We have modeled the time-course of Ca2+ binding to calmodulin, troponin, parvalbumin, and myosin in response to trains of transient increases in the free myoplasmic calcium ion concentration (pCa). A simple mathematical expression was used to describe each pCa transient, the shape and duration of which is qualitatively similar to those thought to occur in vivo. These calculations assumed that all the individual metal binding sites are noninteracting and that Ca2+ and Mg2+ bind competitively to the Ca2+ -Mg2+ sites of troponin, parvalbumin, and myosin. All the on-and-off rate constants for both Ca2+ and Mg2+ were obtained either from the literature or from our own research. The percent saturation of the Ca2+ -Mg2+ sites with Ca2+ was found to change very little in response to each pCa transient in the presence of 2.5 x 10:-3M Mg2+. Our analysis suggests that the Ca2+ content of these sites is a measure of the intensity and frequency of recent muscle activity because large changes in the Ca2+ occupancy of these sites can occur with repeated stimulation. In contrast, large rapid changes in the amount of Ca2+ bound to the Ca2+ -specific sites of troponin and calmodulin are induced by each pCa transient. Thus, only sites of the 'Ca2+ -specific' type can act as rapid Ca2+ -regulatory sites in muscle. Fluctuation in the total amount of Ca2+ bound to these sites in response to various types of pCa transients further suggests that in vivo only about one-half to one-third of the total steady-state myofibrillar Ca2+ -binding capacity exchanges Ca2+ during any single transient.
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