Maxi K channels in the basolateral membrane of the exocrine frog skin gland regulated by intracellular calcium and pH

With the single-channel patch-clamp technique we have identified Ca-sensitive, high-conductance (maxi) K channels in the basolateral membrane (BLM) of exocrine gland cells in frog skin. Under resting conditions, maxi K channels were normally quiescent, but they were activated by muscarinic agonists or by high serosal K. In excised inside-out patches and with symmetrical 140 mmol/l K, single-channel conductance was 200 pS and the channel exhibited a high selectivity for K over Na. Depolarization of the BLM increased maxi K channel activity. Increasing cytosolic free Ca (by addition of 100 nmol/l thapsigargin to the bathing solution of cell-attached patches also increased channel activity, whereas thapsigargin had no effect when added to excised inside-out patches. An increase in cytosolic free Ca directly activated channel activity in a voltage-dependent manner. Maxi K channel activity was sensitive to changes in intracellular pH, with maximal activity at pH 7.4 and decreasing activities following acidification and alkalinization. Maxi K channel outward current was reversibly blocked by micromolar concentrations of Ba from the cytosolic and extracellular site, and was irreversibly blocked by micromolar concentrations of charybdotoxin and kaliotoxin from the extracellular site in outside-out patches. © 1995 Springer-Verlag.