Action of indapamide on excitation-contraction coupling in vascular smooth muscle

The effects of indapamide on electrophysiological and mechanical parameters of longitudinal smooth muscle strips isolated from mammalian portal vein were studied by means of a double sucrose gap method associated with a photoelectric device for recording contraction. Indapamide (10(-4) M) reduced both the amplitude of the action potential and the contraction. The calcium inward current decreased and consequently the phasic contraction was also reduced. The potassium outward current was diminished while the tonic contraction was not modified significantly. The depressant nature of the indapamide response could counterbalance the stimulating action of angiotensin II but not that of noradrenaline. The results suggest that indapamide acts primarily on the plasma membrane of vascular smooth muscle by reducing the transmembrane calcium current, although a secondary decrease in the intracellular bound calcium could not be completely excluded.