The effects of calcium and calcium channel blockers on sodium pump

Abstract

The effects of 10 mM Ca²⁺ and Ca²⁺ channel blockers verapamil, diltiazem and flunarizine on the ouabain-sensitive electrogenic Na⁺, K⁺ pump activity of mouse diaphragm muscle fibres enriched with Na⁺ were compared with the changes in cytosolic [Ca²⁺]. The electrogenic Na⁺ pump activity produced by adding K⁺ to muscles previously bathed for 4 h in a K⁺-free, 2-mM [Ca²⁺] solution increased the resting membrane potential by about 18 mV. This hyperpolarization was completely inhibited after 10 min incubation in 10 mM Ca²⁺. Verapamil 10⁻⁵M, 10⁻⁵M diltiazem and 10⁻⁷ M flunarizine effectively prevented the effect of elevated [Ca²⁺]. At these concentrations, these drugs did not affect the K⁺-induced hyperpolarization. In mouse diaphragm, the basal cytosolic [Ca²⁺] measured by the fluorescent indicator 1-[2-(5-carboxyoxazol-2-yl)-6-aminobenzofuran-5-oxy]2-(2′-amino 5′-methylphenoxy) ethane-N,N,N′,N′-tetraacetic acid acetoxymethyl ester (fura-2/AM) was 261±6 nM. After 4 h in a Liley K⁺-free, 2 mM [Ca²⁺] solution, the cytosolic [Ca²⁺] increased to 314±28 nM. Increase in [Ca²⁺] from 2 to 10 mM caused a twofold increase of cytosolic [Ca²⁺] to 637±26 nM. This rise was, like the Ca²⁺-induced inhibition of electrogenic pump, prevented by 10⁻⁵ M verapamil, 10⁻⁵M diltiazem and 10⁻⁷ M flunarizine. The results suggest that substances which block Ca²⁺ entry into the cell prevent the Ca²⁺ induced inhibition of the Na⁺ pump.