H. Maurice Goodman

Growth Hormone and Dexamethasone Stimulate Lipolysis and Activate Adenylyl Cyclase in Rat Adipocytes by Selectively Shifting Giα2 to Lower Density Membrane Fractions1

GH, in the presence of glucocorticoid, produces a delayed increase in lipolysis in rat adipose tissue, but the biochemical mechanisms that account for this action have not been established. Other lipolytic agents rapidly activate adenylyl cyclase (AC) and the resulting production of cAMP initiates a chain of reactions that culminates in the activation of hormone-sensitive lipase. We compared responses of segments of rat epididymal fat or isolated adipocytes to 30 ng/ml GH and 0.1 μg/ml dexamethasone (Dex) with 0.1 ng/ml isoproterenol (ISO), which evoked a similar increase in lipolysis. All measurements were made during the fourth hour after the addition of GH+Dex or immediately after the addition of ISO to cells or tissues that had been preincubated for 3 h without hormone. Although no significant increases in cAMP were discernible in homogenates of GH+Dex-treated tissues, RP-cAMPS (RP-adenosine 3′5′-phosphothioate), a competitive inhibitor of cAMP, was equally effective in decreasing lipolysis induced by...

Hormonal Control of Growth

Publisher Summary nGrowth of an individual or an organ involves increases, both in cell number and cell size, differentiation of cells to perform highly specialized functions and tissue remodeling. The process is complex and depends on the interplay of genetic, nutritional, and environmental influences as well as actions of the endocrine system. This chapter describes the hormones that play important roles in growth and their interactions at critical times in development. Growth hormone (GH), which is also called somatotropin (STH), is the single most important hormone required for normal growth but appropriate secretion of thyroid and adrenal hormones and insulin is essential for optimal growth. The mode of action of GH involves activation of a signaling cascade by binding sequentially to the two GH receptor molecules to form a receptor dimer that sandwiches the hormone between the two receptor molecules. The physiological effects of GH include growth of skeletal system and body composition. Deficiency of GH results in pituitary dwarfism, while its overproduction leads to gigantism in children and a condition known as acromegaly in adults. GH secretion is affected by age, other neuroendocrine factors, and changes in the internal or external environment. Growth hormone also inhibits its own secretion by a short loop negative feedback effect. Thus, regulation of growth by the endocrine system can be viewed as coordination of local growth processes with overall development of the individual and with external environmental influences.

Growth Hormone Action on Adipocytes

Adipose tissue appears to be one of the physiologically important targets for direct actions of growth hormone. Many of the metabolic responses to growth hormone that can be elicited in appropriately prepared experimental animals in vivo can also be shown in vitro when physiologically relevant concentrations of growth hormone are added to segments of epididymal fat or isolated fat cells. Metabolic responses to growth hormone, which are demonstrable either in vivo or in vitro, however, are complex and time-dependent. The initial responses are insulinlike, but these are short-lived and within a few hours give way to the more typical counterinsulin responses that are generally recognized as the physiological consequences of chronic treatment with growth hormone.(1,2)

Studies on the mechanisms of epinephrine stimulation of lypolysis.

The time course for epinephrine stimulation of lypolysis, cyclic AMP accumulation and activation of protein kinase was studied in adipose tissue from hypophysectomized rats. Triglyceride breakdown, as assessed by glycerol release, increased rapidly in response to epinephrine, maintained a constant rate as long as the hormone was present, and decreased rapidly to basal values when the hormone was removed. Cyclic AMP accumulation was transient peaking within 3 min of exposure to epinephrine and then declining to levels indistinguishable from basal by 9 min. Protein kinase activity in extracts also peaked at 3 min and thereafter declined to a level approximately 25% greater than resting activity. Peak levels of cyclic AMP, steady state levels of protein kinase activity and the rate of glycerol production were all related in a dose dependent manner to the concentration of epinephrine. These observations suggest that the spike in cyclic AMP levels may be necessary to trigger the activation of lipolysis, but was not sufficient to sustain an accelerated rate of triglyceride breakdown. Continued activation of protein kinase, however, may be essential to sustained lipolysis.

Activation of the sodium pump blocks the growth hormone-induced increase in cytosolic free calcium in rat adipocytes.

GH promptly increases cytosolic free calcium ([Ca2+]i) in freshly isolated rat adipocytes. Adipocytes deprived of GH for 3 h or longer are incapable of increasing [Ca2+]i in response to GH, but instead respond in an insulin-like manner. Insulin blocks the GH-induced increase in [Ca2+]i in GH-replete cells and stimulates the sodium pump (i.e. Na+/K+-ATPase), thereby hyperpolarizing the cell membrane. Blockade of the Na+/K+-ATPase with 100 microM ouabain reversed these effects of insulin and enabled GH to increase [Ca2+]i in GH-deprived adipocytes. Both insulin and GH activated the sodium pump in GH-deprived adipocytes, as indicated by increased uptake of 86Rb+. Decreasing availability of intracellular Na+ by blockade of Na+/K+/ 2Cl- symporters or Na+/H+ antiporters abolished the effects of both hormones on 86Rb+ uptake and enabled both GH and insulin to increase [Ca2+]i in GH-deprived adipocytes. The data suggest that hormonal stimulation of Na+/K+-ATPase activity interferes with activation of voltage-sensitive calcium channels by either membrane hyperpolarization or some unknown interaction between the sodium pump and calcium channels.

Growth hormone regulates the distribution of L-type calcium channels in rat adipocyte membranes

Earlier studies demonstrated that deprivation of growth hormone (GH) for ≥3 h decreased basal and maximally stimulated cytosolic Ca2+ in rat adipocytes and suggested that membrane Ca2+channels might be decreased. Measurement of L-type Ca2+ channels in purified plasma membranes by immunoassay or dihydropyridine binding indicated a two- to fourfold decrease after 3 h of incubation without GH. No such decrease was seen in unfractionated adipocyte membrane preparations. The decrease in plasma membrane channel content was largely accounted for by redistribution of channels to a light microsomal membrane fraction. Immunoassay of α1-, α2/δ-, and β-channel subunits in membrane fractions indicated that the channels redistributed as intact complexes. Addition of GH during the 1st h of incubation prevented channel redistribution, and addition of GH after 3 h restored channel distribution to the GH-replete state of freshly isolated adipocytes. The studies suggest that GH may regulate the abundance of Ca2+ channels in the adipocyte plasma membrane and thereby modulate sensitivity to signals, the expression of which is Ca2+dependent.

Hormonal Regulation of Fuel Metabolism

Publisher Summary nThe homeostatic regulation for mammalian survival in a cold, hostile environment demands an uninterrupted supply of metabolic fuels to maintain body temperature, to escape from danger, and to grow and reproduce, which is provided by the endocrine system and the autonomic nervous system. The principal hormones that govern fuel homeostasis are insulin, glucagon, epinephrine, cortisol, growth hormone (GH), thyroxine (T4), and leptin; the principal target organs for these hormones are adipose tissue, liver, and skeletal muscle. This chapter describes the hormonal regulation of fuel metabolism, the characteristics of metabolic fuels, and the intrinsic biochemical regulatory mechanisms, under different physiological circumstances. Hypothalamic centers that regulate energy homeostasis, receive input from multiple sources including signals from adipose tissue, the pancreas, the gastrointestinal tract, and circulating nutrients. Under basal conditions an individual on a typical mixed diet derives about half of the daily energy needs from the oxidation of glucose, a small fraction from consumption of protein, and the remainder from fat. The strategy of hormonal regulation of metabolism during starvation or exercise is to provide sufficient substrate to working muscles while maintaining an adequate concentration of glucose in blood to satisfy the needs of brain and other glucose-dependent cells. Hormonal regulation of energy balance is accomplished largely through adjusting the flux of energy-rich fatty acids and their derivatives to muscle, and the consequent sparing of carbohydrate and protein.

Chapter 11 – Hormonal Control of Reproduction in the Male

This chapter provides an overview of the morphology and development of the testes along with the metabolism, functions, and regulations of its hormones. The testes serve the dual function of producing sperm and hormones. Testicular function is driven by the pituitary through the secretion of two gonadotropic hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Secretion of these pituitary hormones is controlled by the central nervous system through intermittent secretion of the hypothalamic hormone, gonadotropin-releasing hormone (GnRH), and the testes through the secretion of testosterone and inhibin. Testosterone is the principal androgen secreted by the mature testis. Some testosterone is also metabolized to estradiol. Like other steroid hormones, testosterone penetrates target cells and stimulates their growth and function. Testosterone promotes growth, differentiation, and function of accessory organs of reproduction.

Pituitary secretions related to adrenocorticotropic hormone induce sensitivity of adipose tissue to the insulin-like actions of growth hormone.

In its initial encounter with growth hormone (GH) in vitro, epididymal fat excised from GH-deficient rats responds with an insulin-like increase in glucose metabolism. Tissues freshly excised from normal rats are refractory to the insulin-like effects of GH, but become sensitive immediately after surgical stress. Reversal of refractoriness is prevented by administration of the opioid antagonist, naloxone, just prior to stress, suggesting a possible role of beta-endorphin or related peptides. These experiments were undertaken to determine the source of these peptides which might equally well be released from the pituitary, adrenal medullae, or nerve endings in response to stress. Since adrenalectomy, like stress, also results in increased secretion of adrenocorticotropic hormone (ACTH) and related peptides, we studied the effects of GH on glucose oxidation in adipose tissue obtained from adrenalectomized rats and found a significant insulin-like response to GH in tissues studied 4 days after adrenalectomy. This effect was not due to GH deficiency, since plasma concentrations were only slightly reduced by adrenalectomy. Administration of naloxone (250 micrograms/rat), 30 or 60 min before sacrifice, or dexamethasone (100 micrograms/injection), 60 and 120 min before sacrifice, prevented a response to GH without affecting circulating levels of GH. The effects of adrenalectomy could not be reproduced by preincubation of adipose tissue from normal nonstressed rats with ACTH and beta-endorphin, but were duplicated by preincubation of adipose tissue for 15 min in medium in which pituitary glands had previously incubated in the presence of corticotropin-releasing hormone (0.1 microM) and arginine vasopressin (0.2 microM). Addition of naloxone (250 micrograms/ml) blocked this effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Independent modulation of hepatic protein kinase activities.

The effects of hormonal status on protein kinase activity was examined in homogenates of rat liver. Protein kinase activity was evaluated from incorporation of 32P from [gamma-32P]ATP into protamine or histone as receptor substrates. Protamine phosphorylation in the presence or absence of cyclic AMP exceeded histone phosphorylation by at least a factor or two. Hypophysectomy markedly increased protamine phosphorylation in the presence or absence of saturating amounts of cyclic AMP. In contrast, hypophysectomy only slightly increased cyclic AMP independent phosphorylation of histone. These results could not be amounted for by differences in ATPase or protein phosphase activities. Cortisone (2 mg/day x 3) decreased total protein kinase activity in livers of hypophysectomized rats when protamine was substrate, but had no effect on the total activity toward histone. Growth hormone (100 mug/day x 3) significantly increased histone, but not protamine phosphorylation in livers of hypophysectomized rats. Administration of 5 mug of triiodothyonine/day to hypophysectomized rats also markedly increased the phosphorylation of histone, but not protamine when saturating amounts of cyclic AMP were present. These results support the hypothesis that liver may contain more than one type of protein kinase activity and that the different protein kinase activities can be separately affected by hormones. Such control distal to cyclic AMP might allow selective modulation of cyclic AMP-dependent processes in cells which carry out more than one such process.