Journal of Applied Physiology
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J Appl Physiol 88: 180-185, 2000;
8750-7587/00 $5.00
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Vol. 88, Issue 1, 180-185, January 2000

Ca2+ measurements in skinned cardiac fibers: effects of Mg2+ on Ca2+ activation of force and fiber ATPase

Keri Allen, Yuan Yuan Xu, and W. Glenn L. Kerrick

Department of Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida 33101

In contrast to previous studies, a new fluorescent method was used to accurately determine the Ca2+ 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 Ca2+ range responsible for Ca2+ activation of force and ATPase. The dissociation constant (Kd) of calcium green-2 for Ca2+ was determined for three different Mg2+ concentrations in solutions similar to those used in the experiment. Increasing Mg2+ concentration from 1.0 to 8.0 mM had no significant effect on the Ca2+ sensitivity of either force or actomyosin ATPase activity, in contrast to previous reported studies on force. The ATPase activity was activated at lower Ca2+ concentration than the force. The ratio (ATPase/force) is proportional to the dissociation rate of force-generating myosin cross bridges and decreased during Ca2+ activation. These findings are consistent with the hypothesis that cardiac muscle contraction is activated by a single Ca2+-specific binding site on troponin C.

calcium; magnesium; binding constant; dissociation constant; actomyosin


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