Effects of enoximone and isobutylmethylxanthine on contractile tension and cyclic nucleotide levels in isolated blood-perfused dog papillary muscle.

The effects of enoximone (MDL 17,043) and 3-isobutyl-1-methylxanthine (IBMX) on developed tension, blood flow, and cyclic nucleotide levels were investigated in the isolated blood-perfused dog papillary muscle preparation. Intraarterial infusion of each agent, over 15 s, in doses ranging from 0.001 to 3 mg, produced a dose-dependent positive inotropic effect accompanied by increases in rate of contraction, rate of relaxation, and blood flow. The dose-response relationships for enoximone were always less steep than those for IBMX. Enoximone did not enhance automaticity at any dose, whereas the higher doses of IBMX (0.3, 1, and 3 mg) enhanced automaticity and produced arrhythmic preparations. Both agents produced increases in cyclic AMP during the peak positive inotropic effect (45 s); however, only IBMX produced an increase in cyclic GMP. The increase in cyclic AMP produced by enoximone lagged behind tension development by at least 15 s, whereas the increases in cyclic nucleotides produced by IBMX occurred concurrently with the development of the positive inotropic effect. The evalation in cyclic AMP produced by enoximone is consistent with the reported property of this agent to inhibit specifically and selectively the particulate high-affinity cyclic AMP phosphodiesterase from dog heart, and supports a cyclic AMP-dependent mechanism of positive inotropism for enoximone.