Stimulation of steroidogenesis in cultured rat adrenocortical cells by unsaturated fatty acids.

The hypothesis that the stimulatory action of free fatty acids (FFA) in the hypothalamic-pituitary-adrenocortical (HPA) axis occurs in part at the adrenal cortex was evaluated. Pathophysiological concentrations of oleic and linoleic acids, but not stearic or caprylic acid, stimulated steroidogenesis from cultured rat adrenocortical cells (concentrations eliciting 50% of maximal responses, approximately 60 and 120 microM, respectively), with a latency of 90 min. Maximal stimulation of steroidogenesis by both acids was < 50% of that produced by adrenocorticotropic hormone (ACTH) and was blocked by cycloheximide. The maximal steroidogenic response to ACTH was inhibited approximately 50% by oleic acid. The actions of oleic and linoleic acids were not associated with an increase in adenosine 3',5'-cyclic monophosphate (cAMP) secretion but appeared to require intracellular oxidation. None of the lipids influenced cell viability or corticosterone radioimmunoassay. The latency of the steroidogenic response, the putative requirement for intracellular oxidation, and the apparent lack of involvement of cAMP suggest a mechanism of action of FFA distinct from that of ACTH, yet still requiring protein synthesis. It is concluded that the modulation of steroidogenesis by these abundant naturally occurring lipids may be an important component of the control mechanisms within the HPA pathway in disorders of lipid homeostasis (e.g., obesity, starvation, or diabetes).

Regulation of pituitary-adrenocortical activity by free fatty acids in vivo and in vitro.

Virtually every metabolic disorder characterized by elevated plasma free fatty acid (FFA) levels is also associated with hypercorticoidism. For example, the glucocorticoid response to insulin-hypoglycemia is shown in this report to be greatly potentiated in Type I diabetic rats. Since glucocorticoids (corticosterone, in rats) potentiate lipolysis and promote gluconeogenesis, they exacerbate diabetes. We found that elevation of circulating FFA levels in normal rats (via Intralipid/heparin infusion) increased plasma levels of adrenocorticotropic hormone (ACTH) and corticosterone, and resulted in hyperglycemia. In vitro, however, cultured pituitary cells were relatively unaffected by FFA except at very high concentrations. Neither basal ACTH secretion nor the ACTH response to corticotropin-releasing hormone (CRH) was affected by pathophysiological molar ratios of FFA:BSA. Thus, the ACTH secretory response to FFA in vivo likely is mediated via neuroendocrine activation. Cultured adrenocortical cells, however, were stimulated by oleic acid and, to a lesser extent, by linoleic acid; saturated fatty acids were without effect. The latencies of oleic acid-induced steroidogenesis in vitro and Intralipid-induced corticosterone secretion in vivo were both about 60 min. We conclude that pathophysiological levels of circulating FFA (typical of diabetes, obesity, starvation, and consumption of high-fat diets) initiate a positive feedback loop between the adipocyte and the HPA axis, which ultimately exacerbates the symptoms of these disorders.

Monolayer culture of adult rat pancreatic islets on extracellular matrix: modulation of B-cell function by chronic exposure to high glucose.

Studies in vivo indicate that chronic hyperglycemia is deleterious for insulin secretion. We have used an improved islet monolayer culture system to study chronic modulations of B-cell function. Adult rat islets maintained over several weeks on extracellular matrix in the presence of 11.1 mM glucose responded to an acute stimulation with 16.7 mM glucose by a 5- to 8-fold increase in insulin secretion. When cultured in the presence of higher glucose concentrations, the response to an acute glucose stimulus diminished time and dose dependently. In islets desensitized by exposure to 33.3 mM glucose for 1 week, reduction of the glucose level to 11.1 mM reversed the desensitization within 2 weeks. This desensitization was not limited to the glucose stimulus; responses to other nutrient secretagogues, such as glyceraldehyde and alpha-ketoisocaproic acid, were also reduced. In contrast, responses of insulin secretion to nonnutrient stimulators (tolbutamide and quinine) and amplifiers (isobutylmethylxanthine and carbachol) showed no desensitization in islets exposed to 33.3 mM glucose. Desensitization similar to that caused by 33.3 mM glucose could be induced by 11.1 mM glucose together with 0.1 mM isobutylmethylxanthine. High glucose also caused a time-dependent loss in compact monolayer organization with disruption of cell contacts. Our studies suggest that 1) generation of the reduced insulin response may be related to the prolonged high insulin secretion rate; 2) expression of the functional change is specific to the nutrient stimulus-secretion coupling; and 3) modifications in intercellular contacts may be involved in B-cell desensitization.