Subcutaneous treatment with growth hormone-releasing hormone for short stature

In the present study we report the effects of therapy with growth hormone-releasing factor (1-29)NH2 (GRF) on growth rate, plasma levels of insulin growth factor I (IGF-I) and growth hormone (GH) secretion in 11 children who were selected solely on the basis of their short stature and normal GH secretion on standard provocative tests. All children received GRF for 6 months (5 micrograms/kg body weight subcutaneously) each evening. The 24-hour GH secretory profile was studied before and after 6 months of treatment. Simultaneously, GH secretory responses to single intravenous bolus GRF (1.5 micrograms/kg body weight) were also studied before, during, and 6 months off therapy with GRF(1-29)NH2. Plasma levels of IGF-I were measured before, during (1, 2 and 6 months), and after 6 months off therapy with GRF. Statural growth was measured at 3-month intervals. The peak plasma GH level in response to GRF was 56.04 +/- (SD) 24.46 ng/ml before treatment, and similar results were found after therapy. The 24-hour GH secretory profile did not show differences before, during, and after treatment. Comparably, no differences were found in GH pulse frequency, pulse amplitude, pulse height, pulse increment, pulse area and total area before, and 6 months off therapy with GRF. The increments in serum IGF-I achieved were not significantly different at all intervals studied. All patients increased growth velocities (mean +/- SD, cm/year) in response to GRF therapy. Our results demonstrate that GRF administration was effective in accelerating growth velocity in 11 children without GH deficiency.     

Growth hormone secretion during pregnancy: altered effects of growth hormone releasing factor and insulin-like growth factor-I in vitro

Growth hormone (GH) secretion is controlled by growth hormone releasing factor (GRF) but changes in the circulating level of this hormone are difficult to measure. Insulin-like growth factor (IGF-I) is a GH-dependent growth factor which significantly but slightly inhibits stimulated GH release in vitro. We have tested the effects of GRF and IGF-I on GH release in pregnancy, a state in which serum concentrations of GH are elevated and levels of IGF-I are lowered. We have found, in a system of acutely dispersed adenohypophysial cells prepared from pregnant (day 21-23) or control cycling female rats, that adenohypophysial cells from pregnant rats have an increased GH release with GRF. In contrast, IGF-I inhibition is similar but slightly smaller. These altered responses may result in elevated serum GH levels during pregnancy.     

Somatostatin prevents the desensitizing action of growth hormone-releasing factor on growth hormone release

We have investigated the effect of prior exposure to somatostatin (SRIF) alone or in combination with growth hormone-releasing factor (GRF) on the subsequent cyclic AMP and GH responses to GRF in rat anterior pituitary cells in primary culture. The maximal 4.5-fold stimulation of GH release induced by a 3-hr incubation with GRF is reduced by 60% following a prior 3-hr exposure to 30 nM GRF. A 3-hr preincubation with GRF in the presence of 30 nM SRIF doubles spontaneous GH release while the maximal amount of GH released during a subsequent 3-hr exposure to GRF is similar to that measured in cells pretreated with control medium, thus completely preventing the loss of GH responsiveness induced by prior exposure to GRF. The prevention by SRIF of the desensitizing action of GRF on GH release is not observed on the cyclic AMP response which remains almost completely inhibited in GRF-pretreated cells. Similar protective effects are obtained when SRIF is incubated with prostaglandin E2 (PGE2), thus completely preventing the desensitizing action of PGE2 on GH release. Prior treatment with pertussis toxin completely prevents the protective action of SRIF on GH responsiveness. Pretreatment with GRF + SRIF increases by 85 and 60% the maximal amount of GH release induced by cholera toxin and 8-bromoadenosine 3',5'-monophosphate, respectively. The post-SRIF rebound effect on GH release occurs mainly during the first 30 min following withdrawal of the tetradecapeptide. The present data demonstrate that simultaneous preincubation with SRIF and GRF prevents the marked inhibition of GH release during subsequent exposure to GRF.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of testosterone and estrogen on the pituitary growth hormone response to growth hormone-releasing factor

The effects of testosterone and estrogen on the pituitary growth hormone response to hypothalamic growth hormone-releasing factor (GRF) were evaluated in vivo using male and female rats and in vitro using a pituitary cell monolayer culture system. In vivo the increase in plasma growth hormone (GH) concentration in response to a 500 ng/kg dose of GRF was similar in gonadectomized male and female rats. Pretreatment of intact and gonadectomized male rats with testosterone caused significant enhancement of the pituitary GH response to GRF, whereas pretreatment of gonadectomized female rats with 17 beta-estradiol did not alter the response. The GH response to GRF was not different between prepubertal (i.e., 30-day-old) male and female rats. However, following puberty (i.e., by 60 days of age), the response in male rats was significantly greater than that observed in female rats. The in vitro preincubation of anterior pituitary cells with either testosterone or 17 beta-estradiol did not cause any shift in the dose-response curve between GRF and GH. These results demonstrated that androgens play an active role in modulating the pituitary response to GRF in vivo.     

Effect of growth hormone-releasing factor on plasma growth hormone, prolactin and somatomedin C in hypopituitary and short normal children

We studied the effect of a single intravenous bolus of 0.5 microgram/kg of growth hormone-releasing factor (GRF) on plasma GH, prolactin (PRL) and somatomedin C (SMC) in 12 short normal children and 24 patients with severe GH deficiency (GHD), i.e. GH less than 5 ng/ml after insulin and glucagon tolerance tests. GRF elicited an increase in plasma GH in both short normal and GHD children. The mean GH peak was lower in the GHD than in the short normal children (8.2 +/- 2.5 vs. 39.2 +/- 5.1 ng/ml, p less than 0.001). In the GHD patients (but not in the short normals) there was a negative correlation between bone age and peak GH after GRF (r = -0.58, p less than 0.005); GH peaks within the normal range were seen in 5 out of 8 GHD children with a bone age less than 5 years. In the short normal children, GRF had no effect on plasma PRL, which decreased continuously between 8.30 and 11 a.m. (from 206 +/- 22 to 86 +/- 10 microU/ml, p less than 0.005), a reflection of its circadian rhythm. In the majority of the GHD patients, PRL levels were higher than in the short normal children but had the same circadian rhythm, except that a slight increase in PRL was observed 15 min after GRF; this increase in PRL was seen both in children with isolated GHD and in those with multiple hormone deficiencies; it did occur in some GHD patients who had no GH response to GRF. Serum SMC did not change 24 h after GRF in the short normal children. We conclude that: (1) in short normal children: (a) the mean GH response to a single intravenous bolus of 0.5 microgram/kg of GRF is similar to that reported in young adults and (b) GRF has no effect on PRL secretion; (2) in GHD patients: (a) normal GH responses to GRF are seen in patients with a bone age less than 5 years and establish the integrity of the somatotrophs in those cases; (b) the GH responsiveness to GRF decreases with age, which probably reflects the duration of endogenous GRF deficiency, and (c) although the PRL response to GRF is heterogeneous, it does in some patients provide additional evidence of responsive pituitary tissue.     

Growth hormone-releasing factor on growth hormone secretion in prepubertal calves

The effects of iv administration of growth hormone-releasing factor (GRF) on growth hormone (GH) release and on nitrogen metabolism were measured in prepubertal calves. Crossbred beef heifers (111 kg) were used in a Latin square design to test the effects of 0, 0.01, 0.033, 0.067, and 0.1 microgram human pancreatic (hp) GRF [hpGRF (1,40)OH]/kg body wt on plasma GH concentrations. When they were given doses of 0.067 and 0.1 microgram hpGRF/kg body wt, plasma GH increased (P less than 0.05) within 5-15 min, compared with injections of control buffer, and then returned to preinjection concentrations. The response to 0.067 microgram hpGRF/kg body wt every 3 hr for 42 hr was studied in five heifers (137 kg body wt). The animals responded to 50% of the GRF injections with an increase in plasma GH during every 6-hr period measured. Nitrogen retention, hormone concentrations, and weight gain were measured in five bull calves (90 kg body wt) administered 0 or 0.067 microgram Nle rat hypothalamic GRF (1,29)NH2/kg body wt every 4 hr for 10 days. Metabolic parameters were interpreted to indicate an anabolic response to GRF even though increases of 16% in nitrogen retention, 23% in plasma somatomedin C concentrations, and 36% in weight gain with pulsatile GRF treatment were variable and statistically similar to those of controls. These results indicate that GRF induces peak GH secretion within 15 min in prepubertal calves and that calves can respond to multiple injections of GRF with an increase in plasma GH.     

Somatostatin as a physiological regulator of pulsatile growth hormone secretion

Somatostatin plays an important role in the regulation of the episodic and ultradian rhythm of growth hormone (GH) secretion. Passive immunization of rats with specific antibodies to the 14 and 28 amino acid sequences caused a significant GH elevation. The fact that somatostatin antiserum was unable to block episodic GH surges indicates that this hormone's release must be regulated by a dual mechanism. Indeed, GH-releasing factor (GRF) seems to be instrumental in the maintenance of pulsatile GH secretion. Moreover, exogenous GRF induced a further GH increase predominantly during the period of active secretion. Neutralization of endogenous somatostatin eliminated this time-dependent effect, indicating that this peptide blocks periodical spontaneous GH release. Food deprivation and changes in glucose homeostasis virtually obliterate the ultradian GH rhythm. In this context, peripheral somatostatin seems to play an important role. Also the central GRF/somatostatin interplay is responsible for a short-loop feedback control on pituitary somatotrops.     

Dynamics of secretory granules in somatotrophs of rats after stimulation with growth hormone-releasing factor: a stereological analysis

The anterior pituitary tissue of male rats injected with growth hormone-releasing factor (GRF) was either processed for stereology at the light-and electron-microscopic levels, or homogenized for growth hormone (GH) assay 2–60 min after GRF injection. Secretory granules of somatotrophs became smaller but increased in numerical density 2 min after GRF injection. Their volume density began to increase at 5 min. The frequency of exocytosis of the granules was most prominent as early as 2 min after GRF injection and reduced thereafter. GH levels in the tissue were lowest at 2–5 min, and returned to the control value by 60 min. Serum GH levels were highest at 15 min; even at 60 min, this value was higher than in the controls. These findings suggest that secretory granules in somatotrophs are stimulated to divide by GRF, resulting in a decrease in size and an increase in number. The discrepancy between the earlier formation of new secretory granules and the later restoration of intracellular GH levels implies that GRF first stimulates the synthesis of constituents of granules other than GH, and only later the synthesis of GH, and that newly formed small secretory granules contain less GH. From the clearance rate of serum GH and the frequency of granule exocytosis, it can be estimated that about a half million granules are released to maintain 1 ng/ml of serum GH in rats.     

Ontogeny of the response to growth hormone-releasing factor

Primary cell cultures were prepared from fetal, neonatal and adult rat pituitaries and evaluated for their ability to secrete growth hormone (GH) in response to growth hormone-releasing factor (GRF). Pituitary cells prepared from fetuses at days 19 and 21 of gestation, neonatal animals at the day of birth (day 0) or the following day (day 1) and peripubertal male rats showed full dose response curves to GRF with maximal GH release when stimulated with 1 X 10(-10) M rat GRF. At this concentration of GRF, the amount of GH released was not different from that elicited by activation of adenylate cyclase with 1 X 10(-5) M forskolin. In contradistinction, a preparation of cells from fetuses at day 18 of gestation did not show the same release of GH when challenged with 1 X 10(-10) M GRF and forskolin (0.057 +/- 0.001, compared to 0.076 +/- 0.003 micrograms/10(5) cells per 4.5 h), although the cells clearly responded to both secretagogues (basal levels of GH, 0.029 +/- 0.002 micrograms/10(5) cells per 4.5 h). While cells prepared from fetuses at day 21 of gestation or from animals after birth released 5-10% of their total cellular GH content, those prepared from 18- and 19-day fetuses released as much as 40% of their total GH suggesting there is a maturation of intracellular GH processing that occurs late in gestation. The results show that, in late pregnancy, the rat fetal pituitary is highly responsive to growth hormone-releasing factor and suggest that this peptide participates in regulating GH levels during the perinatal period.     

Change in the growth hormone (GH) response to GH-releasing factor (GRF) caused by exogenous GH in short children

To determine whether exogenous GH induces feedback of GH release in children, growth hormone-releasing factor (GRP) tests were performed before and after 10-day GH administration. Sixteen non-obese short boys, aged 5-14 yr, with normal GH response to pharmacological tests were studied. Mean basal and peak serum GH levels in GRF tests before and after exogenous GH were not significantly different. The subjects were divided into two groups, A and B, according to the percent change in integrated areas under the GH curves in GRF tests (GH AUC) before and after 10-day GH administration. Group A consisted of 6 boys with decreased GH AUC and group B consisted of 10 boys with increased GH AUC. Mean peak GH in GRF tests and mean GH AUC were significantly higher before exogenous GH in group A than in group B. The boys in group A were all prepubertal, while 4 boys in group B had begun their early pubertal change. The mean age in group A (7.8 +/- 1.8 yr) was significantly lower than that of group B (11.9 +/- 2.4 yr). GH AUC before exogenous GH showed a significant correlation with the percent change in AUC (= -0.742, p less than 0.01). These data demonstrated that the exogenous GH suppressed the GH response to GRF in prepubertal children with good response to GRF before exogenous GH, while it exaggerated the GH response to GRF in older children with relatively poor response before GH.     

Effects of corticotropin releasing factor and growth hormone releasing factor on pituitary hormone secretion in patients with congenital thyrotropin deficiency. Abnormal response of growth hormone to corticotropin releasing factor

Blood concentrations of anterior pituitary hormones, ACTH, GH, TSH, PRL, LH, and FSH were determined in corticotropin releasing factor (CRF) test (synthetic ovine CRF 1.0 microgram per kg body weight) and growth hormone releasing factor (GRF) test (synthetic human pancreatic GRF-44 100 micrograms) in 2 female sibling patients with congenital isolated TSH deficiency, in their mother, in 2 patients with congenital primary hypothyroidism and in 8 normal controls. The patients with isolated TSH deficiency showed normally increased plasma ACTH and serum GH after CRF and GRF, respectively, and also showed an abnormal GH response to CRF. The serum GH showed a rapid increase to maximum levels (12.9 ng/ml) within 30 to 60 min followed by decrease. The possibility of secretion of abnormal GH could be excluded by the fact that on serum dilution, GH value gave a linear plot passing through zero. In addition, serum PRL, LH and FSH levels after CRF administration in case 1 and PRL after GRF in case 2 were also slightly increased but these responses were marginal. The mother of the patients, patients with congenital primary hypothyroidism, and normal healthy controls showed normal responses of pituitary hormones throughout the experiment. Data from the present study and a previous report show that abnormal GH response to the hypothalamic hormones (CRF, TRH and LHRH) may be observed in patients with congenital isolated TSH deficiency.     

Hypothalamic growth hormone-releasing factor (GRF) participates in the negative feedback regulation of growth hormone secretion

Effects of growth hormone (GH) excess on immunoreactive hypothalamic GH-releasing factor (GRF) and somatostatin (SRIF) were studied in rats. Hypothalamic GRF content significantly reduced after 7-day daily treatment with 160 micrograms of rat GH or after inoculation of GH-secreting rat pituitary tumors, MtT-F4 for 9 or 13 days and GH3 for 3 months. Basal and 59 mM K+-evoked release of GRF from incubated hypothalami diminished, more than the content, by 43-51% in MtT-F4 tumor- or by 67-83% in GH3 tumor-bearing rats. In contrast, there was a small but significant increase in content or release of SRIF in rats harboring the GH3 or MtT-F4 tumor, respectively. These results indicate the existence of a negative feedback loop via hypothalamic GRF as well as SRIF in control of GH secretion.     

Effects of growth hormone-releasing factor, somatostatin and dopamine on growth hormone and prolactin secretion from cultured ovine pituitary cells

A synthetic form of human pancreatic growth hormone releasing factor (GRF-44-NH2) was shown to be a potent stimulator of growth hormone (GH) secretion and cellular cyclic AMP levels in cultured sheep pituitary cells. A small dose-dependent stimulation of prolactin secretion was also observed. Somatostatin (0.5 microM) completely blocked the maximal GRF (1 nM)-stimulated secretion without a significant effect on cyclic AMP levels. Dopamine (0.1 microM) inhibited the GRF-elevated GH secretion by 50% and lowered cyclic AMP levels by 30%. Dopamine (0.1 microM) inhibition of basal prolactin secretion was not affected by GRF (1 nM). The data support the hypothesis that cyclic AMP is involved in the action of GRF but suggest that somatostatin can inhibit GRF-induced secretion of GH independently of cyclic AMP.     

Dynamic of the GRF-induced GH response in genetically obese Zucker rats: influence of central and peripheral factors

To determine the time onset of the growth hormone (GH) alteration in the genetically obese rat, we studied the in vivo and in vitro rat growth hormone releasing factor (rGRF(1-29)NH2)-induced GH secretion in 6- and 8-week-old lean and obese male Zucker rats. Under sodium pentobarbital anesthesia, rGRF(1-29)NH2 (GRF) was injected intravenously at two doses: 0.8 and 4.0 micrograms/kg b.w. Basal serum GH concentrations were similar in lean and obese age-matched animals. The GH response to both GRF doses tested was unchanged in 6-week-old obese rats as compared to their lean litter mates. In contrast, a significant decrease of the GH secretion in response to 4.0 micrograms/kg b.w. GRF was observed in the 8-week-old obese rats. The effect of GRF (1.56, 6.25 and 12.5 pM) was further studied in vitro, in a perifusion system of freshly dispersed anterior pituitary cells of lean and obese Zucker rats. Basal GH release was similar in the 6-week-old animal group. In contrast, it was significantly decreased in 8-week-old obese rats as compared to their lean litter mates. Stimulated GH response to 1.56 and 6.25 pM GRF was significantly greater in the 6-week-old obese group than in the age-matched control group. In contrast, the GH response to all GRF concentrations tested was significantly decreased in the 8-week-old obese rats as compared to their respective lean siblings. In 8-week-old obese rats, a decrease of GH pituitary content and an increase of hypothalamic somatostatin (SRIF) concentration were observed. Insulin and free fatty acid serum were significantly increased in 8-week-old obese rats. In contrast, lower insulin-like growth factor I serum levels were observed in the obese animals as compared to their lean litter mates. Finally, to further clarify the role of the periphery in the inhibition of GH secretion observed in the 8-week-old fatty rats, we exposed cultured pituitary cells of 8-week-old lean animals to 17% serum of their obese litter mates. A significant decrease of GRF-stimulated GH secretion of lean rat pituitary cells exposed to the obese serum was noted (P less than 0.05). This study demonstrates that, in the obese Zucker rat, an alteration of the GH response to GRF is evident by the 8th week of life. This defective GH secretion could be related to peripheral and central abnormalities.     

Administration of a nonpeptidyl growth hormone secretagogue, L-163, 255, changes somatostatin pattern, but has no effect on patterns of growth hormone-releasing factor in the hypophyseal-portal circulation of the conscious pig.

The activity of the growth hormone secretagog, L-163,255, on growth hormone (GH), growth hormone-releasing factor (GRF), and somatostatin (SRIF) levels was evaluated in a porcine model of hypophyseal portal blood (HPB) collection. Young, castrated pigs had HPB and jugular blood collected for approximately 300 min. The blood collection was divided into discrete periods: baseline (BL) approximately 180 min; GH response period (RSP) approximately 90 min; and positive control period following a GRF bolus, 30 min. RSP was divided into a dominant response period (DOM) and a tail (TL). The spontaneous relationship between HPB GRF and SRIF and peripheral GH during BL has been reported (Proc Soc Exp Biol Med 217:188-196, 1998). The apex of the GH pulse resulting from L-163,255 administration was nonrandomly associated (P < 0.05) with descending periods of SRIF troughs. Frequency and amplitude of GRF and SRIF pulses, and frequency and depth of SRIF troughs were not different between BL and the beginning of DOM (the 20-30 min of GH increase). GH AUC was significantly greater (P < 0.05) for DOM compared to BL and TL, and for TL compared to BL. GRF AUC tended to be greater (P < 0.1) for RSP compared to BL, but the majority of the increase was in the TL period. There were no significant differences in the SRIF AUCs between the sampling periods. Furthermore, in a separate experiment, fos activity (a marker of neuronal activation) in the hypothalamus of pigs was examined after either L-163,255 (1x or 4x), isotonic saline (control), or hypertonic saline (positive control) administration. There were no differences in fos activity in the GRF, SRIF, or CRH immunopositive neurons between L-163,255 treatment and control. The pituitaries of the L-163,255-treated pigs showed marked fos activation compared to the controls. In conclusion, L-163,255 in pigs has its primary effect at the level of the anterior pituitary.     

Actions of L-363,586, a cyclic hexapeptide analogue of somatostatin, on GH secretion by human somatotrophinoma cells in vitro.

L-363,586 is a cyclic, hexapeptide analogue of somatostatin-14 with potent inhibitory actions on rat growth hormone (GH) release in vitro. The studies reported here investigate the direct effects of L-363,586 on basal and growth hormone-releasing factor (GRF)-stimulated GH secretion from 3 human somatotrophinomas in dispersed cell culture. 1nM and 10nM L-363,586 inhibited both basal and GRF-stimulated GH release from cells of all 3 somatotrophinomas during a 2h treatment period, whilst 100nM L-363,586 had a prolonged inhibitory action on basal GH secretion from cells of 2 of the tumours throughout treatment and recovery periods. Rebound release of GH was observed with cells of 1 tumour following treatment with L-363,586 plus GRF. The actions of L-363,586 were similar to those of somatostatin-14. These data suggest that L-363,586 may have a role in the treatment of acromegaly.     

The episodic secretory pattern of growth hormone regulates liver carbonic anhydrase III. Studies in normal and mutant growth-hormone-deficient dwarf rats.

Carbonic anhydrase III (CAIII) occurs in male rat liver at concentrations twenty times those in the female, and is sensitive to the pattern of growth hormone (GH) release. Males release GH episodically and have high concentrations of CAIII; females produce GH in a more continuous fashion and have lower CAIII levels. In normal female rats, the endogenous GH secretory pattern was masculinized, either by regular injections of GH-releasing factor (GRF) or by intermittent infusions of somatostatin (90 min on/90 min off). Both treatments induced regular GH pulses and stimulated growth, but only intermittent somatostatin infusions raised CAIII levels (controls, 1.5 +/- 0.5; somatostatin-treated, 9.0 +/- 2.9 micrograms/mg; means +/- S.D.). GRF pulses (4 micrograms every 4 h) did not however raise CAIII levels (controls 1.8 +/- 0.5; GRF-treated 1.4 +/- 0.4 micrograms/mg). Surprisingly, hepatic CAIII is also sexually dimorphic (males, 18.8 +/- 3; females, 2.22 +/- 0.4 micrograms/mg) in a GH-deficient dwarf rat strain which has low plasma GH levels without 3-hourly GH peaks. Intermittent somatostatin infusions in female dwarf rats partially masculinized hepatic CAIII, an effect reduced by co-infusion with GRF. This CAIII response was not secondary to growth induction, since neither somatostatin nor GRF stimulated growth in dwarf rats, and pulses of exogenous GH stimulated growth in female dwarfs without masculinizing CAIII levels. Furthermore, continuous GH infusion in male dwarf rats partially feminized hepatic CAIII levels (to 9.1 +/- 2.4 micrograms/mg), whereas infusions of insulin-like growth factor-1, which induced the same body weight gain, did not affect hepatic CAIII (20.8 +/- 6 micrograms/mg). These results show that hepatic CAIII expression is highly sensitive to the endogenous GH secretory pattern, independent of growth. They also implicate the low basal GH levels between pulses, rather than the peak GH levels, as the primary determinant of the sexually dimorphic hepatic CAIII expression in the rat.     

Inhibition of somatotroph growth and growth hormone biosynthesis by activin in vitro

Activin-A, a homodimeric protein composed of two inhibin beta A-subunits, was first isolated from gonadal fluids based upon its ability to stimulate FSH secretion and biosynthesis, but was also observed to suppress GH secretion. The present report describes the effects of activin on the biosynthesis of GH and the proliferation of pituitary somatotrophs. In pituitary cells cultured in the presence of 0.7 nM activin for 3 days, GH secretion was decreased by 50% compared to the control value. Inhibition of GH biosynthesis, measured by quantitative immunoprecipitation of [35S]methionine-labeled cells, could be observed after 24 h of activin treatment, and maximal (70%) inhibition of GH biosynthesis was observed after 3 days. Activin inhibited basal as well as GH-releasing factor (GRF)-, glucocorticoid-, and thyroid hormone-stimulated GH biosynthesis. Inhibin, which is known to reverse the effect of activin on FSH secretion, did not reverse the effect of activin on GH biosynthesis. Treatment of somatotrophs with activin for 3 days completely inhibited the growth-promoting effect of GRF on somatotrophs. However, no effect of activin on GRF-stimulated expression of the c-fos protooncogene was observed. These data demonstrate that activin, in addition to its stimulatory effect on FSH secretion, is able to inhibit both expression of GH and growth of somatotropic cells.     

Characteristics of phorbol ester stimulated growth hormone release: inhibition by insulin-like growth factor I, somatostatin, and low calcium medium and comparison with growth hormone releasing factor

TPA (12-O-tetradecanoylphorbol 13-acetate) is one of a class of compounds known as tumor promoters which perturb the inositol phosphate pathway in a number of cells. We have used TPA in a dispersed rat adenohypophysial cell system to probe the characteristics of growth hormone (GH) release. In this system we have found that the cells release GH in response to low concentrations of TPA: the EC50 was 0.23 +/- 0.05 nM (n = 6) and the maximal concentration was 5 nM. However, the maximal TPA-induced GH release was only 34 +/- 5% (n = 7) of the GH released by maximal growth hormone releasing factor (GRF) suggesting TPA releases a subpool of stored GH. Both somatostatin and insulin-like growth factor I inhibit GH release stimulated by TPA to the same extent as that stimulated by GRF, showing that the normal inhibitory control mechanism of release is not altered. Incubation in a low calcium medium that totally blocks GRF-stimulated GH release also inhibits TPA-stimulated GH release. The calcium channel blockers nifedipine and diltiazem both partly inhibit GRF- and TPA-stimulated GH release, showing some component of the calcium necessary for GH release arises from influx across the cell membrane.     

In vitro release of growth hormone-releasing factor (GRF) from the hypothalamus: somatostatin inhibits GRF release.

The manner of release of growth hormone-releasing factor (GRF) from the rat hypothalamus was studied in a perifusion system using a highly sensitive radioimmunoassay for rat GRF. The recovery of GRF in this system was 50-60%. The release of GRF from the rat hypothalamic blocks was almost stable for 20-240 min after the start of the perifusion and was stimulated by depolarization induced by high K+ concentration. The release of GRF was inhibited by somatostatin at concentrations of 10(-11) to 10(-8) M with maximum inhibition to 52.5% of the basal release at a concentration of 10(-9) M. These results suggest that this system is useful in studying the regulatory mechanism of GRF release and that, in addition to its action on the pituitary, somatostatin appears to act at the level of the hypothalamus in inhibiting GRF release in the regulation of GH secretion.