Ca(2+)-dependent K(+) current and exocytosis in responses to caffeine and muscarine in voltage-clamped guinea-pig adrenal chromaffin cells.
We characterized changes in membrane currents and the cytosolic Ca(2+) concentration, [Ca(2+)](i), in response to caffeine, and compared them with those in response to muscarine using the perforated patch-clamp technique and fura-2 microfluorimetry in guinea-pig adrenal chromaffin cells. Catecholamine release from single voltage-clamped cells was monitored with amperometry using carbon microelectrodes. Caffeine produced a transient outward current (I(out)) at holding potentials over - 60 mV, increasing in amplitude with increasing the potentials. It also evoked a rapid increase of [Ca(2+)](i) at all potentials examined. The current-voltage relation revealed that the activation of K(+) channels was responsible for the I(out) evoked by caffeine. Both current and [Ca(2+)](i) responses were reversibly abolished by cyclopiazonic acid, an inhibitor of Ca(2+)-pump ATPase. At - 30 mV, the caffeine-induced I(out), but not [Ca(2+)](i), was partly inhibited by either charybdotoxin or apamin. In the majority of cells tested, caffeine induced a larger I(out) but a smaller [Ca(2+)](i) increase than muscarine. Caffeine and muscarine increased catecholamine release from voltage-clamped single cells concomitant with the transient increase of [Ca(2+)](i), and there was a positive correlation between them. These results indicate that caffeine activates Ca(2+)-dependent K(+) channels and catecholamine secretion due to the release of Ca(2+) from internal stores in voltage-clamped adrenal chromaffin cells of the guinea-pig. There seems to be a spatial difference between [Ca(2+)](i) increased by Ca(2+) release from caffeine-sensitive stores and that released from muscarine (inositol 1,4,5-trisphosphate)-sensitive ones.
Ohta
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T
Laboratory of Pharmacology, Department of Biomedical Sciences, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo, Japan. tohta@vetmed.hokudai.ac.jp
Wakade
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AR
Nakazato
Y
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Ito
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D000311:Adrenal Glands / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002395:Catecholamines / Q000378:metabolism; D019439:Chromaffin Cells / Q000187:drug effects / Q000502:physiology*; D006224:Cricetinae; D005089:Exocytosis / Q000502:physiology*; D007425:Intracellular Membranes / Q000378:metabolism; D008297:Male; D009116:Muscarine / Q000494:pharmacology*; D018721:Muscarinic Agonists / Q000494:pharmacology*; D009994:Osmolar Concentration; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000502:physiology*
D002395:Catecholamines; D018721:Muscarinic Agonists; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D002110:Caffeine; D009116:Muscarine; D002118:Calcium