Allosteric activation of Na⁺-Ca²⁺ exchange by L-type Ca²⁺ current augments the trigger flux for SR Ca²⁺ release in ventricular myocytes

¹ Mount Sinai School of Medicine
² University of Auckland
³ University of Utah

^* To whom correspondence should be addressed. E-mail: bridge{at}cvrti.utah.edu.

Submitted on December 17, 2007
Revised on December 30, 2007
Accepted on 2 January 2008

	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⁺] (10 mM) 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.

Key Words: L-type calcium channel, calcium signaling, excitation-contraction coupling, heart, sarcoplasmic reticulum, voltage clamp