Is Ca2+ effect on passive K+ transport mediated by spectrin--dependent ATPase?

The aim of this report was to find which part of the membrane is responsible for the Ca2+ dependence of membrane permeability to K+. We found that the enzyme activity of large contractile complex of membrane proteins, the so called spectrin-dependent ATPase (sp-ATPase) increases at certain Ca2+ concentrations when K+ permeability decreases and vice versa. Ca2+ apparent dissociation constant for sp-ATPase is 6 X 10(-7) M which is the value corresponding to findings of Porzig and Stoffel (1978) for Ca2+ binding to membrane with low K+ permeability. Moreover, 20 chemically quite different substances which inhibit sp-ATPase activity simultaneously increase in the same concentrations K+ permeability and vice versa. No exception was found. The results show that low membrane permeability to K+ occurs at such conformation of sp-ATPase at which its enzyme activity may fully be manifested whereas at other conformations permeability to K+ increases. The conformation of sp-ATPase seems to affect gating mechanism of a respective channel for passive K+ transport through membrane.