Influence of extracellular Na+ removal on cytosolic Ca2+ concentration in smooth muscle cells of rabbit cerebral artery.

There are some controversies over the contribution of Na+/Ca2+ exchanger (NCX) to the regulation of cytosolic Ca2+ concentration ([Ca2+]c) in smooth muscle. To prove the functional role of Na+/Ca2+ exchanger, we examined whether the removal of extracelluar Na+ could affect [Ca2+]c of rabbit cerebral arterial smooth muscle. The fluorescence ratio of fura-2 (R(340/380)) was measured in the single myocyte of rabbit middle cerebral artery and Na+ was substituted with the same concentration of NMDG+ or Li+. In 21 out of 230 cells tested, Na+ removal increased R(340,380) (deltaR(340/380)) by 115 +/- 16.5% of the deltaR(340/380) induced by 10 mM caffeine in the same cell. The Na+ removal-induced deltaR(340/380) was blocked by a selective inhibitor of cardiac type NCX exchanger (KB-R7943, (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea, 10 microM). In those cells where the Na+ removal by itself did not increase R(340/380), the caffeine-induced deltaR(340/380) was increased by Na+-removal (130 +/- 9.8% of control response, n=30). Under the whole-cell patch clamp condition, short application of caffeine induced transient increase of outward current (I(K,Ca)-transient) which reflect the change of subsarcolemmal [Ca2+]. The application of KB-R7943 increased the amplitude of I(K,Ca)-transient (n=4). These results suggest the functional existence of NCX in rabbit cerebral artery smooth muscle.