Mpv. Kanazirska et al., INTRACELLULAR CA2-ACTIVATED K+ CHANNELS MODULATED BY VARIATIONS IN EXTRACELLULAR CA2+ IN DISPERSED BOVINE PARATHYROID CELLS(), Endocrinology, 136(5), 1995, pp. 2238-2243
The modulation of K+ channels by Ca2+ may have important functional im
lications in parathyroid cells, since in most endocrine cells they con
trol 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 intrace
llular Ca2+ concentration (Ca-i(2+)) associated with changes in extrac
ellular Ca2+ concentration (Ca-0(2+)). This study shows that the membr
anes of dispersed bovine parathyroid cells contain a K+ channel that i
s activated by elevated Ca2+, through an indirect mechanism (i.e. expo
sure 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 conduc
tance of 191 pS and is highly selective for K+, similar to the so-call
ed maxi type of Ca2+-activated K+ channel previously defined in a numb
er of other cell types. Like the latter channel, the activity of this
channel in excised patches from parathyroid cells is markedly increase
d when an EGTA-containing buffer on the cytoplasmic face of the membra
ne is replaced with one containing 0.5 mu M Ca2+. Changes in Ca2+ on t
he 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, re
spectively. The latter result suggests that in intact parathyroid cell
s, 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+ concentration
s, values that are in excellent agreement with those previously measur
ed using Ca2+-sensitive fluorescent dyes. Thus, parathyroid cells expr
ess a maxi type of Ca2+-activated K+ channel that is indirectly regula
ted by Ca-0(2+), presumably through concomitant changes in Ca-i(2+). T
he latter maylimit the extent of the cellular depolarization produced
in response to elevated Ca-0(2+) in this cell type.