Characterization of Ca²⁺release channels in fetal and adult rat hearts

The goal of this study was to characterize the Ca²⁺-release channel in whole homogenates of left (LV) and right ventricles (RV) of fetal (22 days in gestation) and adult Sprague-Dawley rat hearts using [³H]ryanodine binding and ⁴⁵Ca²⁺ fluxes. Although many features of the Ca²⁺-release channels were similar in fetal and adult hearts, biochemical assays revealed quantitative differences. Similar properties include 1) Ca²⁺-sensitive cooperative ryanodine binding to Ca²⁺-release channel, measured as Ca²⁺ concentration for half-maximal activation (fetal LV: 0.13 ± 0.02 μM; adult LV: 0.15 ± 0.02 μM) and Hill coefficient (fetal LV: 2.5 ± 0.9; adult LV: 2.7 ± 0.5), and 2) caffeine-sensitive ryanodine binding, measured as the percent increase in ryanodine binding induced by caffeine (fetal LV: 148.8 ± 16.9% vs. adult LV: 171.4 ± 34.9%). The distinguishing property was the lower Ca²⁺-release channel density in the fetal heart (LV: 0.22 ± 0.03 pmol/mg protein) compared with adult heart (LV: 0.59 ± 0.04 pmol/mg protein; P < 0.05), as determined by [³H]ryanodine binding. The lower density of Ca²⁺-release channel is supported by the finding that there is very low ryanodine-sensitive oxalate-supported ⁴⁵Ca²⁺ uptake in the fetal heart. The tested characteristics of the Ca²⁺-release channel were similar between LV and RV in both fetal and adult rat hearts. Our results indicate that expression of Ca²⁺-release channels in sarcoplasmic reticulum increases during postnatal growth in the rat heart. This is consistent with previous physiological reports that Ca²⁺ available for excitation-contraction coupling in the fetal heart is derived mainly from transsarcolemmal Ca²⁺ influx.