Pulsatile Insulin Secretion in Isolated Rat Islets

The pancreas secretes insulin in an oscillatory fashion, but the precise site of the pacemaker for pulsatile insulin secretion has not been identified. These studies were designed to determine whether islets also secrete insulin in a pulsatile fashion if they are isolated from their pancreatic milieu. Isolated rat islets (80–100) were perifused 8 h in culture medium after overnight incubation, and samples were collected at 3.3-min intervals. Insulin secretion was evaluated for pulsatility with the Clifton Cycle Detection Program. Perifusion of islets was associated with a spontaneous, persistent, and regular pulsatility of insulin secretion, which was observed in all conditions tested. Perifusion with medium containing 5.5 mM glucose (n = 11) demonstrated oscillations with a mean periodicity of 17.6 ±1.1 min and a mean amplitude of 4.8 ± 0.4 μU/ml when overall mean insulin concentration was 16.7 ± 2.4 μU/ml. When the glucose concentration was 16.7 mM (n = 9), overall mean insulin concentration was 54.4 ± 2.6 μU/ml, with increases in periodicity (22.0 ±1.3 min) and amplitude (10.7 ± 0.5 μU/ml). All measurements were significantly different from those observed during perifusion with 5.5 mM glucose (P < 0.02−0.001). Theophylline (1 mM) also enhanced the overall mean insulin concentration and amplitude (69.4 ± 10.4 and 14.2 ±1.2 μU/ml, respectively) compared with control studies without theophylline (16.7 ± 5.3 and 4.3 ± 0.5 μU/ml) (P < 0.01). The period of the cycle was also increased from 17.5 ± 1.1 to 26.4 ± 6.3 min, but this was not significantly different from the control group. When glucose was delivered in rhythmic fashion with a period of 40 min, the characteristics of the insulin cycling approximated those of the glucose oscillation rather than the spontaneously generated insulin pulsatility. We conclude that a pacemaker for pulsatile insulin secretion is located within the islets of Langerhans and is subject to influence by both glucose concentration and pattern of delivery and cAMP-mediated mechanisms.