Showing papers by "J. D. McGarry published in 1998"

Chronic exposure to free fatty acid reduces pancreatic beta cell insulin content by increasing basal insulin secretion that is not compensated for by a corresponding increase in proinsulin biosynthesis translation.

Abstract: The pancreatic beta cell normally maintains a stable balance among insulin secretion, insulin production, and insulin degradation to keep optimal intracellular stores of the hormone. Elevated levels of FFA markedly enhance insulin secretion; however, the effects of FFA on insulin production and intracellular stores remain unclear. In this study, twofold elevation in total circulating FFA effected by infusion of lard oil and heparin into rats for 6 h under normoglycemic conditions resulted in a marked elevation of circulating insulin levels evident after 4 h, and a 30% decrease in pancreatic insulin content after a 6-h infusion in vivo. Adding 125 muM oleate to isolated rat pancreatic islets cultured with 5.6 mM glucose caused a 50% fall in their insulin content over 24 h, coupled with a marked enhancement of basal insulin secretion. Both effects of fatty acid were blocked by somatostatin. In contrast to the stimulatory effects of oleate on insulin secretion, glucose-induced proinsulin biosynthesis was inhibited by oleate up to 24 h, but was unaffected thereafter. This result was in spite of a two- to threefold oleate-induced increase in preproinsulin mRNA levels, underscoring the importance of translational regulation of proinsulin biosynthesis in maintaining beta cell insulin stores. Collectively, these results suggest that chronically elevated FFA contribute to beta cell dysfunction in the pathogenesis of NIDDM by significantly increasing the basal rate of insulin secretion. This increase in turn results in a decrease in the beta cell's intracellular stores that cannot be offset by commensurate FFA induction of proinsulin biosynthesis.

Circulating fatty acids are essential for efficient glucose-stimulated insulin secretion after prolonged fasting in humans.

Abstract: In the fasted rat, efficient glucose-stimulated insulin secretion (GSIS) is absolutely dependent on an elevated level of circulating free fatty acids (FFAs). To determine if this is also true in humans, nonobese volunteers were fasted for 24 h (n = 5) or 48 h (n = 5), after which they received an infusion of either saline or nicotinic acid (NA) to deplete their plasma FFA pool, followed by an intravenous bolus of glucose. NA treatment resulted in a fall in basal insulin concentrations of 35 and 45% and in the area under the insulin response curve (area under the curve [AUC]) to glucose of 47 and 42% in the 24- and 48-h fasted individuals, respectively. The 48-h fasted subjects underwent the same procedure with the addition of a coinfusion of Intralipid plus heparin (together with NA) to maintain a high concentration of plasma FFAs throughout the study. The basal level and AUC for insulin were now completely normalized (C-peptide profiles paralleled those for insulin). To assess the effect of an overnight fast, nonobese (n = 6) and obese (n = 6) subjects received an infusion of either saline or NA, followed by a hyperglycemic clamp (200 mg/dl). The insulin AUC in response to glucose was unaffected by lowering of the FFA level in nonobese subjects, but fell by 29% in the obese group. The data clearly demonstrate that in humans, the rise in circulating FFA levels after 24 and 48 h of food deprivation is critically important for pancreatic beta-cell function both basally and during subsequent glucose loading. They also suggest that the enhancement of GSIS by FFAs in obese individuals is more prominent than that seen in their nonobese counterparts.

A fatty acid- dependent step is critically important for both glucose- and non-glucose-stimulated insulin secretion.

Abstract: Lowering of the plasma FFA level in intact fasted rats by infusion of nicotinic acid (NA) caused essentially complete ablation of insulin secretion (IS) in response to a subsequent intravenous bolus of arginine, leucine, or glibenclamide (as previously found using glucose as the beta-cell stimulus). However, in all cases, IS became supranormal when a high FFA level was maintained by co-infusion of lard oil plus heparin. Each of these secretagogues elicited little, if any, IS from the isolated, perfused "fasted" pancreas when tested simply on the background of 3 mM glucose, but all became extremely potent when 0.5 mM palmitate was also included in the medium. Similarly, IS from the perfused pancreas, in response to depolarizing concentrations of KCl, was markedly potentiated by palmitate. As was the case with intravenous glucose administration, fed animals produced an equally robust insulin response to glibenclamide regardless of whether their low basal FFA concentration was further reduced by NA. In the fasted state, arginine-induced glucagon secretion appeared to be independent of the prevailing FFA concentration. The findings establish that the essential role of circulating FFA for glucose-stimulated IS after food deprivation also applies in the case of nonglucose secretagogues. In addition, they imply that (i) a fatty acid-derived lipid moiety, which plays a pivotal role in IS, is lost from the pancreatic beta-cell during fasting; (ii) in the fasted state, the elevated level of plasma FFA compensates for this deficit; and (iii) the lipid factor acts at a late step in the insulin secretory pathway that is common to the action of a wide variety of secretagogues.

Roles of the N- and C-terminal domains of carnitine palmitoyltransferase I isoforms in malonyl-CoA sensitivity of the enzymes: insights from expression of chimaeric proteins and mutation of conserved histidine residues.

Abstract: The mitochondrial outer membrane enzyme carnitine palmitoyltransferase I (CPT I) plays a major role in the regulation of fatty acid entry into the mitochondrial matrix for beta-oxidation by virtue of its inhibition by malonyl-CoA. Two isoforms of CPT I, the liver type (L) and muscle type (M), have been identified, the latter being 100 times more sensitive to malonyl-CoA and having a much higher Km for the substrate carnitine. Here we have examined the roles of different regions of the CPT I molecules in their response to malonyl-CoA, etomoxir (an irreversible inhibitor) and carnitine. To this end, we analysed the properties of engineered rat CPT I constructs in which (a) the N-terminal domain of L-CPT I was deleted, (b) the N-terminal domains of L- and M-CPT I were switched, or (c) each of three conserved histidine residues located towards the N-terminus in L-CPT I was mutated. Several novel points emerged: (1) whereas the N-terminal domain is critical for a normal malonyl-CoA response, it does not itself account for the widely disparate sensitivities of the liver and muscle enzymes to the inhibitor; (2) His-5 and/or His-140 probably play a direct role in the malonyl-CoA response, but His-133 does not; (3) the truncated, chimaeric and point- mutant variants of the enzyme all bound the covalent, active-site- directed ligand, etomoxir; and (4) only the most radical alteration of L-CPT I, i.e. deletion of the N-terminal 82 residues, affected the response to carnitine. We conclude that the N-terminal domain of CPT I plays an essential, but permissive, role in the inhibition of the enzyme by malonyl-CoA. By contrast, the larger C-terminal region dictates the degree of sensitivity to malonyl-CoA, as well as the response to carnitine; it is also sufficient for etomoxir binding. Additionally, further weight is added to the notion that one or more histidine residues may be involved in the CPT I-malonyl-CoA interaction.