Phasic components of calcium binding and release by canine cardiac relaxing system (sarcoplasmic reticulum fragments)

Abstract

Cardiac relaxing system (sarcoplasmic reticulum fragments) isolated from canine myocardium exhibit cyclic changes initiated by ATP. The addition of ATP results in rapid calcium binding followed by a slower spontaneous release. After release has occurred, restimulation of calcium binding can be effected by additional ATP provided the ATP which initiated the first binding is hydrolyzed to a low level. The evidence suggests that calcium binding and calcium release represent two independent phasic changes in a calcium binding site with different responses to temperature ( Q 10 1.5 and 2.6, respectively), pH, Mg 2+ and ATP. The ability of ATP to restimulate binding may represent still a third phasic change. Evidence is also presented that each site must go through each phase independently in order to proceed to the next. The first phase is initiated by ATP and results in calcium binding; the second phase requires calcium binding and results in calcium release; the third phase requires calcium release. The implications of these changes and the source of energy transduction are discussed.