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The beta-adrenergic action is thought to be related to the ability of the beta-adrenergic stimulants to increase the intracellular level of cyclic adenosine 3',5'-monophosphate (cyclic AMP) in the smooth muscle (1). Recently some workers (2,3) have mentioned that papa verine, a smooth muscle relaxant, is found to strongly inhibit cyclic phosphodiesterase in rabbit aorta and rat uterus. On the other hand, Takagi, Takayanagi and Tomiyama (4, 5) have reported that the relaxing effects of papaverine and isoprenaline, a beta-adrenergic stimulant, are identical to exogenous dibutyryl cyclic AMP and that these drugs suppress a supply of calcium to the contractile protein at the membrane, Further, Takagi, Takayanagi and Tsuchida (6) have also indicated using the taenia from the guinea pig caecum that caffeine and imidazole, which influence phosphodiesterase activity, considerably modify

The effects of papaverine, isoprenaline and N6,2'-O-dibutyryl 3',5'cyclic adenosine monophosphate (DiBu.C-AMP) on calcium movements in the taenia from the guinea-pig caecum have been investigated and compared with the effects of phenylephrine. Papaverine and DiBu.C-AMP antagonized the contraction of KC1-depolarized muscle induced by CaCl, and increased 45Ca-efflwr from the taenia. Papaverine also significantly depressed 45Ca-uptake by this preparation. These findings suggest that papaverine impairs the availability of calcium to the contractile system and that there may be a correlation between relaxation and increase of 45Ca-efflux. The action of DiBu.C-AMP resembles that of papaverine but its potency is less.

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