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1. The whole-cell configuration of the patch-clamp recording technique was used to characterize the electrophysiological properties of CRI-G1 insulin-secreting cells. 2. Current-clamp recordings demonstrated the excitable nature of these cells. 3. Voltage-clamp recordings revealed the presence of an inward Na+ current, an inward Ca2+ current and a delayed outward K+ conductance. 4. The electrophysiological properties of CRI-G1 closely resemble those of pancreatic beta-cells, thereby rendering this cell-line as a useful alternative to freshly isolated cells for the study of pancreatic beta-cell electrophysiology and pharmacology.

ATP-sensitive K(+)-channel currents were recorded from isolated membrane patches and voltage-clamped CRI-G1 insulin-secreting cells. Internal Mg2+ ions inhibited ATP-K+ channels by a voltage-dependent block of the channel current and decrease of open-state probability. The run-down of ATP-K+ channel activity was also shown to be [Mg2+]i dependent, being almost abolished in Mg2(+)-free conditions. Substitution of Mn2+ for Mg2+ did not prevent run-down, nor did the presence of phosphate-donating nucleotides, a protease or phosphatase inhibitor or replacement of Cl- by gluconate.

1. The effects of various sulphonylureas and diazoxide on insulin secretion and the activity of various channels have been studied using tissue culture and patch-clamp methods in an insulin-secreting cell line derived from a rat islet cell tumour. 2. Tolbutamide, glibenclamide and HB699 increased the rate of insulin release by 2-5 fold. The concentrations of tolbutamide and glibenclamide giving half-maximum effects on insulin secretion were approximately 40 microM and 0.2 microM, respectively. 3. Diazoxide (0.6-1.0 mM) per se, had either no effect or produced a small increase in insulin secretion, whereas when secretion was maximally stimulated by the combination of glucose (3 mM) and leucine (20 mM), it produced inhibition. Tolbutamide-induced release was also inhibited by diazoxide. 4. Tolbutamide, glibenclamide, HB699 and HB985 reduced the open-state probability of the ATP-K+ channel in a dose-dependent manner. Tolbutamide and glibenclamide were shown to be effective regardless of which side of the membrane they were applied. 5. In whole cell recording, in which the total ATP-sensitive K+ conductance of the cell could be measured, dose-inhibition curves for tolbutamide and glibenclamide were constructed, resulting in Ki values of 17 microM and 27 nM, respectively. The value of Ki for tolbutamide was unchanged when ATP (0.1 mM) was present in the electrode. 6. Diazoxide (0.6 mM) activated the ATP-K+ channels only when they had first been inhibited by intracellular ATP (0.1 mM) or bath applied tolbutamide (3-30 microM). The inhibition produced by glibenclamide could not be reversed by diazoxide. 7. Neither tolbutamide (1.0 mM) nor glibenclamide (10 microM) altered the open-state probability of the Ca2+-activated K+ channel or the Ca2+-activated non-selective cation channel which are present in this cell line. 8. It is concluded that the sulphonylureas and related hypoglycaemic drugs and diazoxide regulate insulin secretion by direct effects on the ATP-K+ channel or a protein closely associated with this channel.

1. The effects of diazoxide on ATP-K+ channel currents, recorded from the insulin-secreting cell line, CRI-G1, were studied using patch-clamp techniques. 2. Under current-clamp recording conditions diazoxide (0.6 mM), inhibited action potential activity and hyperpolarized CRI-G1 cells with a concomitant increase in membrane conductance. Recordings from voltage-clamped whole-cells and isolated patches indicate that activation of ATP-K+ channel currents underlie these effects. 3. Diazoxide elicited an activation of ATP-K+ channels which had been partially inhibited by ATP, on application to either surface of the plasma membrane, although it was more effective when applied directly to the cytoplasmic side. Activation of the ATP-K+ currents involves an increase in the single channel open-state probability and an apparent increase in the number of functional channels. 4. Activation was observed only when Mg-ATP was present in the cytoplasmic bathing solution. There was no activation of currents by diazoxide when ATP, in the absence of Mg2+ ions, or Mg-AMP-PNP was present to inhibit the ATP-K+ channels. 5. In the absence of ATP and Mg2+ ions in the cytoplasmic bathing solution, diazoxide (0.6 mM) produced an inhibition of ATP-K+ currents. 6. Cromakalim (BRL 34915) at 10 microM and 100 microM had no significant effects on ATP-K+ currents. 7. It is concluded that diazoxide-induced activation of ATP-K+ channel currents probably involves phosphorylation of the channel or some closely associated membrane protein.