INOTROPIC EFFECTS OF EJECTION ARE MYOCARDIAL PROPERTIES

Recent studies in isolated and in vivo canine hearts have suggested th at the left ventricular end-systolic pressure (LVP(es)) of ejecting be ats is the net result of a balance between positive and negative effec ts of ejection. At present, it is unknown whether these ejection effec ts are merely a ventricular chamber property or represent a fundamenta l myocardial property. Accordingly, we examined the effects of ejectio n in eight isolated rat cardiac trabeculae at the sarcomere level. We approximated in situ sarcomere shortening patterns using an iterative computer loading system. Six isovolumic contractions were compared wit h four ejecting contractions. The superfusing solution contained eithe r 0.7 mM Ca2+ or 0.65 mM Sr2+ plus 0.15 mM Ca2+. With Ca2+, simulated LVP(es) (''LVP''(es)) of ejecting contractions was significantly lower than isovolumic ''LVP''(es) (-5.3 +/- 5.6%), whereas with Sr2+, eject ing ''LVP''(es) was significantly higher than isovolumic ''LVP''(es) ( +4.5 +/- 7.5%). Contraction duration and time to end systole were mark edly prolonged in ejecting vs. isovolumic contractions with either Ca2 + or Sr2+. AS a consequence, comparison of simulated LVP between eject ing and isovolumic beats throughout the contraction, i.e., at the same simulated LW and time, revealed only a positive effect of ejection wi th either Ca2+ (+18.8 +/- 5.5%) or Sr2+ (+23.4 +/- 9.3%). We conclude that both positive and negative effects of ejection are basic myocardi al properties.