Binding of calcium to myoplasmic buffers contributes to the frequency-dependent inotropy in heart ventricular cells.
In guinea-pig ventricular cells, the Ca2+ buffer capacity of the myoplasm was estimated from the ratio of ionized calcium (from Indo-1 fluorescence) through total calcium (ionized plus bound calcium, from x-ray microprobe analysis). During post-rest potentiation (1 Hz paired-pulses in voltage-clamp), where diastolic sarcomere length remained nearly constant, Ca2+ buffer capacity slowly fell from 5500:1 to 700:1 suggesting that slow Ca2+ binding sites became saturated. We discuss that frequency-inotropy depends not only on the replenishment of intracellular stores with Ca2+, but also on binding of Ca2+ to these slow sites; the slow Ca2+ sites could complete with the fast activator sites on troponin C for systolic Ca2+, or they could enhance the Ca2+ affinity of the fast Ca2+ sites on troponin C by cooperative interaction.
Isenberg
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Wendt-Gallitelli
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D000200:Action Potentials; D000818:Animals; D001665:Binding Sites; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D006168:Guinea Pigs; D006352:Heart Ventricles / Q000166:cytology; D009200:Myocardial Contraction; D014336:Troponin / Q000378:metabolism*; D019209:Troponin C
D002021:Buffers; D014336:Troponin; D019209:Troponin C; D002118:Calcium