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Inotropic effects of ejection are myocardial properties
1Department of Internal Medicine, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27157-1045.
Insights
Ejection in heart muscle has both positive and negative effects on pressure, influencing myocardial contractility. These ejection effects are fundamental properties of the heart muscle itself, not just the chamber.
Area of Science:
- Cardiovascular Physiology
- Muscle Mechanics
- Cardiac Electrophysiology
Background:
- Studies suggest left ventricular end-systolic pressure (LVPes) results from ejection effects.
- It remains unclear if these effects are chamber-specific or fundamental myocardial properties.
Purpose of the Study:
- To investigate the effects of ejection on myocardial contractility at the sarcomere level.
- To determine if ejection effects are basic myocardial properties.
Main Methods:
- Isolated rat cardiac trabeculae were used to simulate in situ sarcomere shortening patterns.
- An iterative computer loading system approximated ejection and isovolumic contractions.
- Experiments were conducted using solutions with Ca2+ or Sr2+ plus Ca2+.
Main Results:
- Simulated LVPes was lower with ejection in Ca2+ but higher in Sr2+ solutions compared to isovolumic contractions.
- Ejection prolonged contraction duration and time to end systole.
- Comparing pressure at equivalent time and volume revealed only a positive ejection effect in both solutions.
Conclusions:
- Both positive and negative ejection effects are fundamental myocardial properties.
- Ejection influences myocardial contractility at the sarcomere level.
Abstract:
Recent studies in isolated and in vivo canine hearts have suggested that the left ventricular end-systolic pressure (LVPes) of ejecting beats is the net result of a balance between positive and negative effects of ejection. At present, it is unknown whether these ejection effects are merely a ventricular chamber property or represent a fundamental myocardial property. Accordingly, we examined the effects of ejection 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 with four ejecting contractions. The superfusing solution contained either 0.7 mM Ca2+ or 0.65 mM Sr2+ plus 0.15 mM Ca2+. With Ca2+, simulated LVPes ("LVP"es) of ejecting contractions was significantly lower than isovolumic "LVP"es (-5.3 +/- 5.6%), whereas with Sr2+, ejecting "LVP"es was significantly higher than isovolumic "LVP"es (+4.5 +/- 7.5%). Contraction duration and time to end systole were markedly prolonged in ejecting vs. isovolumic contractions with either Ca2+ or Sr2+. As a consequence, comparison of simulated LVP between ejecting and isovolumic beats throughout the contraction, i.e., at the same simulated LVV and time, revealed only a positive effect of ejection with either Ca2+ (+18.8 +/- 5.5%) or Sr2+ (+23.4 +/-9.3%). We conclude that both positive and negative effects of ejection are basic myocardial properties.