Angiotensin II effect on ²²Na⁺ transport in vascular smooth muscle cells

It is well established that angiotensin II (AII) rapidly increases free cytosolic Ca²⁺ in vascular smooth muscle cells (VSMCs). Several studies have indicated that the hormone also plays a role in Na⁺-K⁺ regulation of these cells. In this study, we explored the mechanism of AII effect on ²²Na⁺ transport in cultured rat VSMCs. The ²²Na⁺ washout from these cells was described by three exponents with exponential factors k₁>k₂>k₃. In 1.8 mM Ca²⁺ medium, AII (10^-9-10^-6M) increased (in a dose response manner) the k₁ value, and consequently the initial washout rate constant (ki(e)) for the isotope. AII had no effect on ki(e) in Ca²⁺-deficient medium or in the presence of ouabain. Amiloride (10^-3 M) and verapamil (10^-5M) abolished the AII induced increase in ki(e). These findings are consistent with angiotensin II stimulation of an amiloride-sensitive Na⁺ transport, which is likely to represent the Na⁺/H⁺ antiport. In cultured VSMCs, the sustained stimulation by AII of this transport system requires the presence of extracellular Ca²⁺ and its influx into these cells.