Effects of Ca²⁺ and a phorbol ester on insulin secretion from islets of langerhans permeabilised by high‐voltage discharge

Abstract

Isolated rat islets of Langerhans permeabilised by high-voltage discharge secreted insulin in response to elevations in Ca2+ over the range 100 nM to 10 ,uM Ca 2+. The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), had no effects on insulin secretion in the absence of Ca2+. In the presence of Ca2+ concentrations of > 10 nM, TPA produced dose-related shifts in the Ca 2+-activation curve to lower Ca2+ concentrations, together with marked increases in the maximum secretory response to Caz+. These results suggest that, in islets, the activation of protein kinase C is important in modulating both the sensitivity of the exocytotic mechanism to intracellular Ca2+, and the magnitude of the insulin secretory response.

Triggering and amplifying pathways of regulation of insulin secretion by glucose.

“…Under physiological conditions, glucose increases [Ca 2+ ] i in ␤-cells, and all maneuvers interfering with this rise impair the stimulation of insulin secretion. Direct evidence that cytoplasmic Ca 2+ triggers exocytosis of insulin granules has been provided by experiments using permeabilized islet cells (11,12) or capacitance recordings in single ␤-cells (13,14). Although progressive desensitization to Ca 2+ occurs in these preparations (7,15), probably because of the loss of essential cytoplasmic constituents, imposed oscillations of [Ca 2+ ] i are able to entrain oscillations of secretion in permeabilized cells, indicating that Ca 2+ has a minute-to-minute triggering role (7).…”

Oscillatory control of insulin secretion

“…Rapid changes in DAG formation have been detected in rat islets following stimulation with glucose or carbachol (Peter-Riesch et al, 1988;Wolf et al, 1989). Although DAG analogues and phorbol esters have been found to stimulate insulin secretion (Virji et al, 1978;Malaisse et al, 1980;Jones et al, 1985) by sensitizing the secretory machinery to Ca 2+ (Jones et al, 1985;Tamagawa et al, 1985), it is debated whether PKC is involved in nutrient-induced insulin secretion (Metz, 1988;Jones and Persaud, 1998). The use of PKC inhibitors has resulted in conflicting conclusions (Jones and Persaud, 1998).…”

Distinct mechanisms for two amplification systems of insulin release

“…Extrapolation topossible effects of the phorbol ester on cytosolic Ca2+ of normal fl-cells stimulated by tolbutamide would thus be speculative. Experiments using permeabilized islet cells have shown that TPA increases insulin release at a fixed, sub-micromolar, concentration of Ca2+ [38,43] and that this increase is attenuated by polymyxin B, a protein kinase C inhibitor [44]. Taken together, these results and our reports on the interaction of TPA with glucose [10] and tolbutamide (the present study) suggest that the potentiation of insulin release by the phorbol ester results from intracellular events, or non-electrogenic membrane events, that sensitize the releasing machinery to the action of Ca2+.…”