The Functional Analysis of Sarcoplasmic Reticulum in Murine Whole Heart During Developmental Stages

Abstract

Sarcolemmal Ca²⁺ entry has been shown to be the predominant mechanism responsible for Ca²⁺ transient in fetal immature cardiomyocytes, while sarcoplasmic reticulum (SR) Ca²⁺ release is responsible for 90% of Ca²⁺ transient in adult mouse cardiomyocytes. However, most of the studies have been conducted either on primary cultures or on isolated myocytes, in which the SR function has been possibly underestimated. To investigate developmental changes of SR function in immature hearts under physiological condition, we analyzed Ca²⁺ transients in beating whole hearts, stimulated electrically at 0.5 Hz after loading with Fluo-3, in fetal and neonatal mice. Thapsigargin (SR Ca²⁺ ATPase inhibitor) prolonged the time to 50% relaxation of Ca²⁺ transients significantly even at early embryonic stages. After ryanodine (RyR inhibitor) application, the amplitude of the Ca²⁺ transients was reduced by 72.8 ± 4.2%, while after nifedipine (L-type Ca²⁺ channel inhibitor), nickel (T-type Ca²⁺ channel inhibitor), and SEA0400 (reverse mode Na⁺-Ca²⁺ exchanger inhibitor) application, the amplitude was reduced by only 34.4 ± 4.2%, 26.5 ± 4.2%, and 16.7 ± 6.6%, respectively. In addition, the amplitude of caffeine induced Ca²⁺ transient developmentally increased from embryonic to neonatal stages. These results suggest that SR becomes functional in Ca²⁺ homeostasis even on early embryonic hearts.