<b>THE EFFECTS OF SODIUM-FREE SOLUTION ON INTRACELLULAR CALCIUM DETERMINED FROM THE MECHANICAL RESPONSES OF THE GUINEA PIG BASILAR </b><b>ARTERY </b>

Abstract

The properties of the tonic contraction evoked from smooth muscle cells of the guinea pig basilar artery by Na-free and isotonic [K], (143.6 mM K) solutions were investigated. In the Na-free solution (substituted by cyoline, Tris or sucrose), a small phasic contraction was followed by a large tonic one with 3-4 min latency. Diltiazem (1 >< 10*'*M) and MnCl2 (3 mM) had little elfect, while procaine (5 mM) completely relaxed the tissue. The tonic contraction evoked by the isotonic [K], solution was relaxed by diltiazem, MnCl, or procaine, and that evoked by the Nafree solution was relaxed at a very slow velocity by the addition of 1 mM [Na], or the removal of Ca. Caffeine (5 mM) produced a small or large phasic contraction superimposed on the tonic contraction evoked in the Na-free or isotonic [K], solution. The caffeine-induced contraction was followed by complete relaxation of the tissue in isotonic [K], solution, but not in Na-free solution. In saponin treated skinned muscles, the minimum concentration of Ca required to produce the contraction was 1 >< 10""M, and the maximum amplitude of contraction was evoked by application of 3 ><10""M Ca. This pCa-tension relationship was not affected by the presence or absence of Na in the relaxing solution. Furthermore, 10 mM procaine or caffeine only slightly inhibited the above relationship. In skinned muscles, accumulation of Ca into the storage site was slightly inhibited in the Na-free solution (0.85 times the control), as estimated from the amplitude of the resulting caffeine-induced contraction. However, Na-free solution had no effect on the leakage of Ca from the storage site. The present results indicate that the tonic contraction evoked in the Na-free solution is mainly due to inhibition of the Ca efflux at the myoplasmic membrane, while that evoked in the isotonic [K], solution is due to acceleration of the Ca influx.