Potentiation of fractional sarcoplasmic reticulum calcium release by total and free intra-sarcoplasmic reticulum calcium concentration

Contractile reserve and intracellular calcium regulation in mouse myocytes from normal and hypertrophied failing hearts

Mouse myocyte contractility and the changes induced by pressure overload are not fully understood. We studied contractile reserve in isolated left ventricular myocytes from mice with ascending aortic stenosis (AS) during compensatory hypertrophy (4-week AS) and the later stage of early failure (7-week AS) and from control mice. Myocyte contraction and [Ca(2+)](i) transients with fluo-3 were measured simultaneously. At baseline (0.5 Hz, 1.5 mmol/L [Ca(2+)](o), 25 degrees C), the amplitude of myocyte shortening and peak-systolic [Ca(2+)](i) in 7-week AS were not different from those of controls, whereas contraction, relaxation, and the decline of [Ca(2+)](i) transients were slower. In response to the challenge of high [Ca(2+)](o), fractional cell shortening was severely depressed with reduced peak-systolic [Ca(2+)](i) in 7-week AS compared with controls. In response to rapid pacing stimulation, cell shortening and peak-systolic [Ca(2+)](i) increased in controls, but this response was depressed in 7-week AS. In contrast, the responses to both challenge with high [Ca(2+)](o) and rapid pacing in 4-week AS were similar to those of controls. Although protein levels of Na(+)-Ca(2+) exchanger were increased in both 4-week and 7-week AS, the ratio of SR Ca(2+)-ATPase to phospholamban protein levels was depressed in 7-week AS compared with controls but not in 4-week AS. This was associated with an impaired capacity to increase sarcoplasmic reticulum Ca(2+) load during high work states in 7-week AS myocytes. In hypertrophied failing mouse myocytes, depressed contractile reserve is related to an impaired augmentation of systolic [Ca(2+)](i) and SR Ca(2+) load and simulates findings in human failing myocytes.

Evidence for Ca(2+) activation and inactivation sites on the luminal side of the cardiac ryanodine receptor complex

We have used tryptic digestion to determine whether Ca(2+) can regulate cardiac ryanodine receptor (RyR) channel gating from within the lumen of the sarcoplasmic reticulum (SR) or whether Ca(2+) must first flow through the channel and act via cytosolically located binding sites. Cardiac RyRs were incorporated into bilayers, and trypsin was applied to the luminal side of the bilayer. We found that before exposure to luminal trypsin, the open probability of RyR was increased by raising the luminal [Ca(2+)] from 10 micromol/L to 1 mmol/L, whereas after luminal trypsin exposure, increasing the luminal [Ca(2+)] reduced the open probability. The modification in the response of RyRs to luminal Ca(2+) was not observed with heat-inactivated trypsin, indicating that digestion of luminal sites on the RyR channel complex was responsible. Our results provide strong evidence for the presence of luminally located Ca(2+) activation and inhibition sites and indicate that trypsin digestion leads to selective damage to luminal Ca(2+) activation sites without affecting luminal Ca(2+) inactivation sites. We suggest that changes in luminal [Ca(2+)] will be able to regulate RyR channel gating from within the SR lumen, therefore providing a second Ca(2+)-regulatory effect on RyR channel gating in addition to that of cytosolic Ca(2+). This luminal Ca(2+)-regulatory mechanism is likely to be an important contributing factor in the potentiation of SR Ca(2+) release that is observed in cardiac cells in response to increases in intra-SR [Ca(2+)].

Reverse mode of the sarcoplasmic reticulum calcium pump and load-dependent cytosolic calcium decline in voltage-clamped cardiac ventricular myocytes

We have characterized [Ca](i) decline in voltage-clamped rabbit ventricular myocytes with progressive increases in sarcoplasmic reticulum (SR) calcium load. "Backflux" through the SR calcium pump is a critical feature which allows realistically small values for SR calcium leak fluxes to be used. Total cytosolic calcium was calculated from the latter part of [Ca](i) decline using rate constants for cellular calcium buffers. Intra-SR calcium buffering characteristics were also deduced. We found that the net SR calcium pump flux and rate of [Ca](i) decline decreased as the SR free [Ca] rose, with pump parameters held constant. We have therefore characterized for the first time in intact myocytes both forward and reverse SR calcium pump kinetics as well as intra-SR calcium buffering and SR calcium leak. We conclude that the reverse flux through the SR calcium pump is an important factor in comprehensive understanding of dynamic SR calcium fluxes.