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 (A_SK) with pronounced myogenic tone and mesenteric small arteries (A_MS) 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 A_SK developed significant myogenic tone and constricted further in low [Na⁺]o. Nifedipine (1 μM) reduced both myogenic tone and low [Na⁺]o-induced contraction. Because the membrane potential of A_SK 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 A_MS did not develop significant myogenic tone. Although low [Na⁺]o also constricted A_MS, the magnitude of constriction was significantly weaker than that in A_SK (17 ± 4 vs. 47 ± 6%, P < 0.01, at 58 mM Na⁺). With Bay K 8644, A_MS 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.