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To elucidate further the manner in which gonadal steroids influence the secretion of LH, we examined the effects of gonadectomy and the absence of functional androgen receptors on GnRH-induced LH release from dispersed rat anterior pituitary cells. Intact and gonadectomized (GNX) normal rats and androgen-resistant, testicular feminized (Tfm) animals from the King x Holtzman strain (a mutant strain that possesses defective androgen receptors) were used. Dispersed pituitary cells were perifused with Medium 199 during a 4-h equilibration period and then subjected to eight 2.5-min pulses of GnRH introduced at 30-min intervals at concentrations ranging from 0.03-100 nM. Basal LH secretion by cells from intact male and female rats was indistinguishable (P = 0.79) and was substantially lower (P less than 0.0001) than that by cells from GNX male and female animals. Basal LH secretion by cells from Tfm rats was significantly higher (P less than 0.01) than that by cells from intact animals, but lower (P less than 0.005) than that by cells from GNX animals. In response to GnRH, perifused pituitary cells from animals representing all experimental groups demonstrated concentration-dependent LH release. Pituitary cells from intact female rats showed an overall greater (P less than 0.05) response to GnRH than cells from intact male rats. Pituitary cells from Tfm rats demonstrated a greater GnRH-stimulated LH mean response than cells from intact male (P less than 0.0001) or intact female (P less than 0.0001) rats. Gonadectomy of male rats resulted in an overall GnRH-stimulated LH release similar to that exhibited by cells from gonadectomized female rats (P = 0.61). Cells from Tfm animals released more LH in response to GnRH than those from gonadectomized male and female rats (P less than 0.001). These data demonstrate that the release of LH in response to GnRH by pituitary cells from intact male rats (i.e. in the presence of androgen and functional androgen receptors) is less than that seen by cells from intact females rats. Since circulating levels of testosterone and estradiol are known to be elevated in the testicular feminized rat, the heightened GnRH-stimulated LH release by cells from such animals may reflect either the long term lack of androgenic influence and/or the combined effects of androgen resistance and elevated levels of circulating estrogens.

In vitro and in vivo studies of somatotroph responsivity to GHRH stimulation indicate that partial loss of GH responsiveness occurs during constant GHRH stimulation. To determine if these observations reflect either a short term effect of GHRH or if the absence of somatostatin effects somatotroph desensitization (as occurred in in vitro studies), we administered GHRH-40 (10 ng/kg.min) by continuous iv infusion for 14 days to five normal men and one GH-deficient boy. Serum insulin-like growth factor I (IGF-I) concentrations were measured at frequent intervals to assess the biological effect of GHRH on GH secretion. The GH secretory profiles were assessed by measuring serum GH levels every 20 min for 24 h before (day 0), on the 14th GHRH infusion day, and 14 days after discontinuation of the GHRH infusion in the normal men. The GH-deficient boy was studied before and during the 14th GHRH infusion day. A supramaximal iv GHRH dose was administered at the end of the 24-h sampling period, and the GH responses were compared. Serum IGF-I concentrations increased on the 14th day of GHRH infusion in the normal men [day 0 mean, 0.84 +/- 0.14 (+/- SE) X 10(3); day 14, 1.74 +/- 0.20 X 10(3) U/L; P less than 0.05] and from 0.20 X 10(3) on day 0 to a maximum of 0.67 X 10(3) U/L on day 3 in the GH-deficient boy; they declined to pretreatment levels after discontinuation of GDRH. The mean integrated serum GH concentrations in the normal men were 1.44 +/- 0.10 micrograms/L.h on day 0 and 3.11 +/- 0.95 on day 14 of GHRH infusion. The integrated GH concentration in the GH-deficient boy was 1.53 micrograms/L.h on day 0 and 4.23 on day 14 of GHRH infusion. Pulsatile GH secretion, assessed by cluster analysis, was preserved in the normal men and occurred de novo in the GH-deficient boy on the 14th GHRH infusion day. The GH response to bolus GHRH administration was also preserved; no attenuation of the response occurred in the normal men or the GH-deficient boy after 14 days of GHRH infusion. The increase in IGF-I concentrations during 14 days of continuous GHRH administration, the persistence of pulsatile GH release in normal men, the de novo appearance of GH pulses in the GH-deficient boy, and the preservation of the response to a supramaximal GHRH dose indicates that the somatotrophs remain responsive to prolonged constant stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

We tested the hypothesis that selective beta 1-adrenergic blockade will enhance growth hormone (GH) secretion in boys with constitutional delay of growth in response to both exogenously administered growth hormone-releasing hormone as well as to endogenous GH-releasing hormone pulsations. The study group comprised eight healthy, short, prepubertal boys ranging from 7 2/12 to 15 0/12 yr old with bone ages delayed 15 to 42 months. All had demonstrated GH levels of greater than 10 ng/ml following a pharmacologic or physiologic stimulus. During two consecutive nights, blood samples were withdrawn every 20 min for GH determination between 2000 and 0800 h. Immediately after each 0800 h blood withdrawal, 1 microgram/kg of GH-releasing hormone (1-40)-OH was administered intravenously to each subject and blood was withdrawn every 15 min for an additional 2 h. During the day before the second overnight sampling period each subject received atenolol, 25 mg orally, at 1030 and 1600 h to induce beta-adrenergic blockage. The six subjects in whom beta-adrenergic blockade could be documented had enhanced GH release after GH-releasing hormone administration on the atenolol treatment day both in terms of higher peak GH levels achieved (p less than 0.05) as well as greater total GH secretion (3916 +/- 701 versus 5624 +/- 986 ng/ml.min, p less than 0.01). In contrast, there were no differences in endogenous, unstimulated nocturnal GH pulse characteristics between study and control days.(ABSTRACT TRUNCATED AT 250 WORDS)

To investigate the role of somatostatin (SRIF) in regulating sexually dimorphic GH secretion, we used a reverse hemolytic plaque assay and acutely dispersed somatotropes from age-matched normal male, normal female, and androgen receptor-deficient, testicular feminized (Tfm) rats. Hemolytic plaques were developed after a 90-min incubation in the presence of GH antiserum, 10 nM GH-releasing hormone (GHRH), and the following concentrations of SRIF: 0, 0.001, 0.003, 0.01, 0.03, 0.1, 0.3, 1, 3, 10, 30, and 100 nM. Additional studies were performed with 0 or 100 nM SRIF in the absence of GHRH. The absolute number of somatotropes (x10(6); mean +/- SEM) recovered from the pituitaries of Tfm rats (1.73 +/- 0.18) was significantly greater than that from the males (1.11 +/- 0.13; P = 0.01); the number from female rats (1.30 +/- 0.15) was not different from that of either male or Tfm animals. GHRH-stimulated GH secretion, as estimated by the mean GH plaque area (micron2 x 10(4); mean +/- SEM) in the absence of SRIF, was greater for somatotropes from male rats (3.36 +/- 0.41) than that for either Tfm (2.27 +/- 0.32; P = 0.02) or female (1.78 +/- 0.24; P = 0.001) rats; values for the latter two groups did not differ. However, mean GH plaque areas for each group during maximal SRIF inhibition in either the presence or absence of GHRH were indistinguishable from each other and from mean plaque areas obtained under basal conditions. As demonstrated by a lesser EC50 value (0.04 +/- 0.02 nM; mean +/- SEM), somatotropes from female rats were more sensitive to the inhibitory effect of SRIF than were those from either male (EC50 = 1.82 +/- 0.45; P = 0.0001) or Tfm (EC50 = 0.74 +/- 0.22, P = 0.0001) rats; values for the latter two groups were indistinguishable. These observed differences suggest that gender and/or the gonadal hormone environment may be important determinants of the inhibitory effects of SRIF on GH secretion by the somatotrope. While these gender-associated differences may represent effects of the gonadal hormones directly on the somatotrope, they could reflect modulation of the secretion of hypothalamic SRIF and/or GHRH by the prevailing gonadal hormone environment. Such gender-related differences may contribute to the overall sex-dependent patterns of GH secretion in the intact animal.

To determine whether a normal complement of androgen receptors is required to permit full expression of sex-related differences in pituitary GH secretion, we compared the GHRH-stimulated GH secretory responses of continuously perifused anterior pituitary cells from normal male, normal female, and androgen-resistant testicular feminized (Tfm) rats. In each experimental replicate, acutely dispersed pituitary cells were exposed to GHRH (0.03-100 nM) administered as 2.5-min pulses in random order at 30-min intervals. The eluate was collected in 5-min fractions for GH determination by RIA. Basal unstimulated secretion of GH by cells from male rats was greater than that by cells from female (P = 0.007) and Tfm (P = 0.03) rats; basal secretion by the other two groups was similar (P = 0.55). Linear concentration-response relationships between GHRH and GH release were defined for cells from male (P = 0.0002), female (P = 0.0001), and Tfm (P = 0.0002) rats. Overall GHRH-stimulated GH secretion by cells from male rats was greater (P less than 0.0001) than that by cells from female rats. Overall secretion by cells from Tfm rats was less (P less than 0.001) than that by cells from male rats but greater (P less than 0.001) than that by cells from female rats. For all experimental groups, body weight was strongly correlated with both basal (r2 = 0.42; P = 0.001) and GHRH-stimulated (r2 = 0.53; P = 0.0001) GH secretion by the dispersed pituitary cells. These data suggest that a deficiency of androgen receptors results in a diminution of the in vitro GH secretory capability of anterior pituitary cells to a level below that by cells from normal males, but not to the level in normal females. The intermediate position of cells from the Tfm rat may represent a partial masculinization or defeminization within this generally female phenotype.