Allosteric Activation of Na+-Ca2+ Exchange by L-Type Ca2+ Current Augments the Trigger Flux for SR Ca2+ Release in Ventricular Myocytes

Abstract

The possible contribution of Na+-Ca2+ exchange to the triggering of Ca2+ release from the sarcoplasmic reticulum in ventricular cells remains unresolved. To gain insight into this issue, we measured the “trigger flux” of Ca2+ crossing the cell membrane in rabbit ventricular myocytes with Ca2+ release disabled pharmacologically. Under conditions that promote Ca2+ entry via Na+-Ca2+ exchange, internal [Na+] (10mM), and positive membrane potential, the Ca2+ trigger flux (measured using a fluorescent Ca2+ indicator) was much greater than the Ca2+ flux through the L-type Ca2+ channel, indicating a significant contribution from Na+-Ca2+ exchange to the trigger flux. The difference between total trigger flux and flux through L-type Ca2+ channels was assessed by whole-cell patch-clamp recordings of Ca2+ current and complementary experiments in which internal [Na+] was reduced. However, Ca2+ entry via Na+-Ca2+ exchange measured in the absence of L-type Ca2+ current was considerably smaller than the amount inferred from the trigger flux measurements. From these results, we surmise that openings of L-type Ca2+ channels increase [Ca2+] near Na+-Ca2+ exchanger molecules and activate this protein. These results help to resolve seemingly contradictory results obtained previously and have implications for our understanding of the triggering of Ca2+ release in heart cells under various conditions.