Calcium regulation of cardiac myofibrillar activation: effects of MgATP.
In 2 mM MgATP, 0.08 ionic strength and 1 mM free Mg++ cardiac myofibrils bound 3.5 nmoles Ca/mg protein at maximal ATPase activation. Significant amounts of Ca were also bound to cardiac myosin with these same conditions. By subtraction of this myosin-bound Ca we obtained an estimate of 4 moles Ca bound per mole of myofibrillar troponin at maximal ATPase. We found, however, that Ca activation of myofibrillar ATPase could be estimated assuming that only two of troponin's Ca-binding sites are engaged in regulation of crossbridge activity. Increases in MgMTP from 0.3 to 5.0 mM raised the free Ca, giving half-maximal isomteric tension or ATPase. Although part of this shift is most probably due to changes in the number of rigor (nucleotide-free) actin-myosin linkages, the rightward shift of the free Ca++-activation relation with increase in MgATP from 2 to 5 mM appears to be due to effects of active (nucleotide-containing) actin-myosin linkages.
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D004285:Dogs; D008274:Magnesium; D009130:Muscle, Smooth / Q000378:metabolism*; D009200:Myocardial Contraction / Q000187:drug effects; D009210:Myofibrils / Q000201:enzymology / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism; D009994:Osmolar Concentration; D014336:Troponin / Q000378:metabolism
D014336:Troponin; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D009218:Myosins; D008274:Magnesium; D002118:Calcium