The effects of 10 mM Ca2+ and Ca2+ channel blockers verapamil, diltiaz
em and flunarizine on the ouabain-sensitive electrogenic Na-+, K+ pump
activity of mouse diaphragm muscle fibres enriched with Na+ were comp
ared with the changes in cytosolic [Ca2+]. The electrogenic Na+ pump a
ctivity produced by adding K+ to muscles previously bathed for 4 h in
a K+-free, 2-mM [Ca2+] solution increased the resting membrane potenti
al by about 18 mV. This hyperpolarization was completely inhibited aft
er 10 min incubation in 10 mM Ca2+. Verapamil 10(-5) M, 10(-5) M dilti
azem and 10(-7) M flunarizine effectively prevented the effect of elev
ated [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 inobenzofuran-5-oxy]-2-(2'
-amino-5'-methylphenoxy) ethane-N,N,N',N'-tetraacetic acid acetoxymeth
yl ester (fura-2/AM) was 261+/-6nM. After 4 h in a Liley K+-free, 2 mM
[Ca2+] solution, the cytosolic [Ca2+] increased to 314 +/- 28 nM. Inc
rease 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 inhibit
ion of electrogenic pump, prevented by 10(-5) M verapamil, 10(-5) M di
ltiazem and 10(-7) M flunarizine. The results suggest that substances
which block Ca2+ entry into the cell prevent the Ca2+-induced inhibiti
on of the Na+ pump.