Ryanodine Perfusion Decreases Cardiac Mechanical Function Without Affecting Homogenate Sarcoplasmic Reticulum Ca 2+Uptake: Comparison with the Stunned Heart

Abstract

This study tested the hypothesis that perfusion with low concentrations of ryanodine, which opens the sarcoplasmic reticulum (SR) Ca 2+channel in a sub-conducting state, could mimic the effects of stunning on both mechanical and SR activity. Perfusion of isolated rat hearts with 10–160 n mryanodine progressively decreased left ventricular developed pressure (LVDP) and increased end-diastolic pressure (EDP), but LVDP decreased more and EDP increased less than in the stunned heart. The effect of ryanodine perfusion on LVDP and EDP is consistent with the opening of the SR Ca 2+channel by high-affinity ryanodine binding, reducing SR Ca 2+content and interfering with mechanical function. In contrast to stunning, ryanodine perfusion did not affect the homogenate Ca 2+uptake rates measured in the presence or absence of high [ryanodine]. Perfusion with 80 n m 3H-ryanodine resulted in a large decline in LVDP, but only a small degree of ryanodine binding. Thus, prolonged opening of only a few channels affects the SR in situ, whereas this is undetectable in the homogenate. Higher levels of ryanodine binding (0.3 pmol/mg) to the in vitrohomogenate also did not affect the homogenate Ca 2+-uptake rate in the presence or absence of high [ryanodine], whereas it reduced the stimulation of Ca 2+uptake by ruthenium red. High-affinity ryanodine binding to the SR Ca 2+channel, either by perfusion or by binding after homogenisation, did not duplicate the increased Ca 2+efflux observed in the stunned heart, suggesting that the SR defect in the stunned heart is not a prolonged opening of a sub-conducting state of the SR Ca 2+channel.