Effect of cyclic AMP, db-cyclic AMP and phosphodiesterase inhibitors on histamine inhibition of the contractile response of the mouse vas deferens

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Abstract
Histamine inhibited the contractile response of the mouse vas deferens to various frequencies of stimulation, and the inhibition was inversely related to the frequency of stimulation. This effect of histamine was mimicked by cyclic AMP, db-cyclic AMP and various phosphodiesterase inhibitors (IBMX, aminophylline and theophylline). Histamine-produced inhibition, but not that produced by the other compounds, was blocked by cimetidine. The low concentrations (10–100 μM) of various phosphodiesterase inhibitors caused inhibition but failed to potentiate the inhibitory response to histamine. The basal cyclic AMP levels of the tissues were unaffected at these concentrations of aminophylline. At higher concentrations (1 and 5 mM), however, aminophylline significantly elevated the basal cyclic AMP levels of the tissues and markedly inhibited the contractile response to various frequencies of stimulation but still failed to enhance or potentiate the inhibitory response to histamine. In fact, the inhibitory response to histamine at these concentrations of aminophylline was reduced. Based on these and earlier [3] findings, it is concluded that, although histamine increases the accumulation of cyclic AMP in the mouse vas deerens and although its inhibitory effect on the preparation can be mimicked by both the cyclic nucleotides and phosphodiesterase inhibitors, the involvement of cyclic AMP in the mediation of its inhibitory response is still unresolved.