Properties of Ca²⁺ handling in sarcoplasmic reticulum of saponin-treated sarcolipin transgenic mouse myocardium

Abstract

We investigated whether the alteration of Ca²⁺ uptake rate in sarcoplasmic reticulum (SR) would influence the Ca²⁺ content and the leakage of Ca²⁺ in SR. In this study, we used saponin-treated thin trabeculae obtained from the hearts which overexpresses sarcolipin (SLN). We reported that SLN reduced the activity of SR Ca²⁺-ATPase. The advantage of saponin-treated preparation is that integrated structure around SR and Ca²⁺ handling functions are well preserved. Thus, the functions of SR to release and uptake Ca²⁺ can be measured under more physiological conditions compared to isolated vesicular SR. Ca²⁺ content was measured by releasing all Ca²⁺ from SR by caffeine (50 mM) after loading Ca²⁺ (pCa 6.2) into SR in the presence of ATP (4 mM) for different periods. Leaked Ca²⁺ was estimated by measuring the remaining Ca²⁺ in SR after washing for various durations, following Ca²⁺ loading. Ca²⁺ uptake rate was slower in SLN transgenic (SLN-TG) than that in non-transgenic (SLN-NTG) myocardium. The maximal Ca²⁺ content and leakage of Ca²⁺ in SLN-TG and SLN-NTG were almost identical. The results suggest that the modulation of Ca²⁺ uptake rate could not affect maximal Ca²⁺ content and Ca²⁺ leakage measured at steady state. The modulation of Ca²⁺ uptake rate influences Ca²⁺ handling in each heartbeat but does not alter the maximal Ca²⁺ content and leakage at steady state. [J Physiol Sci. 2006;56 Suppl:S125]