Pharmacological properties of contraction caused by sodium removal in muscle strips isolated from canine coronary artery.

Contractions produced by Na+ removal were studied in muscle strips isolated from canine coronary artery. In the presence of 20 mM K+ and 0.5 mM Ca2+, rapid contractions were observed repeatedly on complete replacement of NaCl with sucrose. This contraction in the absence of Na+ (0-Na) was not affected by phentolamine but was strongly inhibited by verapamil. Ouabain slowly potentiated the O-Na contraction and markedly reduced the inhibition due to verapamil. The O-Na contraction was dependent on external Ca2+ both with and without ouabain. Bepridil had effects very similar to those of verapamil. Amiloride and excess Mg2+ reduced the O-Na contraction and the degree of their inhibition was similar after ouabain treatment. The decrease in verapamil susceptibility could suggest that the O-Na contraction has verapamil-sensitive and -insensitive components. The former is probably due to Ca2+ influx through voltage-dependent channels and the latter to Ca2+ influx through an Na(+)-Ca2+ exchange process. Ouabain is considered to increase the contribution of Na(+)-Ca2+ exchange to the O-Na contraction. Mg2+ may inhibit both verapamil-sensitive and -insensitive pathways. Amiloride probably exerts its inhibitory effect on the contractile machinery.