Ca2+‐dependent K+ current and exocytosis in responses to caffeine and muscarine in voltage‐clamped guinea‐pig adrenal chromaffin cells

• Published in: Journal of neurochemistry 2001-09, Vol.78 (6), p.1243-1255
• Main Authors: Ohta, Toshio, Wakade, Arun R., Nakazato, Yoshikazu, Ito, Shigeo
• Format: Article
• Language: English
• Subjects: amperometry; Biological and medical sciences; Ca2+‐dependent K+ channels; caffeine; Cell physiology; chromaffin cells; Fundamental and applied biological sciences. Psychology; Molecular and cellular biology; Secretion. Exocytosis; whole‐cell voltage‐clamp
• ISSN: 0022-3042; 1471-4159
• DOI: 10.1046/j.1471-4159.2001.00502.x

We characterized changes in membrane currents and the cytosolic Ca2+ concentration, [Ca2+]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 (Iout) at holding potentials over − 60 mV, increasing in amplitude with increasing the potentials. It also evoked a rapid increase of [Ca2+]i at all potentials examined. The current‐voltage relation revealed that the activation of K+ channels was responsible for the Iout evoked by caffeine. Both current and [Ca2+]i responses were reversibly abolished by cyclopiazonic acid, an inhibitor of Ca2+‐pump ATPase. At − 30 mV, the caffeine‐induced Iout, but not [Ca2+]i, was partly inhibited by either charybdotoxin or apamin. In the majority of cells tested, caffeine induced a larger Iout but a smaller [Ca2+]i increase than muscarine. Caffeine and muscarine increased catecholamine release from voltage‐clamped single cells concomitant with the transient increase of [Ca2+]i, and there was a positive correlation between them. These results indicate that caffeine activates Ca2+‐dependent K+ channels and catecholamine secretion due to the release of Ca2+ from internal stores in voltage‐clamped adrenal chromaffin cells of the guinea‐pig. There seems to be a spatial difference between [Ca2+]i increased by Ca2+ release from caffeine‐sensitive stores and that released from muscarine (inositol 1,4,5‐trisphosphate)‐sensitive ones.