Ion Channels of Glucose-Responsive and -Unresponsive β-cells

To assess whether different electrophysiological characteristics could account for the heterogeneous secretion of individual β-cells in vitro, we used patch-clamp configurations to study currents in plaque-forming (insulin-secreting) and non–plaque-forming rat pancreatic β-cells that were distinguished in a reverse hemolytic plaque assay (RHPA) after a 30-min stimulation by 16.7 mM glucose. RHPA showed that the population of single β-cells under study was stimulated (P < 0.01–0.001) to secrete insulin by 16.7 mM glucose, 100 μM tolbutamide, 20 μM glyburide, or 30 mM KCl but, under these conditions, also comprised β-cells that did not secrete detectable amounts of insulin. Under current clamp conditions, secreting and nonsecreting β-cells showed analogous resting membrane potentials (∼60 mV) and were similarly depolarized by 30 mm KCl and 100 μM tolbutamide. Under voltage-clamp conditions, total membrane conductance (∼6 nS) was also similar in the glucose-responsive and -unresponsive β-cells, which, when monitored in the whole-cell configuration after RHPA, showed the following currents: a voltage-dependent Na⁺ current, a voltage-activated Ba²⁺ current, a voltage-dependent K⁺ delayed-rectifier current, a voltage-dependent Ca²⁺-activated K⁺ current, and a voltage-independent and tolbutamide-sensitive K⁺ current. In the cell-attached configuration and the presence of 2.8 mM glucose, secreting and nonsecreting β-cells displayed a similar single-channel activity that was abolished when glucose concentration was raised to 16.7 mM. We conclude that β-cells studied after RHPA have an electrically normal membrane whether they release insulin in response to 16.7 mM glucose or not.