Glucose induces insulin release and a rise in cytosolic calcium concentration in a transplantable rat insulinoma

An important role for calcium in the cellular events leading to insulin secretion is supported by many studies. However, simultaneous measurements of changes in intracellular free Ca²+ concentrations ([Ca²+]i) and insulin release in response to secretagogues have not been performed. Using cells isolated from a glucose-responsive insulinoma, changes in [Ca²+]i were measured with the fluorescent calcium probe quin2. With the nutrient secretagogues glucose (30 m) and -glyceraldehyde (GA; 20 m), [Ca²+]i increased slowly, reaching a peak approximately 15 min after addition of the stimulus, while KC1 (25 m) and carbachol (2 m) led to a rapid but transient increase in [Ca²+];. Glucose increased [Ca²+]i from 104 ± 6 (mean ±) to 248 ± 31 m (n = 13), and GA caused a rise in [Ca²+]i from 96 ± 6 to 280 ± 39 n (n = 4). KC1 and carbachol caused a rise from 107 ± 6 to 184 ± 5 nM and from 98 ± 5 to 157 ± 5 n, respectively (n = 5 each). When insulin release was measured simultaneously with changes in [Ca²+]; and compared to unstimulated cells, the following results were obtained. During the first 5 min of stimulation, high glucose caused a 90 ± 12% increase in insulin release and a 72 ± 11% rise in [Ca²+]i (n = 5). GA evoked a 122 ± 30% increase in insulin secretion, with a 82 ± 17% rise in [Ca²+]i (n = 3). Both KC1 and carbachol caused a 58 ± 9% increase in insulin release, with 7 ± 4% and 50 ± 2% rises in [Ca²+]i(respectively (n = 4 each). Insulin release was also measured in a perifusion system. It was shown that glucose (30 m), GA (20 m), and a-ketoisocaproate (30 m) caused a biphasic release of insulin, while KCl (25 m) and carbachol (2 m) caused a monophasic release. The results show that [Ca²+]i increases during the stimulation of insulin secretion when measured simultaneously on the same β-cells. However, while these changes coincide, a simple direct quantitative relationship between insulin release and the rise in [Ca²+]i could not be demonstrated.