveness, and intracellular cyclic AMP accumulation to GRH, while somatostatin (SRIF) suppressive effects on GH secretion were increased. Hypothalamic GRH content was also markedly reduced. T4Rx completely restored hypothalamic GRH content and spontaneous GH secretion despite only partial recovery of pituitary GH content, GRH and SRIF sensitivity, and intracellular cyclic AMP response to GRH. The results indicate multiple effects of hypothyroidism on GH secretion and suggest that a critical role of T4 in maintaining normal GH secretion, in addition to restoring GH synthesis, is related to its effect on hypothalamic GRH.

Receptor-associated resistance to growth hormone-releasing factor in dwarf "little" mice

Anterior pituitaries from the dwarf mouse strain "little" did not release growth hormone or accumulate adenosine 3',5'-monophosphate (cyclic AMP) in response to human and rat growth hormone-releasing factor (GRF). Dibutyryl cyclic AMP, as well as the adenylate cyclase stimulators forskolin and cholera toxin, markedly stimulated growth hormone (GH) release. The basis of the GH deficiency in the little mouse may therefore be a defect in an early stage of GRF-stimulated GH release related either to receptor binding or to the function of the hormone-receptor complex.

Hyperprolactinemia associated with chronic renal failure in the rat

Patients with CRF exhibit hyperprolactinemia and resistance to the prolactin-suppressive effects of dopamine. In order to explore the pathogenetic mechanisms involved, an animal model of CRF was developed in the adult male rat bearing an indwelling right atrial catheter by performing a two stage 5/6 nephrectomy (NX). Following NX, serum creatinine levels rose to a value of 1.36 +/- 0.2 mg/dl at 8 weeks as compared to sham-operated controls (0.31 +/- 0.1, P less than 0.01). There was a parallel increase in plasma prolactin levels in NX animals with values significantly greater than in controls by 8 weeks (49 +/- 11 vs 17 +/- 2 ng/ml, P less than 0.02). At 8 weeks, the plasma prolactin responses to metoclopramide (500 micrograms/kg, iv) were similar in unanesthetized NX and sham-operated control animals. The prolactin-suppressive effects of an iv dopamine infusion (6 micrograms/kg/min X 30 min) was also similar in the two groups (46 +/- 8% vs 40 +/- 10% suppression). The responses of lactotrophs in vitro were compared in NX and control animals at 8 weeks. Basal prolactin release during 4 h was similar in the two groups as were the suppressive responses to dopamine and bromocriptine. The results indicate that the rat with CRF, like human develops hyperprolactinemia. In contrast to the human, however, responses to dopaminergic agonists and antagonists in vivo and in vitro are unimpaired, indicating that hyperprolactinemia in rats with CRF occurs on a non-dopaminergic basis.

Identification of a putative GH-releasing factor (GRF) batch as predominantly ovine CRF with a small quantity of human GRF

The existence of discordant results regarding the effects of intracerebroventricular (icv) administration of GRF on GH secretion prompted a reexamination of the central actions of GRF and a detailed chemical characterization of the peptide designated as hpGRF-44-NH2. The icv injection of 10 micrograms rat (r) GRF to freely-moving rats caused an acute stimulation of GH release, whereas 10 micrograms of the putative hpGRF peptide icv continued to suppress spontaneous GH secretion. Through a series of biochemical and immunologic studies we demonstrate that peptide hpGRF-44-NH2, code number 92-81-5G-41-47, is predominantly ovine (o) CRF and also contains a small amount (3-5%) human (h) GRF-44-NH2. We conclude that the major effect of icv administered GRF, at high doses, is to stimulate GH release and that the central actions previously attributed to the hpGRF peptide are, in fact, due to the oCRF component of this compound.

Ontogenesis of growth hormone-releasing hormone neurons in the rat hypothalamus

The ontogenesis of growth hormone releasing hormone (GH-RH) containing neurons in the rat hypothalamus has been studied by immunohistochemistry, using a specific anti-rat GH-RH serum. Immunoreactive fibers were first detected in the prospective median eminence on day 18 of gestation. During the subsequent 3 days, they rapidly increased in distribution and intensity of staining within this structure. On day 21, positive fibers were also visible in a plexus within the arcuate nucleus. In 1-day-old rats treated with colchicine, positive perikarya were distributed in several hypothalamic nuclei, including the arcuate nucleus, dorsomedial nucleus, basal lateral hypothalamus, and perifornical region. The distribution was similar to that previously described in adult rats. The intensity of staining in the various hypothalamic regions increased during early postnatal life to levels nearly comparable to those in adult rats by 30 days. These findings showing the early appearance of GH-RH-positive terminals in the median eminence and the wide distribution of the perikarya at an early stage of postnatal life support the view that hypothalamic GH-RH serves an important role in the regulation of growth hormone secretion during late prenatal and early neonatal periods.

Involvement of hypothalamic somatostatin in the suppression of growth hormone secretion by central corticotropin-releasing factor in conscious male rats

The role of central corticotropin-releasing factor (CRF) in the regulation growth hormone (GH) secretion was studied in freely moving conscious male rats with indwelling intra-atrial and intracerebroventricular (i.c.v.) cannulae. GH measurements in blood samples obtained every 20 min from 10.00 to 14.00 h in control animals injected with saline either intravenously (i.v.) or into the lateral cerebral ventricle revealed that spontaneous GH secretion was pulsatile, and occurred regularly at around 12.00 h. When ovine CRF (10 micrograms) was injected i.c.v., spontaneous GH secretion was inhibited (mean plasma GH [11.20-13.00 h]: 20 +/- 7 ng/ml vs. control: 126 +/- 22 ng/ml, p less than 0.01). In contrast, the intravenous injection of CRF (10 micrograms) did not affect spontaneous GH secretion (mean plasma GH [11.20-13.00 h]: 162 +/- 25 ng/ml vs. control: 193 +/- 31 ng/ml). This GH suppressive action of central CRF was blocked by the i.v. injection (0.5 ml) of antisomatostatin serum (AS), but not of normal sheep serum (NS), (mean plasma GH [11.20-13.00 h]: NS + CRF: 15 +/- 2 ng/ml vs. AS + CRF: 202 +/- 30 ng/ml, p less than 0.01). The mean plasma GH value [11.20-13.00 h] in animals receiving AS and CRF was not significantly different from those in animals receiving saline (i.v.) together with AS. These results suggest a potential inhibitory role of central CRF in the regulation of spontaneous GH secretion in the rat which is mediated by the stimulation of hypothalamic somatostatin.

Effects of intravenous, subcutaneous, and intranasal administration of growth hormone (GH)-releasing hormone-40 on serum GH concentrations in normal men

In addition to stimulating GH release in normal subjects, GH-releasing hormone-40 (GHRH-40) stimulates GH secretion in some adults and children with GH deficiency. Recognizing that GHRH-40 may have potential as a therapeutic agent for the treatment of GH deficiency, we examined the effects of iv, sc, and intranasal (in) GHRH-40 administration on GH secretion and measured the plasma levels of immunoreactive GHRH achieved after the administration of the peptide via these different routes. Normal men were given vehicle or GHRH-40 iv (0.003, 0.01, 0.03, and 0.1 micrograms/kg; n = 10), sc (1, 3.3, and 10 micrograms/kg; n = 8), or in (3, 10, 30, and 100 micrograms/kg; n = 5). No subject had any symptoms after administration of vehicle or GHRH-40. During the 2-h period after iv administration of GHRH-40, the maximal increment in serum GH levels above basal (nanograms per ml; mean +/- SD) after the 0.1 micrograms/kg dose was 15.5 +/- 10.4 compared to 2.4 +/- 4.1 after vehicle (P = 0.0017). During the 3-h period after sc administration, when compared to the maximal increment in serum GH above basal after vehicle alone (10.2 +/- 12.9), the maximal increments above basal in serum GH were increased after both the 3.3 micrograms/kg (26.2 +/- 23.1; P = 0.022) and 10 micrograms/kg (63.6 +/- 53.5; P = 0.0003) doses. During the 3-h period after in administration, when compared to the maximal increment in serum GH above basal after vehicle alone (2.8 +/- 6.4), the maximal increments above basal in GH were higher after both the 30 micrograms/kg (18.5 +/- 10.4; P = 0.0053) and 100 micrograms/kg (21.7 +/- 8.1; P = 0.0028) doses. In addition, significant dose-response relationships were documented between the maximal increments above basal in serum GH and GHRH-40 administered by all routes. The mean (+/- SEM) peak plasma level of IR-GHRH (nanograms per ml) achieved after administration of 10 micrograms/kg GHRH-40, iv, as reported previously (66.6 +/- 17.6), was approximately 60- and 500-fold higher than the mean levels in the current study after administration of the same dose sc (1.11 +/- 0.39) or in (0.14 +/- 0.02), respectively. In summary, although GHRH-40 stimulates GH release when administered iv, sc, or in, significantly higher doses were required using the sc and in routes to achieve responses comparable to those obtained with iv administration.

Hypothalamic somatostatin mediates the suppression of growth hormone secretion by centrally administered thyrotropin-releasing hormone in conscious rats

The effect and mechanism of action of central TRH on the regulation of GH secretion was studied in conscious male rats with indwelling intraatrial and intracerebroventricular (icv) cannulae. Plasma GH was measured every 10-20 min from 1000 h-1400 h by repeated blood sampling. In animals that received saline iv or icv, GH secretion was pulsatile, with peak hormone levels occurring at 1120-1200 h. TRH (10 micrograms), injected icv at 1100 h, inhibited spontaneous GH secretion, and mean plasma GH levels remained suppressed (less than 20 ng/ml) for at least 3 h after injection. In contrast, an iv injection of the same dose of TRH at 1100 h did not significantly affect spontaneous GH secretion. Intravenous injection of human GH-releasing factor [1-40] (hGRF, 1 micrograms) at 1100 h in animals injected 5 min earlier with saline (10 microliters, icv) stimulated GH release, with peak values (748 +/- 63 ng/ml, mean +/- SE) observed 10 min after injection. However, animals injected icv with TRH (10 micrograms) 5 min before the iv injection of hGRF exhibited an attenuated GH response to hGRF (peak values, 115 +/- 28 ng/ml; P less than 0.001 vs. saline icv + hGRF). The inhibition of GH secretion by central TRH was abolished by pretreatment of animals with antisomatostatin serum (0.5 ml, iv) but not with normal serum (P less than 0.001). These results suggest an inhibitory role of central TRH in the regulation of spontaneous GH secretion in the rat that is mediated by stimulation of hypothalamic somatostatin.

Blunted growth hormone (GH) response to GH-releasing hormone in hypothyroidism resolves in the euthyroid state

GH secretion is dependent upon thyroid hormone availability. In this study, the GH response to GH-releasing hormone (GHRH) was studied in a group of patients when they were hypothyroid and also when they were euthyroid. Hypothyroidism was associated with a significant reduction in both the peak GH response and the integrated GH secretory response to GHRH compared to those in the euthyroid state [4.7 +/- 1.6 (+/- SEM) vs. 12.2 +/- 3.9 ng/ml (P less than 0.025), and 349 +/- 116 ng vs. 986 +/- 304 ng ml-1 min-1 (P less than 0.025), respectively]. GH responsiveness was impaired within 2 weeks of discontinuation of T3 treatment in athyreotic subjects and was restored within 4 weeks of T4 treatment in one chronically hypothyroid subject. The results imply that a blunted GH response to GHRH in hypothyroidism is attributable to a primary pituitary defect that occurs rapidly and is reversible with attainment of the euthyroid state.

Modulation of somatostatin binding to rat pituitary membranes by exogenously administered growth hormone

The effect of exogenously administered GH on somatostatin (SRIF) receptor regulation was studied in rat anterior pituitary membranes. A single class, high affinity specific receptor for SRIF was identified by binding studies with [125I-Tyr11]SRIF [binding capacity (mean +/- SD), 129.4 +/- 23.3 fmol/mg protein; binding affinity, 2.8 +/- 0.6 X 10(10) M-1]. A single injection of rat GH (150 micrograms) caused a significant reduction in capacity, but not in affinity, of SRIF receptors 2 and 6 h after injection (mean decrease, 23% and 24%, respectively) from that in controls. In contrast, mean SRIF binding capacity 24 h after a single injection of rat GH was increased 48% above control values, but affinity was unaffected. Measurement of membrane SRIF content indicated that these changes could not be explained by alterations in receptor occupancy. When rat GH was injected repeatedly for 3 days (150 micrograms/rat X day), the mean binding capacity, though not the affinity, of SRIF receptors was decreased 23% from that in controls 24 h after the last injection. The results can be explained by stimulation of SRIF release from the hypothalamus by GH and somatomedins, with subsequent internalization of the pituitary plasma membrane SRIF receptor. They suggest yet another level of neuroendocrine regulation of GH secretion.

Nonirritative lesions of VMH: effects on plasma insulin, obesity, and hyperreactivity

Obesity resulting from lesions of the ventromedial hypothalamus (VMH) has often been attributed to ablation-induced disinhibition of insulin release. However, lesion studies have generally employed electrolyzing anodal current with stainless steel electrodes, which results not only in tissue ablation but deposits of metallic ions that can chronically irritate surrounding tissue. The present study compared the effects of irritative and nonirritative VMH lesions on plasma insulin levels and obesity in female rats. Blood samples were obtained after a 4-h fast and 17 min after the initiation of a meal (6 ml of sweetened milk in 7 min) during a period when VMH rats were food restricted to the level of sham-operated animals and again when all animals were fed ad libitum. Irritative lesions (anodal electrolytic with stainless steel electrodes) caused heavy metallic ion deposition at the lesion site, marked obesity, and hyperinsulinemia both during food restriction and ad libitum feeding. Nonirritative lesions (cathodal electrolytic with platinum electrodes) resulted in no metallic ion deposition in seven of nine animals. These seven rats, which displayed 65% of the weight gain of animals with irritative lesions (significantly greater than sham rats), had significantly elevated insulin levels only under the postabsorptive condition during ad libitum feeding. In addition, only the animals with irritative lesions displayed emotional hyperreactivity to capture and handling. It is concluded that obesity produced by anodal electrolytic lesions with stainless steel electrodes is a result of both a destructive component resulting in hyperphagia with secondary hyperinsulinemia and an irritative component (accounting for up to 40% of the weight gain in female rats) resulting in basal hyperinsulinemia independent of hyperphagia.(ABSTRACT TRUNCATED AT 250 WORDS)

Two distinct insulin-related molecules in the guinea pig: immunological and biochemical characterization of insulin-like immunoactivity from extrapancreatic tissues of the guinea pig

In this study we extracted guinea pig brain and testis with; the extract was adsorbed to and eluted from cartridges (the Sep-Pak C18 procedure). We found this procedure superior for recovering crystalline insulin added to buffers or tissues, and for recovering endogenous insulin from plasma, but inferior for recovery of insulin from tissues. However, we did find 'rat/pork' type-insulin in guinea pig brain and testis (5-50 pg/g wet weight tissue). Our results with the Sep-Pak C18 procedures were reproduced by four other laboratories (who found 4-60 pg/g wet weight of tissue) and similar findings were also obtained by an independent investigator. Thus, we conclude that extrapancreatic tissues of guinea pigs have a second type of insulin-related material that is more typical of other mammalian insulins, but that the amount recovered is dependent upon the extraction procedure utilized.

Diethylstilbestrol inhibits tumor growth and prolactin production in rat pituitary tumors

Treatment of rats bearing transplantable MtT/W15 tumors with 10 mg of diethylstilbestrol (DES) for 3 weeks led to inhibition of tumor growth. The inhibition of tumor growth was reversible after removal of the DES. Histologic examination revealed decreased mitotic activity; however, DES did not produce cell necrosis. Concomitantly, the anterior pituitary glands of animals treated with DES became hyperplastic, with an increased number of prolactin (PRL)-producing cells. DES resulted in a decreased number of PRL cells in the tumor and decreased serum PRL/tumor weight, compared with that of control rats. There was also an increase in the number of growth hormone (GH) tumor cells and an increased serum GH/tumor weight. 17 beta-Estradiol had an effect similar to that of DES, while progesterone did not inhibit tumor growth or cause pituitary cell hyperplasia. Ovariectomy resulted in a decrease in the tumor growth rate, compared with that of control animals, suggesting that the MtT/W 15 tumors are relatively dependent on estrogens for optimal growth. These results indicate that DES inhibition of MtT/W 15 tumor growth is an excellent model for study of the mechanism of the inhibition of tumor growth and the modification of GH and PRL expression by the tumor cells.

Human pancreatic tumor GH-releasing factor

Within the past year, three similar peptides with specific growth hormone (GH) releasing effects have been extracted from human tissue, identified, and synthesized. Human pancreatic tumor GH releasing factor (I-40)-OH (hpGRF-40) was the sole hpGRF isolated from the pancreatic tumor of a patient in Charlottesville and was the predominant peptide isolated from the pancreatic tumor of a patient in Lyon. The Lyon tumor also contained hpGRF(1-37)-OH and hpGRF(1-44)-NH2. Both immunological and biochemical data suggest that hpGRF-40 and hpGRF-44 are present in the human hypothalamus and may be the human GH releasing hormone(s) (GHRH). In cultures of rat pituitary cells, hpGRF stimulates GH but affects neither basal and dopamine-inhibited prolactin release nor basal and gonadotropin releasing hormone (GnRH)-stimulated luteinizing hormone (LH) release. hpGRF stimulates cyclic AMP production within seconds, an effect which is blocked by somatostatin. In contrast, while hpGRF stimulates phosphatidylinositol turnover in the pituitary, the effect is not inhibited by somatostatin. In the human, hpGRF-40 (1 microgram/kg) given intravenously (i.v.) stimulates GH release within 5 minutes. hpGRF-40 does not elevate serum prolactin levels, thyrotropin (TSH), LH, or corticotropin (measured indirectly through plasma cortisol), or blood glucose or plasma concentrations of insulin, glucagon, pancreatic polypeptide, cholecystokinin, gastrin, gastric inhibitory peptide, motilin, or somatostatin. When graded doses of hpGRF (0.1-10 micrograms/kg) are given i.v., no differences are noted in the maximal levels of serum GH achieved.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a limited growth hormone (GH)-releasing hormone (GHRH)-releasable quantity of GH: effects of 6-hour infusions of GHRH on GH secretion in normal man

Human GH-releasing hormone [hGHRH-40 (GHRH)] stimulates GH release in a dose-dependent fashion when administered as single iv bolus doses or as continuous 90-min infusions. However, there has been variability in the GH responses, and it appears that there are waxing and waning effects of GHRH. To address whether these are a result of the dose of GHRH, time, or intermittent changes in sensitivity of the somatotrophs, we administered 6-h infusions of vehicle and different doses of GHRH to six normal men. In addition, an iv bolus injection of GHRH was given after 5.5 h of infusion to evaluate residual GH secretory capacity. The subjects were given infusions of either vehicle or GHRH (1, 3.3, and 10 ng/kg X min), followed by an iv bolus injection of 3.3 micrograms/kg on four separate occasions. GHRH infusions stimulated GH secretion compared to basal secretion. The changes from basal GH secretion (mean +/- SEM) were 2.0 +/- 1.6, 4.6 +/- 1.5, 12.7 +/- 5.1, and 8.2 +/- 1.8 ng/ml X h during the vehicle and GHRH (1, 3.3, and 10 ng/kg X min) infusions, respectively. The changes from basal GH secretion for 2 h after the iv bolus dose (after 5.5 h of infusion) were 33.3 +/- 8.7, 22.4 +/- 3.8, 14.0 +/- 3.6, and 10.5 +/- 2.0 ng/ml X h on the vehicle and GHRH (1, 3.3, and 10 ng/kg X min) infusion days, respectively. The magnitude of the GH response was inversely related to the GHRH infusion dose. The total amount of GH released during the 7.5-h study periods was not different among the vehicle and 3 GHRH infusion days. Thus, it appears that a finite amount of GH is released by GHRH. There was variability in the degree of responsiveness to the continuous infusions of GHRH. Surges of GH release occurred during the GHRH infusions, which may be attributed to intermittent secretion of a GH inhibitor, such a somatostatin.

Impaired growth hormone responses to growth hormone-releasing factor in obesity. A pituitary defect reversed with weight reduction

To investigate whether the impaired growth hormone secretion associated with obesity is a result of a hypothalamic or a pituitary disorder and whether it is a cause or a consequence of obesity, we studied plasma growth hormone responses to growth hormone-releasing factor in morbidly obese patients before gastrointestinal surgical therapy, in formerly obese subjects who had lost considerable weight postoperatively, and in non-obese controls. Growth hormone secretion was also assessed in response to insulin-induced hypoglycemia (in seven patients preoperatively and four postoperatively). In patients studied preoperatively, growth hormone responses to growth hormone-releasing factor were markedly impaired (P less than 0.001 as compared with controls), whereas in patients studied postoperatively they were partially restored to normal (P less than 0.05 as compared with those studied preoperatively). Growth hormone responses to insulin-induced hypoglycemia were similarly diminished in obese patients studied before operation (P less than 0.02). The growth hormone response to growth hormone-releasing factor was inversely correlated with the percentage of ideal body weight (P less than 0.01) and directly correlated with the growth hormone response to insulin (P less than 0.01). The impaired responsiveness to growth hormone-releasing factor suggests that the diminished response to insulin hypoglycemia is mediated by an impaired pituitary response to endogenous growth hormone-releasing factor. The reversibility of the defect after weight reduction suggests that it is a consequence rather than a cause of obesity.

Extrahypothalamic growth-hormone-releasing factor (GRF) secretion is a rare cause of acromegaly: plasma GRF levels in 177 acromegalic patients

To assess the frequency with which acromegaly is caused by ectopic secretion of GRF, we collected plasma samples from 177 unselected acromegalic patients. The samples together with those of three acromegalic patients with previously diagnosed tumors secreting GRF and of normal subjects were assayed in 3 independent GRF RIAs. Plasma immunoreactive GRF (IR-GRF) levels in normal subjects were either undetectable or detectable at levels up to 62.5 pg/ml. In none of the 177 specimens from acromegalic patients were IR-GRF values detectable in all assays, and in the most sensitive assay, the levels were similar to those in normal subjects, with the highest level measuring 82 pg/ml. In contrast, plasma IR-GRF found in the 3 patients with tumors that secreted GRF ranged from 2.0-24.4 ng/ml. These data suggest that extrahypothalamic GRF secretion is a rare cause of acromegaly. However, it is important that this rare cause of acromegaly be diagnosed before the patient has unnecessary surgery and/or irradiation directed at the pituitary. We recommend that plasma IR-GRF be measured in each new acromegalic patient.

Plasma growth hormone responses to constant infusions of human pancreatic growth hormone releasing factor. Intermittent secretion or response attenuation

Administration of human pancreatic tumor growth hormone (GH) releasing factor (hpGRF[1-40]) as a single injection to normal human subjects stimulates the secretion of GH in a dose-responsive manner. In the present studies, hpGRF(1-40) was infused in a graded stepwise manner over a 6-h period in order to determine whether the GH secretory response would be sustained. Normal adult males received four consecutive 90-min infusions of hpGRF(1-40) at doses of 1, 3.3, 10, and 33 ng/kg per min, preceded and followed by a 90-min saline infusion; and the plasma GH responses were compared with those during a separate control infusion. Plasma GH levels were significantly elevated by each hpGRF(1-40) infusion; and dose responsiveness was evident for the lowest three doses. Mean integrated GH secretory rates for the four doses were 1.95, 3.29, 4.29, and 3.65 times those of the respective control study. Plasma GH responses exhibited considerable variability, frequently decreasing during the latter part of each infusion; and at the highest dose, they decreased continuously beginning shortly after the onset of infusion. Episodic GH secretion occurred in individual subjects during each of the infusion periods. The possible contribution of hypothalamic somatostatin secretion to the diminished GH responsiveness was evaluated by determining plasma thyroid stimulating hormone (TSH) levels during the infusions and the TSH responses to thyrotropin-releasing hormone (500 micrograms i.v.) during a separate hpGRF(1-40) infusion of 2 ng/kg per min. Neither basal nor stimulated TSH levels differed between GRF-infused and control groups. The results indicate that GH secretion is dose responsive to hpGRF(1-40) infusions, though the response to hpGRF(1-40) infusions, though the response is complex. The absence of impaired TSH secretion provides evidence against a mediating role of somatostatin. The explanation for the loss of GH responsiveness remains undetermined but could include GRF-induced receptor down-regulation, a postreceptor effect, or, in spite of our negative results, a somatostatin-mediated inhibition.