Evidence for the involvement of calmodulin in the operation of Ca-activated K channels in mouse fibroblasts

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Abstract
Summary The oscillation of membrane potential in fibroblastic L cells is known to result from periodic stimulation of Ca2+-activated K+ channels due to the oscillatory increase in the intracellular Ca2+ concentration. These repeated hyperpolarizations were inhibited by putative calmodulin antagonists, trifluoperazine (TFP), N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and promethazine (PMZ), and the concentrations required for half-maximal inhibition were 25, 30 and 300 μm, respectively. These doses were lower than those for reducing the membrane resistance due to nonspecific cell damages. Another calmodulin antagonist, chlorpromazine (CPZ), was also effective, but CPZ-sulfoxide was not. Intracellular pressure injections of calmodulin-interacting divalent cations, Ca2+, Sr2+, Mn2+ and Ni2+, elicited slow hyperpolarizations, whereas Mg2+ and Ba2+, which are known to be essentially inert for calmodulin, failed to evoke any responses. The injection of purified calmodulin also brought about a similar hyperpolarization. Quinine, an inhibitor of Ca2+-activated K+ channels, abolished both Ca2+-and calmodulin-induced hyperpolarizations. TFP prevented Ca2+-induced hyperpolarizations. The TFP effect was partially reversed by the calmodulin injection. It is concluded that calmodulin is involved in the operation of Ca2+-activated K+ channels in fibroblasts.