Fetal and postnatal development of Ca2+ transients and Ca2+ sparks in rat cardiomyocytes.
OBJECTIVE
The aim of this study was to characterize the spatio-temporal dynamics of [Ca(2+)](i) in rat heart in the fetal and neonatal periods.


METHODS
Using confocal scanning laser microscopy and the Ca(2+) indicator fluo-3, we investigated Ca(2+) transients and Ca(2+) sparks in single ventricular myocytes freshly isolated from rat fetuses and neonates. T-tubules were labeled with a membrane-selective dye (di-8-ANEPPS). Spatial association of dihydropyridine receptors (DHPR) and ryanodine receptors (RyR) was also examined by double-labeling immunofluorescence.


RESULTS
Ca(2+) transients in the fetal myocytes were characterized by slower upstroke and decay of [Ca(2+)](i) compared to those in adult myocytes. The magnitude of fetal Ca(2+) transients was decreased after application of ryanodine (1 microM) or thapsigargin (1 microM). However, Ca(2+) sparks were rarely detected in the fetal myocytes. Frequent ignition of Ca(2+) sparks was established in the 6-9-day neonatal period, and was predominantly observed in the subsarcolemmal region. The developmental change in Ca(2+) sparks coincided with development of the t-tubule network. The immunofluorescence study revealed colocalization of DHPR and RyR in the postnatal period, which was, however, not observed in the fetal period. In the adult myocytes, Ca(2+) sparks disappeared after disruption of t-tubules by glycerol incubation (840 mM).


CONCLUSIONS
The sarcoplasmic reticulum (SR) of rat ventricular myocytes already functions early in the fetal period. However, ignition of Ca(2+) sparks depends on postnatal t-tubule formation and resultant colocalization of DHPR and RyR.
Seki
Sumihiko
S
Department of Cellular Physiology and Signal Transduction, Sapporo Medical University School of Medicine, Sapporo 060, Japan. sseki@kanazawa-med.ac.jp
Nagashima
Masato
M
Yamada
Yoichi
Y
Tsutsuura
Masaaki
M
Kobayashi
Takeshi
T
Namiki
Akiyoshi
A
Tohse
Noritsugu
N
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000704:Analysis of Variance; D002118:Calcium / Q000378:metabolism*; D002120:Calcium Channel Agonists / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D019084:Fluorescent Antibody Technique, Indirect; D006321:Heart / Q000196:embryology* / Q000254:growth & development; D006329:Heart Conduction System / Q000502:physiology*; D006801:Humans; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D032383:Myocytes, Cardiac / Q000378:metabolism* / Q000648:ultrastructure; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D019284:Thapsigargin / Q000494:pharmacology
D002120:Calcium Channel Agonists; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D004791:Enzyme Inhibitors; D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D019284:Thapsigargin; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D002118:Calcium