Binding of calcium to myoplasmic buffers contributes to the frequency-dependent inotropy in heart ventricular cells

Summary

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, fron 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.