We examined the effects of a selective kappa opioid receptor agonist (U-50,488H) on the contractile properties of single ventricular myocytes from 127 day old control (F1B) and cardiomyopathic (BIO 14.6) hamsters. Myocytes in bicarbonate buffered solution with 1.5 mM [Ca2+] were electrically stimulated with field electrodes in the bath. Length changes were monitored via myocyte edge tracking. Twitch amplitude and the velocity of cell shortening were less in the cardiomyopathic hamster myocytes than in age-matched hamsters (P less than or equal to 0.05). There was a concentration-dependent effect of U-50,488H (0.1-20 microM) to decrease twitch amplitude and shortening velocity in both control and cardiomyopathic myocytes (P less than or equal to 0.001). In cells loaded with the Ca2+ indicator indo-1 the negative inotropic action of U-50,488H was associated with a decreased indo-1 fluorescence transient amplitude. There was no difference in the negative inotropic effect of U-50,488H on control and cardiomyopathic cells. Thus, the CM hamster does not demonstrate a different contractile response to U-50,488H.