Inotropic effects of (+/-)-higenamine and its chemically related components, (+)-R-coclaurine and (+)-S-reticuline, contained in the traditional sino-Japanese medicines "bushi" and "shin-i" in isolated guinea pig papillary muscle.

(+/-)-Higenamine (Hig. demethylcoclaurine) is a cardiotonic principle from aconite root. (+)-R-Coclaurine (Coc) and (+)-S-reticuline (Ret) are compounds contained in the dried buds of Magnolia salicifolia MAXIM. All of these alkaloids possess a common chemical structure: tetrahydroisoquinoline. Coc and Ret showed negative inotropic effects in contrast to the positive inotropic effects of Hig in papillary muscles of guinea pigs. Coc and Ret shifted to the right the concentration-contraction curves of Hig. Hig shifted in parallel to the left the Ca2+ curve, and it tended to shift to the left the isoproterenol (Isp)-induced response curve. In contrast, Coc and Ret inhibited the Ca2+ curve and the low concentration range of the Isp-induced curve, and it potentiated the high concentration ranges of Ca2+ and Isp. Coc and Ret showed actions that were reversed in direction to those of Hig, as clearly demonstrated in the Ca2+ curve.