Calcium and delayed potassium currents in mouse pancreatic beta‐cells under voltage‐clamp conditions.

Pancreatic islets of NMRI mice were dissociated into single cells which were kept in tissue culture for 1‐3 days. The whole‐cell configuration of the patch‐clamp technique was used to study inward and delayed outward currents of beta‐cells under voltage‐clamp conditions at 20‐22 degrees C. Outward currents were suppressed by substituting the impermeant cation N‐methyl‐D‐glucamine for intracellular K+. The remaining inward current had a V‐shaped current‐voltage relation reaching a peak value of 39 +/‐ 4 pA (mean +/‐ S.E. of mean) around ‐15 mV. It was identified as a Ca2+ current, because the peak amplitude was increased 1.6 times by increasing external [Ca2+] ([Ca2+]o) from 2.6 mM to 10 mM and it was blocked by Co2+ (5 mM) or nifedipine (5 microM) but not by TTX (20 microM). The activation time constant of the inward current at ‐10 mV was 1.28 +/‐ 0.08 ms. The relation between the degree of activation (estimated from the size of the tail currents) and membrane potential V followed the sigmoidal function f = 1/(1 + exp [(Vh‐V)/k]) with half‐maximal activation potential, Vh = 4 +/‐ 1 mV and slope factor, k = 14 +/‐ 1 mV (for [Ca2+]o 10 mM). The inward current inactivated only weakly during depolarizing pulses of 0.1‐1 s duration. The delayed outward current (in experiments with 155 mM‐internal [K+] ([K+]i)) had a linear voltage dependence at potentials above ‐20 mV; its amplitude at ‐10 mV was 210 +/‐ 30 pA. Tail currents related to the activation of the outward current had K+‐dependent reversal potentials. The current was blocked by extracellularly applied tetraethylammonium (20 mM) and 4‐aminopyridine (2 mM). It was not affected by glibenclamide (3 microM), tolbutamide (0.2 mM) and alterations of intracellular [Ca2+] (1 nM‐1 microM). The activation time constant of the outward current at ‐10 mV was 21 +/‐ 3 ms. The voltage dependence of activation could be described by the sigmoidal function (see above) with Vh = 19 +/‐ 1 mV and k = 5.6 +/‐ 0.4 mV. The outward current inactivated during long (15 s) depolarizing pre‐pulses (time constant at ‐10 mV: 2.6 +/‐ 0.6 s). 50% inactivation occurred at Vh = ‐36 +/‐ 2 mV, k was ‐4.1 +/‐ 0.2 mV. Inward and outward currents during depolarizing voltage pulses in beta‐cells are similar to Ca2+ and delayed K+ currents in other cell types. These currents seem sufficient to generate the action potentials of the beta‐cell.