Immediate and time-dependent effects of glucose on insulin release: differential calcium requirements

Publisher Website
Google Scholar
Abstract
Glucose regulates insulin release in a complex manner; apart from its acute secretory action it induces time-dependent effects which modulate subsequent islet responses. The Ca2+ sensitivities of the diverse secretory events generated by glucose were investigated in the perfused rat pancreas. First- and second-phase insulin responses to 16.7 mmol/l glucose were obliterated in the presence of 5 mmol/l EGTA; threshold Ca2+ concentrations for significant responses were 0.25 mmol/l for second-phase, and ''O'' (no Ca2+ added, approx 20 .mu.mol/l) for first-phase release (both around 10% of control). The apparent Km of the Ca2+ dependencies were 0.6 mmol/l for first-phase, and 1.25 mmol/l for second-phase release. Time-dependent potentiation was demonstrated by subjecting the pancreas to two 40-min 16.7 mmol/l glucose stimuli separated by a 30-min rest period; this amplified the first-phase response to the second stimulus 2.5 .+-. 0.9-fold. Also the generation of potentiation was Ca2+ dependent, with characteristics similar to those of the acute second-phase insulin response (apparent Km .apprx. 1.0 mmol/l Ca2+). In contrast, the amplified first-phase response to the second glucose pulse retained its high sensitivity to Ca2+, thus resembling the unprimed first-phase. The inhibitory message of glucose was demonstrated by applying two sequential 5-min pulses of 8.3 mmol/l glucose: the insulin response to the second stimulus was reduced by 43 .+-. 9%. Addition of EGTA to the first glucose pulse had no effect on the inhibition of the second insulin response. Thus: 1. Despite its high sensitivity to Ca2+, also first-phase release is fully dependent on extracellular Ca2+. 2. The amplifying effect of glucose priming is Ca2+ dependent, with requirements similar to those of the acute second-phase release. 3. Amplified insulin release does not involve changes in beta cell sensitivity to extracellular Ca2+. 4. The inhibitory effect of a short glucose stimulus is not a Ca2+ dependent event. 5. These findings may relate to the Ca2+ sensitivity of distal steps in insulin release rather than reflecting changes in the Ca2+ economy of the beta cell.