Forskolin effects on slow inward current and phasic tension of frog atrial fibres: modulation by adenosine and phosphodiesterase inhibitors.

The effects of forskolin, which is known as a direct activator of adenylate cyclase were studied on the slow inward calcium current (Isi) and phasic tension of frog atrial fibres. Forskolin induced a dose-dependent positive inotropic effect related to an increase in the slow inward calcium current. These effects, which were not reproduced by 1,9-dideoxyforskolin, seemed to result from an activation of adenylate cyclase. The action of forskolin was antagonized by adenosine and potentiated by phosphodiesterase inhibitors with the following order of potency: rolipram greater than theophylline greater than dipyridamole; M & B 22,948 was without influence. This study suggests that adenosine and rolipram might be suitable tools for studying the implication of cAMP in the modulation of contraction in frog atrium.