Inotropic effects of ryanodine and nicardipine on fetal, neonatal and adult guinea-pig myocardium.

The effects of extracellular Ca2+ and inotropic agents on contractile force were examined in left atrial and right ventricular myocardia isolated from fetal (35-55 days after conception), neonatal (0-5 days after birth) and adult (30-90 days after birth) guinea-pigs. In both atrial and ventricular muscles, the contractile force increased with increasing extracellular calcium concentration ([Ca2+]o) and the sensitivity to [Ca2+]o was higher in the fetus than in the neonate and adult. Nicardipine almost abolished the contractile force in all groups examined. The sensitivity to nicardipine was similar among the three age groups and also between the two tissues. Ryanodine almost abolished the contractile force in atrial muscles from all age groups. In ventricular muscles it partially reduced the contractile force to the same extent in the neonate and adult while it slightly increased the contractile force in the fetus. In both atrial and ventricular muscles, isoproterenol shortened the relaxation time to the same extent in the neonate and adult, but little or not in the fetus. Taken together, our present results suggest that the sarcoplasmic reticulum function in regulating myocardial contraction is less developed in the fetal guinea-pig when compared with the neonate and adult, and that the atrial contraction is more dependent on Ca2+ release from the sarcoplasmic reticulum than the ventricular contraction is. Thus, it appears that the development of contractile function of the guinea-pig myocardium is mostly completed by birth.