Myocellular calcium regulation by the sarcolemmal membrane in the adult and immature rabbit heart

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Abstract
Previous studies have suggested ontogenic differences in Ca-mediated excitation-contraction coupling in mammalian heart. Sarcolemmal (SL) Ca regulation may predominate prior to the development of the specialized Ca-regulatory properties of the sarcoplasmic reticulum (SR). The effect of development on selected Ca-regulatory properties of cardiac SL was evaluated utilizing membrane vesicles obtained from immature (14 to 21-day-old) and adult rabbit heart. Methods were adapted to comparably enrich SL membrane vesicles from immature and adult rabbit heart. The global fluidity characteristics were determined by the polarized fluorescence of diphenylhexatriene passively incorporated into enriched SL membrane vesicles. No age-related differences in either the membrane microviscosity of the lipid-order parameter between 10°C and 37°C were observed. The membrane characteristics of the voltage-gated Ca channel were determined by the membrane binding characteristics of3[H]-nitrendipine. Scatchard analysis of high affinity specific nitrendipine binding demonstrated comparable binding affinity (KD; 511±40 vs 484±40 pM) and theoretical maximal binding site density (Bmax; 218±19 vs 240±40 fmoles/mg prot.) in immature and adult respectively. ATP-independent Ca binding to SL membrane vesicles was determined between 1.5 and 10 mM [Ca]. Ca binding was greater in the immature at 10 mM [Ca] as compared to the adult (840±120 vs 350±30 nmoles/mg). Ca bound to SL over this Ca concentration range is indicative of a “pool” of Ca for cellular influx across the SL by the Na−Ca exchange mechanism and the voltage-gated Ca channel. In view of electrophysiologic evidence also suggesting that Ca-channel-mediated Ca conductance is greater in the immature than the adult, it is proposed that the number of voltage-activatable Ca channels localized to the SL is greater in the immature than the adult. The larger transsarcolemmal Ca fluxes plays a larger role in the beat-to-beat- regulation of cardiac contraction in the developing mammalian heart prior to full expression of the specialized Ca regulatory properties of the SR.