Inactivating and noninactivating Ca(2+)- and voltage-dependent K+ current in rat adrenal chromaffin cells

Abstract

The properties of Ca(2+)- and voltage-dependent K+ currents and their role in defining membrane potential were studied in cultured rat chromaffin cells. Two variants of large-conductance, Ca2+ and voltage-dependent BK channels, one noninactivating and one inactivating, were largely segregated among patches. Whole-cell noninactivating and inactivating currents resulting from each of these channels were segregated among different chromaffin cells. Cell-to-cell variation in the rate and extent of whole-cell current decay was not explained by differences in cytosolic [Ca2+] regulation among cells; rather, variation was due to differences in the intrinsic properties of the underlying BK channels. About 75% of rat chromaffin cells and patches express inactivating BK current (termed BKi) while the remainder express noninactivating BK current (termed BKs). The activation time course of both currents is similar, as is the dependence of activation on [Ca2+] and membrane potential. However, deactivation of BKi channels is slower than that of BKs channels. The functional role of these BK channel variants was studied in current-clamp recordings. Although both BKi and BKs currents contribute to action potential repolarization, cells expressing BKi current are better able to fire repetitively in response to constant current injection. Blockade of BKi current by charybdotoxin abolishes this behavior, showing that afterhyperpolarizations mediated by BKi current are permissive for repetitive firing. Thus, important properties of chromaffin cell membrane excitability are determined by the type of BK current expressed.

[Ca2+]iElevations Detected by BK Channels during Ca2+Influx and Muscarine-Mediated Release of Ca2+from Intracellular Stores in Rat Chromaffin Cells

“…(2) Properties of BK i current in cells recorded with the perforated-patch method are identical immediately before and after activation of phospholipase C by mAChR activation (data not shown), implying that there are no persistent effects on BK channels of any mAChR-activated kinases. (3) The limiting BK i inactivation rates are similar both in perforated-patch and whole-cell recording, irrespective of the methods of [Ca 2ϩ ] elevation (Neely and Lingle, 1992b;Herrington et al, 1995;Solaro et al, 1995). Although these are indirect arguments, there is no observation that requires us to reject this assumption.…”

“…Methods of rat chromaffin cell isolation and maintenance of chromaffin cell cultures were as described in earlier reports (Neely and Lingle, 1992a;Herrington et al, 1995;Solaro et al, 1995). These were based on procedures described in several earlier studies (Fenwick et al, 1978;Role and Perlman, 1980;Livet, 1984).…”

“…Whole-cell and perforated-patch recordings were performed on cells 2-14 d after plating the cells, as previously described Lingle, 1992a, Herrington et al, 1995;Solaro et al, 1995). In experiments in which an intracellular solution of defined [Ca 2ϩ ] i was used, the standard whole-cell recording procedure was used (Hamill et al, 1981).…”

“…10). At the peak of the muscarinic response, the inactivation time constants of BK i currents are typically 40 -60 msec (Herrington et al, 1995;Solaro et al, 1995). Based on inactivation time constants from Figure 1, the implication is that the submembrane [Ca 2ϩ ] i reaches 3-5 M during the peak of the response.…”

Molecular Basis for the Inactivation of Ca2+- and Voltage-Dependent BK Channels in Adrenal Chromaffin Cells and Rat Insulinoma Tumor Cells

“…The most studied form of inactivation of BK channels has been in rat adrenal chromaffin cells (Solaro and Lingle, 1992;Solaro et al, 1995Solaro et al, , 1997Ding et al, 1998). The inactivation of chromaffin cell BK channels (BK i channels) shares some features with N-terminal type inactivation of voltage-dependent K ϩ channels (Hoshi et al, 1990).…”