Contribution of Na+/Ca2+ exchanger to the regulation of myogenic tone in isolated rat small arteries.

The contribution of the Na+/Ca2+ exchanger to the myogenic vascular tone was examined in rat isolated skeletal muscle small arteries (ASK) with pronounced myogenic tone and mesenteric small arteries (AMS) with little myogenic tone. Myogenic tone was assessed by the vascular inner diameter at transmural pressures of 40 and 100 mmHg. To depress the Na+/Ca2+ exchanger, the extracellular Na+ concentration (Na+]o) was lowered from 143 to 1.2 mM by substituting choline-Cl for NaCl. The ASK developed significant myogenic tone and constricted further in low Na+]o. Nifedipine (1 microM) reduced both myogenic tone and low Na+]o-induced contraction. Because the membrane potential of ASK was not changed by low Na+]o (-35 +/- 2 mV at 143 mM Na+]o, -37 +/- 3 mV at 1.2 mM Na+]o), depolarization-induced Ca2+ influx was not a cause of the low Na+]o-induced contraction. The AMS did not develop significant myogenic tone. Although low Na+]o also constricted AMS, the magnitude of constriction was significantly weaker than that in ASK (17 +/- 4 vs. 47 +/- 6%, P < 0.01, at 58 mM Na+). With Bay K 8644, AMS developed myogenic tone, and low Na+]o-induced constriction was significantly increased. In conclusion, Na+/Ca2+ exchanger may play an important role in regulating myogenic tone, likely via mediating Ca2+-extrusion.