Isolated Rat Hearts

The effect of perfusate [Ca2+] on the function of cardiac sarcoplasmic reticulum (CSR) was assessed by the oxalate-supported Ca2' uptake rate of ventricular homogenates of isolated rat hearts maintained in a modified Langendorif preparation. The total Ca2' pumping activity of the CSR was determined by using 20 FM ruthenium red or 625 j±M ryanodine to close the CSR Ca2+ release channel. The homogenate Ca2' uptake rate in the absence of ruthenium red or ryanodine decreased progressively with increasing perfusate [Ca211 (25.7±1.2, 21.4±1.5, 17.2±1.1, and 163±1.2 [mean+SEMI nmol Ca 2+ * min-' * mg-' for hearts perfused for 5 minutes with 0.2, 1.4, 2.8, and 5.6 mM Ca2+, respectively; p=0.0001; n=8). This depression was not observed when Ca2+ uptake was assayed in the presence of ryanodine or ruthenium red. Since the Ca2+ uptake in the presence of ryanodine or ruthenium red is determined by the Ca2+-ATPase, this result suggests that perfusion with varying [Ca2+] did not affect the Ca2+-ATPase. The observed decrease in Ca2+ uptake in the absence of ryanodine or ruthenium red is caused by an increased efflux through the ryanodine-sensitive Ca2+ release channel. When hearts perfused for 5 minutes with 0.2 or 5.6 mM Ca2+ were reperfused for 10 minutes with 1.4 mM Ca2+, homogenate Ca2+ uptake rates were restored to near control levels. These effects of perfusate Ca24 were not direct effects, because changes in the [Ca21] of the homogenization medium did not alter the homogenate Ca24 uptake activity in either the presence or absence ofryanodine. The homogenate Ca24 uptake rates were unaffected by prior active loading ofthe CSRwith Ca2+. These results suggest a regulatory role ofperfusate Ca2+ in increasing the open state of the ryanodine-sensitive Ca24 release channel that is distinct from the beat-to-beat regulation of Ca2+ release from the CSR by Ca2+ (Ca2+4-induced Ca24 release). (Circultion Research 1992;71:1049-1058)