Mechanism of the potentiating effect of NH4Cl on vasoconstriction in rat aorta.

1. In medium containing 10 or 20 mM KCl, NH4Cl (10 mM) addition significantly diminished tetraphenylphosphonium bromide (TPP+) uptake in A7r5 vascular smooth muscle cells. 2. Addition of 10 mM NH4Cl significantly potentiated the 45Ca2+ uptake of A7r5 cells in medium containing 20 mM KCl; this increased uptake could be reduced to the basal level by pretreatment with 0.1 microM verapamil. 3. In rat aortic strips, KCl at 10 mM induced a slight contraction, that was greatly potentiated by the simultaneous addition of 10 mM NH4Cl. In this case, also, pretreatment with verapamil (1 microM) completely abolished the NH4Cl-potentiating effect. 4. The results suggest that ammonium ion amplifies the K(+)-induced contraction of rat aorta by facilitating transmembraneous Ca2+ influx through the voltage-dependent calcium channel, which may be due to potentiation of KCl-induced depolarization of the plasma membrane.