Cr. Solaro et al., INACTIVATING AND NONINACTIVATING CA2- AND VOLTAGE-DEPENDENT K+ CURRENT IN RAT ADRENAL CHROMAFFIN CELLS(), The Journal of neuroscience, 15(9), 1995, pp. 6110-6123
The properties of Ca2+- and voltage-dependent K+ currents and their ro
le in defining membrane potential were studied in cultured rat chromaf
fin cells. Two variants of large-conductance, Ca2+ and voltage-depende
nt 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 ext
ent of whole-cell current decay was not explained by differences in cy
tosolic [Ca2+] regulation among cells; rather, variation was due to di
fferences in the intrinsic properties of the underlying BK channels. A
bout 75% of rat chromaffin cells and patches express inactivating BK c
urrent (termed BKi) while the remainder express noninactivating BK cur
rent (termed BKs). The activation time course of both currents is simi
lar, as is the dependence of activation on [Ca2+] and membrane potenti
al. However, deactivation of BKi channels is slower than that of BKs c
hannels. The functional role of these BK channel variants was studied
in current-clamp recordings. Although both BKi and BKs currents contri
bute to action potential repolarization, cells expressing BKi current
are better able to fire repetitively in response to constant current i
njection. Blockade of BKi current by charybdotoxin abolishes this beha
vior, showing that afterhyperpolarizations mediated by BKi current are
permissive for repetitive firing. Thus, important properties of chrom
affin cell membrane excitability are determined by the type of BK curr
ent expressed.