Calcium oxalate and calcium phosphate capacities of cardiac sarcoplasmic reticulum.

Both oxalate-supported and phosphate-supported calcium uptake by canine cardiac sarcoplasmic reticulum initially increase linearly with time but fall to a steady-state level within 20 min. The departure from linearity could be due to a decrease in influx or to an increase in efflux of calcium. Because Ca2+-ATPase activity is linear, a decrease in the influx of calcium is an unlikely cause of the non-linear calcium uptake curves. A possible cause of an increase in calcium efflux is rupture of the vesicles. This hypothesis was tested by investigating the amount of calcium which could be released upon addition of 5 mM EGTA. The amount of rapidly releasable calcium was zero until a threshold calcium uptake of about 4-6 mumol calcium oxalate or calcium phosphate per mg was reached. After that point the rapidly releasable calcium continued to increase with calcium oxalate to reach more than 23 mumol/mg, but stayed constant at about 0.7 mumol/mg for calcium phosphate. The rapidly releasable calcium was attributed to calcium oxalate or calcium phosphate crystals externalized by vesicle rupture. The differences in the amounts of rapidly releasable calcium were attributed to different kinetics of calcium phosphate and calcium oxalate dissolution. Addition of ryanodine caused a marked increase in the threshold for rapidly releasable calcium oxalate. Transmission electron micrographs showed that vesicles can become filled with calcium oxalate crystals, but the vesicles were heterogeneous with respect to their size and their sensitivity to ryanodine. These observations support the hypothesis that calcium oxalate and calcium phosphate capacities are limited by vesicle rupture and that ryanodine increases the capacity by closing a calcium channel in a subpopulation of vesicles that otherwise would not accumulate calcium.