Metabolic interfaces between growth and reproduction. IV. Chronic pulsatile administration of growth hormone and the timing of puberty in the female sheep.

Puberty in the female lamb is accompanied by an increased frequency of LH pulses, and during normal development this is preceded by a decline in GH. Conversely, in the growth-retarded lamb, when LH levels are depressed by low nutrition, GH secretion is elevated. Based upon this inverse relationship, we tested the hypothesis that GH may act as a metabolic signal from the brain to inhibit the secretion of LH, and that the decline in GH times puberty. Our approach was to extend high circulating GH levels far beyond the early postnatal period, in a physiological pattern and level, in an attempt to block the pubertal LH rise. To evaluate the pattern of LH as a continuous variable under conditions of constant estradiol negative feedback, the gonadotropin was measured in blood samples collected by jugular venipuncture twice weekly; the lambs were ovariectomized and treated chronically with estradiol (Silastic capsule) beginning at 3 weeks of age. Nine lambs served as untreated controls, and 7 were infused iv with pituitary-derived bovine GH (bGH) between 5 and 28 weeks of age. A programmable backpack infusion pump delivered bGH as hourly pulses, with a total dose of 18 micrograms/kg.24 h, to maintain a physiological pattern and level of GH. At various ages, blood samples were collected at 12-min intervals for 6 h to monitor patterns and levels of peripheral LH and GH. Circulating GH in untreated and treated lambs averaged 7.7 +/- 1.5 ng/ml over a 6-h period at 4 weeks of age and declined to 1.1 +/- 0.2 ng/ml by 19 weeks in the untreated lambs; in contrast, bGH-infused lambs averaged 10.4 +/- 0.9 ng/ml at 19 weeks. Although body weights did not differ, back fat depth and quantity of perirenal fat were reduced in bGH-treated females compared to that in controls. Moreover, insulin-like growth factor-I levels were higher in bGH-treated compared with control lambs, and the bGH-treated lambs exhibited glucose intolerance, thus confirming that infused bGH was biologically active. Neuroendocrine sexual maturity, however, was not different in bGH-treated and control lambs, and it occurred at 21-22 weeks of age. The results do not support our hypothesis that decreasing GH secretion is a requirement for puberty in the sheep. Moreover, unlike in children with delayed puberty, exogenous bGH did not advance normal puberty in the lamb.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth hormone inhibits activation of phosphatidylinositol phospholipase C in adipose plasma membranes: evidence for a growth hormone-induced change in G protein function.

Pituitary growth hormone (GH) functions physiologically to oppose the actions of insulin on carbohydrate and lipid metabolism by interfering with metabolic events that occur after insulin binds to its receptor. Which postreceptor effects are involved is presently unknown. Recently, we found that insulin rapidly stimulates a phosphatidylinositol phospholipase C (PI-PLC) in adipose tissue of obese (ob/ob) mice and that this effect of insulin is blocked by treatment of the animals with S-carboxymethylated human GH (RCM-hGH), a derivative having mainly anti-insulin activity. The activation of this PI-PLC by insulin is also inhibited by pertussis toxin. Thus, this study was performed to examine whether the inhibitory effect of GH on the activation of this PI-PLC is exerted at the level of signal transmission by guanine nucleotide binding proteins (G proteins). We found that the nonhydrolyzable GTP analogue, guanosine 5'-[gamma-thio]triphosphate, stimulated basal PI-PLC activity in plasma membranes of adipose tissue of saline-treated ob/ob mice, but it did not stimulate the enzyme in adipose membranes from RCM-hGH-treated mice. Also, RCM-hGH treatment markedly inhibited pertussis toxin-catalyzed ADP ribosylation of G protein alpha subunits in the membranes, suggesting some modification of the G proteins by GH. Immunoblot analysis of adipose membranes from saline- and RCM-hGH-treated mice using antiserum AS/7 (anti-Gi1 alpha and anti-Gi2 alpha) or antiserum EC/2 (anti-Gi3 alpha) showed no difference in the amount of Gi alpha-like protein between the groups. These findings suggest that GH interferes with the ability of a putative Gi-like protein to mediate the activation of PI-PLC in adipose membranes without altering the expression of the G protein.

Growth hormone receptors and action in BC3H-1 myocytes.

The muscles of the body are generally regarded as major targets for the actions of growth hormone (GH), but these tissues are difficult to utilize for in vitro studies of rapid cellular events. Therefore, the present study was conducted to determine whether the BC3H-1 cultured myocyte cell line possesses GH receptors and whether it exhibits metabolic responsiveness to GH. It was found that the myocytes possess receptors, which appear to be specific for GH and GH derivatives but which do not bind prolactin. The degree of GH binding observed was small, suggesting that receptors for GH are present in rather low abundance on these cells. To determine whether BC3H-1 myocytes are metabolically responsive to GH, the insulin-like activity of GH (i.e. the ability of GH to stimulate glucose metabolism) was examined, since this effect of GH is readily demonstrable when isolated rat muscle is incubated directly with the hormone. GH was found to stimulate glucose oxidation by the myocytes indicating that the receptors for GH on these cells are functional.

Isolated adipocytes from growth hormone-treated obese (ob/ob) mice exhibit insulin resistance.

The genetically obese (ob/ob) mouse is a useful model for the study of the diabetogenic action of growth hormone (GH), because treatment of these animals with GH results in decreased responsiveness of their adipose tissue to insulin in vitro. Studies of the mechanisms involved in GH-induced insulin resistance using isolated adipocytes of ob/ob mice have not been possible, however, because of their extreme fragility and the lack of an adequate system for the maintenance of these cells. This study describes a new method for the isolation of ob/ob mouse adipocytes. The isolated cells are stable, viable and metabolically responsive to insulin. In addition, these adipocytes have been maintained in primary culture, in serum-free medium, for up to 3 days. During culture, the cells exhibit large increases in 125I-hGH binding (10-20-fold) and porcine 125I-insulin binding (5-10-fold). The induction of insulin resistance by GH has also been demonstrated in these freshly isolated ob/ob mouse adipocytes. The studies to date indicate that the ob/ob mouse adipocyte system should provide a useful model for detailed studies of the cellular and molecular mechanisms of GH induced insulin resistance.