Functional expression and inactivation of L-type Ca2+ currents during murine heart development - implications for cardiac Ca2+ homeostasis.
OBJECTIVE
The expression and regulation of Ca2+ signaling in embryonic cardiomyocytes has been shown to be different from those in adult heart cells, particularly the L-type Ca2+ channel current (I(CaL)) increases during development. However, little is known about the underlying reasons for this increase of I(CaL ) density and developmental changes in the process of I(CaL ) inactivation, a critical regulator of intracellular Ca2+ homeostasis. In the present work, we therefore studied functional differences of I(CaL) between embryonic and fetal cardiomyocytes and its interaction with intracellular Ca2+ homeostasis and Ca2+-induced Ca2+ release (CICR). Moreover, we examined the process of voltage- (VDI) and Ca2+-dependent inactivation (CDI) of I(CaL) during murine embryonic heart development.


METHODS
The electrophysiological characteristics of I(CaL ) inactivation were analyzed in embryonic ventricular cardiomyocytes of early (E9.5-11.5) and late developmental, fetal (LDS, E16.5-18.5) stages and of adult mice using the whole-cell patch-clamp technique.


RESULTS
Fast, Ca2+-dependent inactivation kinetics (tau(f)) were significantly accelerated in LDS-derived cardiomyocytes (2.53 +/- 1.43 ms, n=9) as compared to EDS (5.09 +/- 2.19 ms, n=8, p>=0.009), whereas slow, voltage-dependent inactivation time constants (tau (s)) were unchanged. In cardiomyocytes derived from LDS we observed an increase in the maximal gating charge (Q(max)), suggesting an increase in the number of L-type Ca2+ channels at the sarcolemma, whereas the channel open probability (P(o)) was unchanged. Accordingly a significantly higher I(CaL ) density was found in LDS (-14 +/- 2.26 pA/pF, n=14) versus> EDS-derived cardiomyocytes (-10.03 +/- 1.43 pA/pF, n=13, p)( 0.05). Since ryanodine (10 microM) failed to alter tau(f) at both developmental stages, a major contribution of CICR and bulk Ca2+ to the acceleration of the fast inactivation kinetics during heart development seems to be unlikely.


CONCLUSIONS
Our data suggest that the increase of local subsarcolemmal Ca2+, evoked by the higher expression of L-type Ca2+ channels at the sarcolemma, rather than bulk Ca2+ accelerates I(CaL ) inactivation during embryonic heart development.
Nguemo
Filomain
F
Institute of Neurophysiology, University of Cologne, Cologne, Germany.
Fleischmann
Bernd K
BK
Schunkert
Heribert
H
Hescheler
Juergen
J
Reppel
Michael
M
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D047108:Embryonic Development / Q000187:drug effects; D005333:Fetus / Q000166:cytology / Q000187:drug effects; D006321:Heart / Q000187:drug effects / Q000196:embryology*; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects / Q000196:embryology; D006706:Homeostasis / Q000187:drug effects; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D051379:Mice; D032383:Myocytes, Cardiac / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D012433:Ryanodine / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D013997:Time Factors
D020746:Calcium Channels, L-Type; D012433:Ryanodine; D002118:Calcium