Effects of ranitidine on contraction and action potentials in normal and potassium depolarized isolated papillary muscles of the guinea pig

<正> The purpose of these experiments was to investigate the possible ionicbases for ranitidine （Ran） in isolated papillary muscles of the guinea pig.Ran,when the concentration was below 300μmol/L,had no influence on contraction ofnormal muscles and only above 0.3 mmol/L,it showed weak negative inotropic ef-fect.The contractile activity elicited by histamine in potassium depolarized mus-cles was depressed by Ran markedly and dose-dependently.The IC50 of Ran was0.273μmol/L.This depressant effect was reversed by an elevation of externalCa2+concentration to 7.7mmol/L.The contraction induced by isoproterenol inpotassium depolarized preparations was not altered by Ran.Ran at theconcentrations of 1,10 and 100μmol/L failed to show influences on fast actionpotentials.However,Ran at 0.5μmol/L time-dependently inhibited slow action po-tentials.It is concluded that the ionic mechanism of Ran might be attributed toantagonized H2 receptor of myocardium which in turn decreases Ca2+ inward cur-rent.