Effects of quinidine on the electrical activity and contractile responses of the guinea-pig ureter smooth muscle.

Quinidine and its stereoisomer quinine suppress various membrane conductances in a variety of cells Recently quinidine has been classified as a putative blocker of the Na-Ca exchange in heart muscle (Mentrard et al. 1984; Chapman et al. 1984). The effects of quinidine on the contractile responses of the ureter muscle of the guinea-pig associated with both the entry of Ca via voltage-operated Ca channels and the Na-Ca exchange operating in Na-loaded ureter in Na-free solutions (Aickin et al. 1984) were studied and low-Na contracture was observed in the latter case. The present experiments were performed on isolated segments of whole ureter using the double sucrose-gap method for simultaneous recording of electrical and mechanical activity (Biilbring and Tomita 1969) and the superfusion technique for tension recording (Brading and Sneddon 1980). Loading of the tissue with Na ions was produced through inhibition of the Na-K pump by treatment of the ureter with ouabain as described ealier (Aickin et al. 1984). Fig. 1 shows the effects of quinidine on the evoked action potentials, phasic contractions and potassium contracture (126 mmol/1-K +). Fig. 1A shows that 10"" mol/1 quinidine caused a shortening of the plateau component of the action potential and a small decrease in the amplitude of the spike component (Fig. 1 A, b, c). Simultaneously, the amplitude of the phasic contraction was decreased. The effects of quinidine on the action potential and phasic contraction were reversible (Fig. 1A, d). The dose-response curve of the inhibition of the phasic contractions by quinidine is shown in Fig. 1C (curve b). Quinidine caused a dose-dependent