INOTROPIC EFFECTS OF RYANODINE AND CALCIUM-ANTAGONISTS ON EMBRYONIC AND HATCHED CHICK MYOCARDIUM

Effects of extracellular Ca2+ and inotropic agents on contractile forc e were examined in myocardial preparations from embryonic and hatched chicks. Measurement of contractile force was performed in an organ bat h with whole hearts for the young embryo (5 to 6 days old) and with is olated strips from the right ventricles for the old embryos (16 to 18 days old), hatched chicks (within 24 hours after hatching) and 1 week old chicks. The extracellular Ca2+ concentration contractile force cur ve was in a lower concentration range in young embryonic heats when co mpared with older ones. 2 mM Ca2+ and 8 mM Ca2+ produced about 60% max imum contraction in preparations from young embryos and the older ages , respectively. The sensitivity to nicardipine and diltiazem was simil ar among all ages examined under 2 mM Ca2+. When the two drugs were ap plied to preparations from the older ages under 8 mM Ca2+, the sensiti vity was lower than that of the young embryo under 2 mM Ca2+. Ryanodin e produced a negative inotropic response at all ages but the effect wa s smaller in the young embryo when compared with those of older ages. Mn2+ produced a negative inotropic effect at all ages. In the older th ree ages, Mn2+ produced a late augmentation of the contractile force i n addition to the initial negative inotropic response, while such augm entation was not observed in the young embryo. In conclusion, the chic k myocardium was shown to undergo developmental changes in excitation- contraction mechanisms including increase in sarcoplasmic reticulum fu nction during the embryonic period, and thus provides an interesting m odel for studies on excitation-contraction mechanisms.