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.