Effects of beta-casomorphin in isolated heart preparations.

beta-Casomorphin-5, a novel opioid peptide Tyr-Pro-Phe-Pro-Gly, was tested in isolated heart preparations of guinea pigs in comparison with other morphine related peptides and cardioactive drugs. The substance induced a dose-dependent change in the cardiac contraction force, showing a positive inotropic effect in low concentrations (10(-9) - 10(-7) mol/l) but a cardiodepressive action at higher doses (10(-7) - 10(-5) mol/l). Similarly, potentiation phenomena due to varied pacing rates of right-ventricular strips were enhanced at 10(-8) mol/l and lowered at 10(-6) mol/l beta-casomorphin-5. The coronary flow continuously increased. The observed changes of heart action were not mediated by opioid receptors: specifically acting opioid peptides were without influence and naloxone had no inhibitory effect. Peptides with similar sequences may exert a regulatory role in cardiac function, possibly mediated by an influence on the cellular calcium fluxes. The results may prove to be useful for the detection and estimation of more stable and cardioactive peptides.