Voltage-activated calcium and potassium currents in human pancreatic beta-cells.
Kelly RP., Sutton R., Ashcroft FM.
1. The whole-cell configuration of the patch clamp technique was used to study inward and delayed outward currents in beta-cells isolated from human pancreatic islets. 2. The delayed outward current activated at about -20 mV and increased linearly with further depolarization. The instantaneous current-voltage (I-V) relation, measured by tail current analysis, reversed at -70 mV. This is close to the K+ equilibrium potential and suggests the outward current is carried primarily by potassium ions. In support of this idea, outward currents were abolished when internal K+ was replaced by the impermeant cation N-methyl-D-glucamine (NMG). 3. The voltage dependence of K+ current activation could be fitted by a sigmoidal function with a mid-point at +1 mV. K+ currents showed voltage-dependent inactivation which was half-maximal at -25 mV. 4. Inward currents were studied after outward currents were suppressed by replacing internal potassium with NMG. In 5 mM [Ca2+]o, the inward current activated between -50 and -40 mV, had a peak amplitude at -10 mV and reversed at potentials positive to +60 mV. The voltage dependence of inward current activation was sigmoidal with half-maximal activation at -10 mV in 5 mM [Ca2+]o and at -22 mV in 5 mM [Ba2+]o. 5. Inward currents were unaffected by tetrodotoxin (TTX), but could be blocked by cadmium ions. Barium was also capable of carrying inward current. This pharmacology is consistent with inward currents flowing through Ca2+ channels. 6. The inactivation of the inward current was dependent on calcium entry. In two-pulse experiments, the voltage dependence of inactivation was U-shaped, and resembled that of the calcium current. Barium currents showed little inactivation. 7. In two-pulse experiments the degree of inward current inactivation during the test pulse was related to the amount of calcium entry during the first pulse. Calcium entering at more positive potentials was less effective at producing inactivation. 8. Calcium and barium currents also showed a slow, voltage-dependent inactivation when the holding potential was changed between -100 and -40 mV. This inactivation developed with a time course of seconds. 9. The Ca2+ and K+ currents described here are similar to those reported for rodent beta-cells and indicate the rodent beta-cell provides a good model for that of man.