Intracellular Ca(2+)-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 implications 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+ channels at different levels of intracellular Ca2+ concentration (Ca2+i) associated with changes in extracellular Ca2+ concentration (Ca2+o). This study shows that the membranes of dispersed bovine parathyroid cells contain a K+ channel that is activated by elevated Ca2+o through an indirect mechanism (i.e. exposure of the entire cell to high Ca2+o 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 Ca(2+)-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 microM 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 microM Ca2+ to 79 mV and 54 mV at 0.25 and 0.5 microM 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 Ca2+o, 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 Ca2+i concentrations of about 0.15-0.2, 0.4 and 0.6-0.7 microM at these three extracellular Ca2+ concentrations, values that are in excellent agreement with those previously measured using Ca(2+)-sensitive fluorescent dyes. Thus, parathyroid cells express a maxi type of Ca(2+)-activated K+ channel that is indirectly regulated by Ca2+o, presumably through concomitant changes in Ca2+i. The latter may limit the extent of the cellular depolarization produced in response to elevated Ca2+o in this cell type.
Kanazirska
M P
MP
Endocrine-Hypertension Division, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.
Vassilev
P M
PM
Ye
C P
CP
Francis
J E
JE
Brown
E M
EM
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002417:Cattle; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004533:Egtazic Acid / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D010280:Parathyroid Glands / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*
D015221:Potassium Channels; D004533:Egtazic Acid; D002118:Calcium