Characterization of a Ca2+-activated K+ current in insulin-secreting murine betaTC-3 cells.

1. The whole-cell perforated-patch recording mode was used to record a Ca2+-dependent K+ current (IK(Ca)) in mouse betaTC-3 insulin-secreting cells. 2. Depolarizing voltage steps (to potentials where Ca2+ currents are activated) evoked a slowly activating, outward current, which exhibited a slow deactivation (in seconds) upon subsequent hyperpolarization. 3. This current was shown to increase with progressively longer depolarizing voltage steps. It could be reversibly abolished by the removal of Ca2+ from the external medium or by application of Ca2+ channel blockers, such as Cd2+ and nifedipine. It was concluded that the depolarization-evoked current was activated by Ca2+. 4. Variations in external K+ concentration led to shifts in the reversal potential of the Ca2+-dependent current as predicted by the Nernst equation for a K+-selective current. 5. The Ca2+-activated K+ current was insensitive to external TEA (10 mM), a concentration sufficient to block the large-conductance Ca2+-dependent (maxi-KCa) channel in beta-cells. It was also insensitive to apamin, tubocurarine and scyllatoxin (leiurotoxin I), specific blockers of small-conductance KCa channels. 6. The current was blocked by quinine, a non-specific KCa channel blocker and, surprisingly, by charybdotoxin (ChTX; 100 nM) but not iberiotoxin, a charybdotoxin analogue, which blocks the maxi-KCa channel. It was sensitive to block by clotrimazole and could be potently and reversibly potentiated by micromolar concentrations of niflumic acid. Thus, the current exhibited unique pharmacological characteristics, not conforming to the known KCa channel classes. 7. The ChTX-sensitive KCa channel was permeable to Tl+, K+, Rb+ and NH4+ but not Cs+ ions. 8. The ChTX-sensitive IK(Ca) could be activated by the muscarinic agonists in the presence or absence of external Ca2+, presumably by releasing Ca2+ from internal stores. 9. Acutely isolated porcine islet cells also exhibited a slow IK(Ca) resembling that described in betaTC-3 cells in kinetic properties, insensitivity to TEA (5 mM) and sensitivity to quinidine, an analogue of quinine. The porcine IK(Ca), however, was not sensitive to block by 100-200 nM ChTX. It is likely, that species differences account for pharmacological differences between the mouse and porcine slow IK(Ca).