Abstract
Contractions of the guinea pigs vas deferens were induced by field stimulation and the effects of derivatives of cGMP were tested on the contraction-relaxation cycle. Relaxation was induced by 8-Br-cGMP and O2'-mb-cGMP. On the contrary, db-cGMP and N2-mb-cGMP potentiated the contraction. The relaxant effect of 8-Br-cGMP was not mimicked by 8-Br-5'GMP, instead the latter caused the opposite effect. Since neither 8-Br-cGMP nor db-cGMP affected the release of norepinephrine on field stimulation the cGMP derivatives induced their effects at a postsynaptic site. 8-Br-cGMP was found to be 100-fold more potent than cGMP to simulate protein kinase activity in crude extracts from guinea pig was deferens. N2-mb-cGMP and O2'-mb-cGMP were approximately equipotent to cGMP while db-cGMP was less active. 8-Br-5'-GMP did not stimulate the protein kinase. It is suggested that the results support the hypothesis that cGMP acts as a relaxant in the smooth muscle of the vas deferens. The potentiation caused by db-cGMP and N2-mb-cGMP on contractions could be dependent on the substitution in the guanine moiety of the molecules with the butyryl group. Their effect was thus probably unrelated to a specific cGMP effect.
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