Papaverine-induced inhibition of electrical and mechanical activity and calcium movements of rat ileal smooth muscle.

1. Papaverine, in the concentration range 50-150 microM, inhibited rhythmic spontaneous contraction and caused relaxation of KCl-induced contractures of rat ileal smooth muscle. 2. Spontaneous slow waves and spikes declined at low papaverine concentrations. 3. The normal 45Ca uptake of the ileum was reduced by papaverin. Although papaverine caused only a small inhibition of normal 45Ca efflux from the ileum, it strongly inhibited the KCl-induced stimulation of 45Ca efflux. 4. Calcium uptake by the isolated ileal smooth-muscle microsomal fraction was slightly increased by papaverine, but mitochondrial calcium uptake was unaffected. 5. Ileal smooth muscle incubated in calcium-free media showed little response to either the reintroduction of calcium or KCl in the presence of papaverine. 6. It is suggested that the inhibitory actions of papaverine on ileal smooth muscle result from a blockade of the calcium influx which triggers the excitation-contraction coupling sequence.