Influences of aminophylline and reduction in external Na on the antispasmodic action of isoproterenol in the isolated rat rectum.

Effects of aminophylline and the reduction in external Na on the antispasmodic action of isoproterenol were investigated in relation to the mobilization of Ca in the isolated rat rectum. The antispasmodic action of isoproterenol on the phasic contractions by acetylcholine and K both in Ca-free and in Ca-free and Na-poor media was potentiated by treatment with aminophylline, however the antispasmodic action was attenuated by reducing Na in the Ca-free medium. Dibutyryl cyclic AMP inhibited media and the inhibitory action was also potentiated by treatment with aminophylline, while the inhibitory action of dibutyryl cyclic AMP was attenuated by reducing Na in the Ca-free medium. From these findings, it appears that isoproterenol inhibits the release of Ca from storage sites induced by acetylcholine and K via the increase of intracellular cyclic AMP content and that the external Na may play an important role in the Ca release-inhibiting effect of cyclic AMP.