Contribution of the ryanodine-sensitive fraction to capabilities of cardiac SR

The contribution of the ryanodine-sensitive fraction of canine cardiac sarcoplasmic reticulum to the total issue calcium uptake was estimated by the oxalate-supported calcium uptake rate in canine whole heart homogenates. Ryanodine stimulated this uptake rate nearly three-fold. Ryanodine stimulated this same activity in isolated SR vesicles only two-fold. An analysis of the yield of calcium uptake activity throughout the isolation procedure showed that the largest discrimination between ryanodine-sensitive and ryanodine-insensitive activity occurs at the first centrifugation step. In isolated vesicles, the initial rate of uptake in the absence of oxalate correlated well with the sustained oxalate-support calcium uptake rate, suggesting that oxalate-supported calcium uptake is a good indicator of in vivo SR function. The similarity between the effects of ryanodine on the rate and capacity of calcium uptake in the presence or absence of oxalate is consistent with earlier observations that ryanodine adds effective volume by closing a calcium release channel in a subpopulation of SR. The data also suggest that the ratio of calcium pumping activity to volume in these two populations is not greatly different.