Effect of phosphodiesterase inhibitors and dBcAMP on the inotropic and relaxing actions of histamine in cardiac muscle.

The influence of phosphodiesterase inhibitors, imidazole and dBcAMP on the action of histamine on peak tension, on rate of tension development and rate of relaxation of ventricular muscle of guinea pig's heart was investigated. It was found that theophylline plus dBcAMP enhanced the peak tension, (dF/dt)max and the rate of relaxation and increased the relaxing action of histamine. Imidazole, an activator of phosphodiesterase, also potentiated the action of histamine on peak tension, (dF/dt)max and rate of relaxation. Caffeine, a well-known inhibitor of phosphodiesterase, suppressed the relaxing action of histamine and epinephrine. These discrepant results could be explained by assuming the possibility that these drugs have effects other than those involving inhibition and activation of phosphodiesterase.