Activation of contraction of arterial smooth muscle in the presence of nitrate and other anions

Contraction strength was measured in ear artery and aorta of the rabbit and coronary artery of the pig during replacement of chloride with anions of the lyotropic series. Contractions induced by depolarization with elevated external potassium were potentiated in the presence of foreign anions, while those induced by noradrenaline were affected only at higher concentrations of the drug. Drug induced contractions in calcium-free solutions were not affected by the anions. In the presence of nitrate and excess K the change of membrane potential per tenfold change in (K)₀ was 56 mV in chloride and 50 mV in nitrate solution. Enhancement of contraction strength by nitrate was not associated with alterations in membrane potential. Stimulation of aorta with 29.5 mM K resulted in a larger net uptake of⁴⁵Ca in nitrate than in chloride. K-stimulated⁴⁵Ca efflux was greater in the ear artery in NO₃ solution than Cl solution while⁴⁵Ca efflux induced by noradrenaline was not effected by anion replacement. These results suggest that in these arteries anions potentiate contraction by enhancing the influx of external calcium, and do not modify the release of internal stores of calcium.