Stoichiometry of NA⁺−CA²⁺ exchange is 3:1 in guinea‐pig ventricular myocytes

Abstract

In single guinea‐pig ventricular myocytes, we examined the stoichiometry of Na+‐Ca2+ exchange (NCX) by measuring the reversal potential (ENCX) of NCX current (INCX) and intracellular Ca2+ concentration ([Ca2+]i) with the whole‐cell voltage‐clamp technique and confocal microscopy, respectively. With given ionic concentrations in the external and pipette solutions, the predicted ENCX were −73 and −11 mV at 3:1 and 4:1 stoichiometries, respectively. ENCX measured were −69 ± 2 mV (n= 11), −47 ± 1 mV (n= 14) and −15 ± 1 mV (n= 15) at holding potentials (HP) of −73, −42 and −11 mV, respectively. Thus, ENCX almost coincided with HP, indicating that [Ca2+]i and/or [Na+]i changed due to INCX flow. Shifts of ENCX (ΔENCX) were measured by changing [Ca2+]o or [Na+]o. The measured values of ΔENCX were almost always smaller than those expected theoretically at a stoichiometry of either 3:1 or 4:1. Using indo‐1 fluorescence, [Ca2+]i measured under the whole‐cell voltage‐clamp supported a 3:1 but not 4:1 stoichiometry. To prevent Ca2+ accumulation, we inhibited INCX with Ni2+ and re‐examined ENCX during washing out Ni2+. With HP at predicted ENCX at a 3:1 stoichiometry, ENCX developed was close to predicted ENCX and did not change with time. However, with HP at predicted ENCX for a 4:1 stoichiometry, ENCX developed initially near a predicted ENCX for a 3:1 stoichiometry and shifted toward ENCX for a 4:1 stoichiometry with time. We conclude that the stoichiometry of cardiac NCX is 3:1.

Mechanism of Na+/Ca2+ Exchanger Activation by Hydrogen Peroxide in Guinea-Pig Ventricular Myocytes

“…They found that eliminating external Na + to inhibit NHE induced a larger change in the Ca 2+ current due to a more profound change in pH. We also experienced that intracellular Na + concentration cannot be controlled when NCX is active (7). Therefore, although we did not demonstrate it experimentally in the present paper, it can be expected that under the present ionic conditions, activation of NHE could change intracellular pH and intracellular Na + concentration.…”

Comprehensive Analyses of Arrhythmogenic Substrates and Vulnerability to Ventricular Tachycardia in Left Ventricular Hypertrophy in Salt-Sensitive Hypertensive Rats

“…16 We generated ramp pulses to induce INa and ICa using a function generator (NF, Tokyo) as described previously. 17 AP was recorded at 10-s intervals using a K + -pipette solution and Tyrode external solution as described previously. 16 Ito was elicited by depolarizing square pulses of 200-ms duration to the voltage range between -40 mV and +60 mV with 10 mV steps from the holding potential of -40 mV.…”

Alteration in NCX-3 immunoreactivity within the gerbil hippocampus following spontaneous seizures

“…This exchanger transports three Na + ions for one Ca 2+ ion, and is driven by the Ca 2+ and Na + gradients over the plasma membrane (1,2). In fact, Ca 2+ -dependent neurotransmitter release is affected by local Ca 2+ homeostasis according to NCX activity (3,4), thus NCX plays an important role in the intracellular Ca 2+ concentration, which represent a major event in many pathophysiologic processes.…”