Influence of Extracellular Na^+ Removal on Cytosolic Ca^<2+> Concentration in Smooth Muscle Cells of Rabbit Cerebral Artery

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There are some controversies over the contribution of Na^+ / Ca^<2+> exchanger(NCX)to the regulation of cytosolic Ca^<2+> concentration([Ca^<2+>]_C)in smooth muscle. To prove the functional role of Na^+ / Ca^<2+> exchanger, we examined whether the removal of extracelluar Na^+ could affect[Ca^<2+>]_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>(ΔR_<340 / 380>)by 115±16.5% of the ΔR_<340 / 380> induced by 10 mM caffeine in the same cell. The Na^+ removal-induced ΔR_<340 / 380> was blocked by a selective inhibitor of cardiac type NCX exchanger(KB-R7943, (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea, 10μM). In those cells where the Na^+-removal by itself did not increase R_<340 / 380>, the caffeine-induced ΔR_<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 [Ca^<2+>]. 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.
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