Free fatty acid-glucagon feed-back mechanism

Palmitate stimulation of glucagon secretion in mouse pancreatic alpha-cells results from activation of L-type calcium channels and elevation of cytoplasmic calcium

We have investigated the short-term effects of the saturated free fatty acid (FFA) palmitate on pancreatic alpha-cells. Palmitate (0.5 or 1 mmol/l bound to fatty acid-free albumin) stimulated glucagon secretion from intact mouse islets 1.5- to 2-fold when added in the presence of 1-15 mmol/l glucose. Palmitate remained stimulatory in islets depolarized with 30 mmol/l extracellular K(+) or exposed to forskolin, but it did not remain stimulatory after treatment with isradipine or triacsin C. The stimulatory action of palmitate on secretion correlated with a 3.5-fold elevation of intracellular free Ca(2+) when applied in the presence of 15 mmol/l glucose, a 40% stimulation of exocytosis (measured as increases in cell capacitance), and a 25% increase in whole-cell Ca(2+) current. The latter effect was abolished by isradipine, suggesting that palmitate selectively modulates l-type Ca(2+) channels. The effect of palmitate on exocytosis was not mediated by palmitoyl-CoA, and intracellular application of this FFA metabolite decreased rather than enhanced Ca(2+)-induced exocytosis. The stimulatory effects of palmitate on glucagon secretion were paralleled by a approximately 50% inhibition of somatostatin release. We conclude that palmitate increases alpha-cell exocytosis principally by enhanced Ca(2+) entry via l-type Ca(2+) channels and, possibly, relief from paracrine inhibition by somatostatin released by neighboring delta-cells.

Effect of free fatty acids and amino acids on glucagon and insulin secretions in normal and diabetic ducks

The relationship between two metabolites free fatty acids (FFA) and amino acids (AA), and the two main pancreatic hormones, insulin and glucagon, was studied by infusing small amounts of these metabolites into normal and diabetic Peking ducks, i.e. two days after subtotal pancreatectomy. Infusion of oleic acid (0.365 g/kg/30 min as an emulsion in plasma) indicated a suppressive effect of free fatty acids on glucagon secretion, but was without effect on insulin secretion, in normal as well as in diabetic ducks, indicating that insulin might not be directly involved in the FFA-glucagon feedback in the duck. Infusions of arginine for one hour (1 g/kg/h) into normal ducks, hyperglycaemic normal birds (as a result of glucose infusion: 1 g/kg/h) and diabetic ducks, suggested the persistence of amino acid effect on glucagon secretion, and a slight reduction of the effect on insulin secretion in diabetes. This suggests that insulin may not be involved in amino acid-induced glucagon secretion in the duck.

Roles of insulin and glucose in the regulation of plasma FFA in the duck. I. In the absence of pancreatic glucagon

The present study was undertaken to determine how free fatty acid (FFA) release by adipose tissue can occur in the absence of pancreatic glucagon, the most potent lipolytic hormone in the duck. Two possible explanations have successively been investigated. Replacement therapy experiments showed that glucose uptake was implicated in the phenomenon; indeed, when pre-operative insulin and glucose levels were restored after total pancreatectomy, plasma FFA progressively fell to a level significantly lower than the pre-operative value. The same effect was observed when the ducks were kept in a continuous state of hyperglycaemia, whereas "normal" levels of insulin or glucose alone were shown to be ineffective. On the other hand a compensatory mechanism for the lack of glucagon, by other lipolytic agents [with the intestine or the anterior pituitary as possible sources] has been excluded by associating the removal of these two organs with pancreatectomy. These experiments suggest that the lack of pancreatic glucagon induced by total pancreatectomy is essentially masked by a failure in glucose uptake due to the operation.

Comparison of the suppressive effects of elevated plasma glucose and free fatty acid levels on glucagon secretion in normal and insulin-dependent diabetic subjects. Evidence for selective alpha-cell insensitivity to glucose in diabetes mellitus

To examine whether abnormal pancreatic alpha-cell function found in human diabetes mellitus may represent a selective insensitivity to glucose, plasma glucagon responses to hyperglycemia and elevation of plasma free fatty acid levels (both known suppressors of glucagon secretion) were compared in juvenile-onset, insulin-requiring diabetic subjects, and in normal nondiabetic subjects. In the latter, both elevation of plasma free fatty acid levels induced by heparin administration of hyperglycemia produced by intravenous infusion of glucose resulted in a comparable 30--40% suppression of circulating glucagon levels (P less than 0.01). In the diabetic subjects, glucagon suppression by hyperglycemia (less than 20%) was less than that occurring in normal subjects (P less than 0.01), even when accompanied by infusion of supraphysiologic amounts of insulin. However, suppression of glucagon levels by elevation of plasma free fatty acids in the diabetic group was similar to that found in normal subjects and of comparable magnitude to that due to hyperglycemia in the normal subjects. These results thus demonstrate a selective impairment of the diabetic alpha-cell response to glucose and provide further evidence for the presence of an abnormal alpha-cell glucoreceptor in human diabetes mellitus.