INTRACELLULAR CA2+-ACTIVATED K+ CHANNELS MODULATED BY VARIATIONS IN EXTRACELLULAR CA2+ IN DISPERSED BOVINE PARATHYROID CELLS

The modulation of K+ channels by Ca2+ may have important functional imlications in parathyroid cells, since in most endocrine cells they control membrane voltage regulating Ca2+ influx and hormone secretion. To characterize specific channel mechanisms regulating membrane voltage in parathyroid cells, the patch-clamp technique was used to determine the activities of K-divided by channels at different levels of intracellular Ca2+ concentration (Ca-i(2+)) associated with changes in extracellular Ca2+ concentration (Ca-0(2+)). This study shows that the membranes of dispersed bovine parathyroid cells contain a K+ channel that is activated by elevated Ca2+, through an indirect mechanism (i.e. exposure of the entire cell to high Ca-0(2+) activates the channel despite a low Ca2+ concentration within the pipette solution on the external side of the channel under study). This K+ channel has a unitary conductance of 191 pS and is highly selective for K+, similar to the so-called maxi type of Ca2+-activated K+ channel previously defined in a number of other cell types. Like the latter channel, the activity of this channel in excised patches from parathyroid cells is markedly increased when an EGTA-containing buffer on the cytoplasmic face of the membrane is replaced with one containing 0.5 mu M Ca2+. Changes in Ca2+ on the intracellular side of the membrane also shift the level of voltage necessary for half-maximal activation of the channel from 103 mV at 0.1 mu M Ca2+ to 79 mV and 54 mV at 0.25 and 0.5 mu M Ca2+, respectively. When similar studies were carried out using cell-attached patches on parathyroid cells exposed to 0.5, 1.5, or 2.0 mM Ca-0(2+), the values for half-maximal activation were approximately 105, 56, and 29 mV, respectively. The latter result suggests that in intact parathyroid cells, the channel is exposed to Ca-i(2+) concentrations of about 0.15-0.2, 0.4 and 0.6-0.7 mu M at these three extracellular Ca2+ concentrations, values that are in excellent agreement with those previously measured using Ca2+-sensitive fluorescent dyes. Thus, parathyroid cells express a maxi type of Ca2+-activated K+ channel that is indirectly regulated by Ca-0(2+), presumably through concomitant changes in Ca-i(2+). The latter maylimit the extent of the cellular depolarization produced in response to elevated Ca-0(2+) in this cell type.