Effect of Mn2+ on neonatal and adult rat heart: initial depression and late augmentation of contractile force.

In the present study, we examined the inotropic effect of Mn2+ on adult and neonatal rat myocardia, contraction of which is known to be highly dependent on Ca2+ release from the sarcoplasmic reticulum and trans-sarcolemmal Ca2+ influx, respectively. Mn2+ produced an initial negative inotropic effect followed by a late augmentation of contractile force in both neonatal and adult preparations, accompanied by marked prolongation of the contraction duration. The attenuation of the late augmentation by ryanodine was greater in the adult while the effect of nicardipine was greater in the neonate, which was similar to the effects of the two drugs in the absence of Mn2+. We tentatively concluded that Mn2+ produces late augmentation of the contractile force in neonatal and adult rat myocardia through some action on the general mechanism of force development, rather than by acting specifically on the sarcoplasmic reticulum.