The effects of papaverine on the electrical and mechanical activity of the guinea-pig taenia coli.

1. Effects of papaverine (5 x 10(-6)-5 x 10(-5)M) were studied on the spontaneous and evoked electrical and mechanical activity of the guinea-pig taenia coli. In normal Locke solution, papaverine slowed and finally stopped the spontaneous spike discharge, usually with a small hyperpolarization of a few mV.2. Papaverine did not change the membrane resistance in normal Locke solution. It abolished the repetitive firing of spikes during the depolarization in response to a current pulse, but a single spike of normal amplitude could always be evoked.3. The mechanical response elicited by a single spike was reduced by papaverine in normal Locke solution. During the maintained depolarization produced by a long current pulse the tonic component of the contraction was more reduced than the phasic component related to the initial spike activity.4. The effects of papaverine were essentially the same as in normal Locke solution when NaCl was replaced with LiCl, or in excess K (24 mM), except that the membrane resistance was increased by papaverine in the presence of excess K.5. Excess Ca (6 mM) antagonized the effect of papaverine on the mechanical response, but not on the electrical response. When the muscle was depolarized in Ca-free solution, papaverine caused repolarization of the membrane and electrical activity reappeared. This effect was similar to that of Mg (4 mM).6. The mechanism by which papaverine suppresses the spontaneous spike activity is discussed and is compared with that of isoprenaline, from which it appears to differ. The observations indicate that papaverine, like Mg, may bind with a site at the membrane with which Ca normally binds, thereby modifying the electrical and mechanical activity controlled by Ca.