Calcium transients in the embryonic chick heart: contributions from
calcium channels and the sarcoplasmic reticulum.
Brotto, Marco A. De Paula, and Tony L. Creazzo.
Department of Cellular Biology and Anatomy, Medical College of
Georgia, Augusta, GA 30912-2000
APStracts 2:0332H, 1995.
In the embryonic mammalian heart virtually all the Ca2+ available for
the calcium transient comes through sarcolemmal Ca2+ influx. However,
several studies in avian species indicate that the sarcoplasmic
reticullum (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 myocyte from the day
11 embryonic chick heart to ascertain directly the roles of the SR
and Ca 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 [mu]M ISO. Nifedipine (10 [mu]M)
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 about 70% with
nifedipine suggesting that most, but not all, of the transient is
associated with L-type Ca current. Thus, a portion of the transient
may result from T-type Ca channels and/or reverse Na/Ca exchange.
Calculations based on integration of the Ca currents and cell volume
indicate that as much as 1/4 th of the Ca2+ entering via sarcolemmal
Ca channels is from T-type channels. Ryanodine at high concentrations
(10 - 100 [mu]M) 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. 10 mM CaCl2 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 Ca channels and possibly reverse Na/Ca exchange.
Received 1 August 1994; accepted in final form 26 July 1995.
APS Manuscript Number H679-4.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 14 August 1995.