Magnitude Of Insulin Release Research Articles

Glucose-stimulated insulin secretion of insulinoma INS-1E cells is associated with elevation of both respiration and mitochondrial membrane potential

Increased ATP/ADP ratio resulting from enhanced glycolysis and oxidative phosphorylation represents a plausible mechanism controlling the glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells. Although specific bioenergetics might be involved, parallel studies of cell respiration and mitochondrial membrane potential (Δ Ψ m) during GSIS are lacking. Using high resolution respirometry and parallel Δ Ψ m monitoring by two distinct fluorescence probes we have quantified bioenergetics in rat insulinoma INS-1E cells representing a suitable model to study in vitro insulin secretion. Upon glucose addition to glucose-depleted cells we demonstrated a simultaneous increase in respiration and Δ Ψ m during GSIS and showed that the endogenous state 3/state 4 respiratory ratio hyperbolically increased with glucose, approaching the maximum oxidative phosphorylation rate at maximum GSIS. Attempting to assess the basis of the “toxic” effect of fatty acids on insulin secretion, GSIS was studied after linoleic acid addition, which diminished respiration increase, Δ Ψ m jump, and magnitude of insulin release, and reduced state 3/state 4 dependencies on glucose. Its effects were due to protonophoric function, i.e. uncoupling, since without glucose, linoleic acid accelerated both state 3 and state 4 respiration by similar extent. In turn, state 3 respiration increased marginally with linoleic acid at 10–20 mM glucose. We conclude that upon glucose addition in physiological range, the INS-1E cells are able to regulate the oxidative phosphorylation rate from nearly zero to maximum and that the impairment of GSIS by linoleic acid is caused by mitochondrial uncoupling. These findings may be relevant to the pathogenesis of type 2 diabetes.

Tumor necrosis factor-alpha in sera of obese patients: fall with weight loss.

In view of the recent demonstration that obesity in animals and humans is associated with an increase in tumor necrosis factor-alpha (TNFalpha) expression, that this expression falls with weight loss, and that TNFalpha may specifically inhibit insulin action, the possibility that TNFalpha may be a mediator of insulin resistance has been raised. We have undertaken this study to investigate whether serum TNFalpha concentrations are elevated in obese subjects, whether they fall after weight loss, and whether this fall parallels the fall in insulin release after glucose challenge. Obese patients (age range: 25-54, weight mean +/- SD: 96.4 +/- 13.8 kg, body mass index: 35.7 +/- 5.6 kg/m2) were started on a diet program. The mean weight fell to 84.5 +/- 11.3 (P < 0.0001) and body mass index to 31.3 +/- 4.9 (P < 0.0001). Plasma TNFalpha concentrations were markedly elevated in the obese (3.45 +/- 0.16 pg/mL), when compared with controls (0.72 +/- 0.28 pg/mL), and fell significantly (2.63 +/- 1.40 pg/mL) after weight loss (P < 0.02). The magnitude of insulin release after glucose (75 g) challenge (area under the curve) also fell significantly (P < 0.01) after weight loss. The magnitude of weight loss and fall in TNFalpha were related to basal body weight (r = 0.57, P < 0.001) and basal TNFalpha (r = 0.55, P < 0.001) concentrations, respectively, but not to each other or to the glucose-induced insulin release (area under the curve). We conclude that obesity is associated with increased plasma TNFalpha concentrations, which fall with weight loss. Because circulating TNFalpha may mediate insulin resistance in the obese, a fall in TNFalpha concentrations may contribute to the restoration of insulin resistance after weight loss, Thus, TNFalpha may be an important circulating cytokine, which may provide a potentially reversible mechanism for mediating insulin resistance.

Mode of action of clonidine upon islet function: dissociated effects upon the time course and magnitude of insulin release.

Clonidine (0.08 to 80.0 ng/ml) caused a dose-related inhibition of glucose-stimulated insulin release, but failed to affect glucose oxidation, glucose-stimulated 45Ca net uptake, and adenylate cyclase activity in isolated rat islets. Phentolamine antagonized the effect of clonidine upon insulin release. Despite profound inhibition of insulin secretion, the drug failed to affect the time course for the changes evoked by glucose in either 45Ca fractional outflow rate from perfused islets or insulin release from the isolated perfused pancreas. The latter changes were multiphasic, revealing an initial secretory peak, a period of low secretory activity, and a second secretory elevation before establishing a period characterized by a steadily and slowly increasing insulin output. In the clonidine-treated islets, the secretory rate was not significantly different from the basal value during the period after the initial secretory response. Thus, despite continuous stimulation with glucose, insulin release appears as a discontinuous phenomenon, even when little insulin is secreted during the initial phase of stimulation.

Mode of action of clonidine upon islet function: dissociated effects upon the time course and magnitude of insulin release

Mode of action of clonidine upon islet function: dissociated effects upon the time course and magnitude of insulin release