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Developmental changes of intracellular Ca²⁺ transients in beating rat hearts

¹Department of Physiology, Health Sciences Center, Texas Tech University, Lubbock, Texas 79430; and ²Department of Physiology, Stritch School of Medicine, Loyola University Chicago, Maywood, Illinios 60153

Submitted 3 April 2003 ; accepted in final form 7 November 2003

Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca²⁺-induced Ca²⁺ release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca²⁺ that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca²⁺ entry and the Ca²⁺ released from the SR to the Ca²⁺ transient during the first 3 wk of postnatal development. To this end, intracellular Ca²⁺ transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca²⁺ flux to the global intracellular Ca²⁺ transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca²⁺ release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca²⁺ current, Ni²⁺ for the current resulting from the reverse-mode Na⁺/Ca²⁺ exchange, and mibefradil for the T-type Ca²⁺ current. Our results showed that the relative contribution of each Ca²⁺ flux changes considerably during the first 3 wk of postnatal development. Early after birth (1–5 days), the sarcolemmal Ca²⁺ flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.

neonate; ryanodine receptor; cardiac excitation-contraction coupling; intact heart

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