Papaverine enhances the effect of adenosine in guinea-pig atria.

Papaverine, while enhancing the force of contraction of guinea-pig atria, remarkably and dose-dependently enhanced the negative inotropic response of the atria to adenosine. It also enhanced the actions of ATP and other adenine nucleotides, but not those of 2-chloroadenosine and ACh. At similar concentrations, papaverine inhibited the uptake of adenosine by the atrial tissue during incubation with adenosine. Adenosine in the medium was degraded to inactive inosine during incubation with the atrial tissue, and papaverine reduced its degradation. The enhancing effect of papaverine on the action of adenosine on guinea-pig atria was like those of dipyridamole, 6-(2-hydroxy-5-nitrobenzyl)thioguanosine and cinepazide. The effect seemed to be due mainly to inhibition of adenosine uptake into the tissue. Inhibition of adenosine degradation may also have contributed to the action of papaverine, but this action was probably much less important than inhibition of adenosine uptake.