title: The cardiac relaxing system. Its nature, calcium ion capacity, and influence of hydrogen and magnesium ions on initial velocity of calcium binding.
abstract: The functional and structural properties of Ca2+ ATPases isolated from heart and skeletal muscles were compared. The pH and Ca2+ dependences of the activities as well as amino acid and phospholipid composition of the enzymes are similar. On the other hand, specific activities of Ca2+ ATPases and their abilities to pump calcium in the reconstituted proteoliposomes differ. The sarcoplasmic reticulum (SR) extracted completely from 1 g of pigeon and guinea pig hearts is able to bind up to 65 and 76 nmol Ca2+/sec, respectively, at 37 degrees (24 microM concentration of free Ca2+ ions). In the absence of oxalate, the process of calcium binding is nonlinear in a time interval of 100 msec to 5 min. One-third and half of the quantity of calcium consumed at 1 sec is bound at 100 and 200 msec, respectively. Judging by steady-state levels of the phosphorylated intermediate of Ca2+ ATPase in highly purified and completely extracted preparations of SR, an estimate was made that 1 g of heart contains from 2 to 3 mg of SR protein. The rate of energy-dependent calcium binding by isolated cardiac SR depends on pH and the concentrations of free calcium and magnesium. At calcium concentrations above 5 microM, the rate of calcium accumulation is higher at pH 6.2 than at pH 7.2. Increase in magnesium ion concentration from 0.5 to 6.0 mM leads to a significant inhibition of calcium binding at calcium concentrations 0.1 to 10 microM. The data obtained show that at physiological concentrations of calcium, the ability of SR to accumulate calcium is close to that postulated for a system of calcium transport providing relaxation for heart muscle.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D010856:Columbidae; D006168:Guinea Pigs; D006863:Hydrogen-Ion Concentration; D007473:Ion Channels / Q000201:enzymology*; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism*; D008861:Microsomes / Q000201:enzymology; D009119:Muscle Contraction; D009126:Muscle Relaxation; D009200:Myocardial Contraction; D009206:Myocardium / Q000201:enzymology; D010743:Phospholipids / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000201:enzymology
chemical_list: D002135:Calcium-Binding Proteins; D007473:Ion Channels; D010743:Phospholipids; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium