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Excitation-contraction coupling in the day 15 embryonic chick heart with persistent truncus arteriosus
T L Creazzo1, M A Brotto, J Burch
1Institute of Molecular Medicine and Genetics, Development Biology Program, Medical College of Georgia, Augusta 30912-2640, USA.
Insights
Cardiac neural crest defects, like persistent truncus arteriosus (PTA), severely impair calcium (Ca2+) transients in embryonic hearts. This dysfunction in cardiac excitation-contraction coupling contributes to poor embryonic viability.
Area of Science:
- Cardiovascular Research
- Developmental Biology
- Molecular Cardiology
Background:
- Neural crest-related heart defects, such as persistent truncus arteriosus (PTA), are associated with high prenatal mortality.
- Previous studies suggest reduced L-type Ca2+ current in PTA hearts, hinting at impaired cardiac excitation-contraction coupling.
- Altered myocardial development is suspected in these defects, beyond structural anomalies.
Purpose of the Study:
- To investigate the role of calcium (Ca2+) transients in the pathophysiology of persistent truncus arteriosus (PTA).
- To determine if impaired cardiac excitation-contraction coupling contributes to the poor viability observed in PTA embryos.
Main Methods:
- Direct measurement of systolic Ca2+ transients in fura-2-loaded cardiomyocytes from normal and PTA embryonic chick hearts.
- Assessment of L-type Ca2+ current and response to isoproterenol (ISO).
- Evaluation of caffeine-stimulated Ca2+ transients and sarcoplasmic reticulum (SR) function.
Main Results:
- Ca2+ transients were significantly depressed in hearts with PTA, often requiring signal averaging or ISO treatment for detection.
- Reduced L-type Ca2+ current was confirmed as a partial cause of depressed Ca2+ transients.
- ISO normalized L-type Ca2+ current but did not fully restore Ca2+ transients; caffeine-stimulated transients were diminished and slowed, indicating impaired SR function.
Conclusions:
- Persistent truncus arteriosus (PTA) is associated with severely impaired cardiac excitation-contraction coupling.
- Both reduced L-type Ca2+ current and sarcoplasmic reticulum (SR) dysfunction contribute to the depressed Ca2+ transients in PTA hearts.
- Impaired cardiac excitation-contraction coupling is a likely cause of poor embryonic viability in PTA.
Abstract:
Ca2+ transients were examined in embryonic chick hearts with an experimentally induced cardiac neural crest-related outflow tract defect known as persistent truncus arteriosus (PTA). In all of the animal models of neural crest-related heart defects, prenatal mortality is too high to be attributed to structural defects of the heart alone, suggesting that there is altered development of the myocardium. Earlier reports indicating reduced L-type Ca2+ current in hearts with PTA suggest that poor viability may be related to impairment of cardiac excitation-contraction coupling. To test this hypothesis, direct measurements of the systolic Ca2+ transient in fura-2-loaded myocytes from normal hearts and hearts with PTA were carried out. We found that Ca2+ transients were severely depressed in hearts with PTA and difficult to measure above background noise unless signal averaged or treated with isoproterenol (ISO). We confirmed that the reduced Ca2+ transients were due, at least partly, to a reduction in L-type Ca2+ current. In addition we found that although ISO could raise the L-type current in hearts with PTA to the level found in normal hearts in the absence of ISO, it could not fully restore the Ca2+ transient. Furthermore, caffeine-stimulated Ca2+ transients were diminished in size and the time-to-peak and the decaying phase were significantly slowed. Interestingly, these observations were not accompanied by a reduction in the number of Ca2+ release channels. These results indicated an impairment of SR function in addition to the reduction in L-type Ca2+ current. These results strongly support our hypothesis that the poor viability of embryos with PTA is due to impaired cardiac excitation-contraction coupling.