Boucek R J, Shelton M E, Artman M, Landon E
Basic Res Cardiol. 1985 May-Jun;80(3):316-25. doi: 10.1007/BF01907907.
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 degrees C and 37 degrees C were observed. The membrane characteristics of the voltage-gated Ca channel were determined by the membrane binding characteristics of 3[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.
先前的研究表明，哺乳动物心脏中钙介导的兴奋-收缩偶联存在个体发育差异。在肌浆网（SR）特化的钙调节特性发育之前，肌膜（SL）钙调节可能占主导地位。利用从未成熟（14至21日龄）和成年兔心脏获得的膜囊泡，评估发育对心脏SL特定钙调节特性的影响。采用相应方法从幼年和成年兔心脏中富集SL膜囊泡。通过被动掺入富集的SL膜囊泡中的二苯基己三烯的偏振荧光来测定整体流动性特征。在10℃至37℃之间，未观察到脂质序参数的膜微粘度存在年龄相关差异。通过3[H]-尼群地平的膜结合特性来测定电压门控钙通道的膜特性。对高亲和力特异性尼群地平结合的Scatchard分析表明，未成熟和成年兔心脏中的结合亲和力（KD；511±40对484±40 pM）和理论最大结合位点密度（Bmax；218±19对240±40 fmol/mg蛋白）相当。在1.5至10 mM [Ca]之间测定与SL膜囊泡的非ATP依赖性钙结合。在10 mM [Ca]时，未成熟心脏中的钙结合比成年心脏更大（840±120对350±30 nmol/mg）。在此钙浓度范围内与SL结合的钙表明存在一个钙“池”，可通过钠-钙交换机制和电压门控钙通道使细胞外钙流入。鉴于电生理证据也表明未成熟心脏中钙通道介导的钙电导比成年心脏更大，因此提出未成熟心脏中定位于SL的电压可激活钙通道数量比成年心脏更多。在SR特化的钙调节特性完全表达之前，较大的跨肌膜钙通量在发育中的哺乳动物心脏的逐搏收缩调节中发挥更大作用。
