SUMMARY1. Papaverine, up to 150 /M, was without effect on resting tension of vas deferens and urinary bladder smooth muscle strips, but caused a dose-dependent inhibition of the phasic component of the 100 mM-K contracture. At 150 FM, papaverine caused an 80 and 60 % inhibition ofphasic tension in vas deferens and urinary bladder muscle respectively.2. Papaverine caused a marked dose-dependent reduction in the tonic component of 100 mM-K contractures in both preparations. At 150 /M, papaverine often relaxed preparations below initial starting tension.3. Addition of 150 /ZM-papaverine to incubation media caused a fall of 25-30 % in 45Ca uptake by both preparations.4. Papaverine at 150 /LM caused a small fall in resting 45Ca efflux from vas deferens and a larger fall in efflux from bladder muscle. However, this papaverine concentration caused a large inhibition of the 45Ca uptake by and 45Ca efflux from both preparations when stimulated by high-K depolarization.5. Papaverine at 150 ,lM was without significant effect on Ca binding by either microsomes or mitochondria isolated from both vas deferens and urinary bladder smooth muscle.6. The main site of papaverine action appears to be on the Ca-influx mechanism responsible for regulating Ca entry during prolonged K depolarization.

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