Ca(2+) measurements in skinned cardiac fibers: effects of Mg(2+) on Ca(2+) activation of force and fiber ATPase.
In contrast to previous studies, a new fluorescent method was used to accurately determine the Ca(2+) concentration in test solutions used to activate skinned rat cardiac cells. This method used the calcium green-2 fluorescent indicator, which is shown to change its fluorescence over the Ca(2+) range responsible for Ca(2+) activation of force and ATPase. The dissociation constant (K(d)) of calcium green-2 for Ca(2+) was determined for three different Mg(2+) concentrations in solutions similar to those used in the experiment. Increasing Mg(2+) concentration from 1.0 to 8.0 mM had no significant effect on the Ca(2+) sensitivity of either force or actomyosin ATPase activity, in contrast to previous reported studies on force. The ATPase activity was activated at lower Ca(2+) concentration than the force. The ratio (ATPase/force) is proportional to the dissociation rate of force-generating myosin cross bridges and decreased during Ca(2+) activation. These findings are consistent with the hypothesis that cardiac muscle contraction is activated by a single Ca(2+)-specific binding site on troponin C.
Allen
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Department of Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida 33101, USA.
Xu
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Kerrick
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D000205:Actomyosin / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002413:Cations, Divalent / Q000378:metabolism / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation / Q000187:drug effects; D005456:Fluorescent Dyes / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D008954:Models, Biological; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism*; D009200:Myocardial Contraction; D009206:Myocardium / Q000166:cytology / Q000201:enzymology* / Q000378:metabolism; D009218:Myosins / Q000378:metabolism*; D009930:Organic Chemicals; D051381:Rats; D013816:Thermodynamics
D002413:Cations, Divalent; D005456:Fluorescent Dyes; D009930:Organic Chemicals; C071336:calcium green; D000255:Adenosine Triphosphate; D000205:Actomyosin; D009218:Myosins; D008274:Magnesium; D002118:Calcium