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Am J Physiol Heart Circ Physiol 270: H518-H525, 1996;
0363-6135/96 $5.00
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AJP - Heart and Circulatory Physiology, Vol 270, Issue 2 518-H525, Copyright © 1996 by American Physiological Society

ARTICLES

Ca2+ transients in embryonic chick heart: contributions from Ca2+ channels and the sarcoplasmic reticulum

M. A. Brotto and T. L. Creazzo
Department of Cellular Biology and Anatomy, Medical College of Georgia, Augusta 30912, USA.

In the embryonic mammalian heart, virtually all the Ca2+ available for the Ca2+ transient comes through sarcolemmal Ca2+ influx. However, several studies in avian species indicate that the sarcoplasmic reticulum (SR) is functional relatively early in development. For the present report we studied fura 2 Ca2+ transients elicited by field stimulation in single isolated ventricular myocytes from the day 11 embryonic chick heart to ascertain directly the roles of the SR and Ca2+ channels. A positive staircase phenomenon was observed at higher frequencies of stimulation (1 Hz). Isoproterenol (Iso) increased the peak of the transient in a dose-dependent manner with a maximum increase of 93% in 100 microM Iso. Nifedipine (10 microM) reduced the transient such that is was not observable above background noise. However, Ca2+ transients were visible when the myocytes were stimulated by Iso. These were blocked by approximately 70% with nifedipine, suggesting that most, but not all, of the transient is associated with L-type Ca2+ current. Thus a portion of the transient may result from T-type Ca2+ channels and/or reverse Na+/Ca2+ exchange. Calculations based on integration of the Ca2+ currents and cell volume indicate that as much as one-fourth of the Ca2+ entering via sarcolemmal Ca2+ channels is from T-type channels. Ryanodine at high concentrations (10-100 microM) inhibited the transients by 30%. Both Iso and ryanodine reduced the time to peak, the time constant of the exponential decay, and the total duration of the transients. Depolarizing the myocytes with high KCl induced a large and partially sustained transient when the external solution contained 1.8 mM CaCl2. CaCl2 (10 mM) in the external solution induced large cyclic Ca2+ oscillations. These results suggest that the SR is functional in the embryonic chick heart well before hatching at day 22, although most of the Ca2+ associated with the transient comes through the sarcolemmal Ca2+ channels and possibly reverse Na+/Ca2+ exchange.


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 Am. J. Physiol. Heart Circ. Physiol.Home page
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Am J Physiol Heart Circ Physiol, March 1, 2005; 288(3): H1173 - H1178.
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Calcium Buffering and Excitation-Contraction Coupling in Developing Avian Myocardium
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