Characterization of Ca(2+)-release channels in fetal and adult rat hearts

The goal of this study was to characterize the Ca(2+)-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 Ca(2+)-release channels were similar in fetal and adult hearts, biochemical assays revealed quantitative differences. Similar properties include 1) Ca(2+)-sensitive cooperative ryanodine binding to Ca(2+)-release channel, measured as Ca2+ concentration for half-maximal activation (fetal LV: 0.13 +/- 0.02 microM; adults LV: 0.15 +/- 0.02 microM) 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 Ca(2+)-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 Ca(2+)-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 Ca(2+)-release channel were similar between LV and RV in both fetal and adult rat hearts. Ou 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.