A significant fraction of calcium transients in intact guinea pig ventricular myocytes is mediated by Na⁺Ca²⁺ exchange

Ca²⁺ mobilization elicited by simulation with brief pulses of high K + were monitored with confocal laser scanned microscopy in intact, guinea pig cardiac myocytes loaded with the calcium indicator fluo-3. Single wavelength ratioing of fluorescence images obtained after prolonged integration times revealed non-uniformities of intracellular Ca²⁺ changes across the cell, suggesting the presence of significant spatial Ca²⁺ gradients. Treatment with 20 μM ryanodine, an inhibitor of Ca²⁺ release from the SR, and 10 μM verapamil, a calcium channel blocker, reduced by 42% and 76% respectively the changes in [Ca²⁺]_i elicited by membrane depolarization. The overall spatial distribution of [Ca²⁺]_i changes appeared unchanged. Ca²⁺ transients recorded in the presence of verapamil and ryanodine (about 20% of the size of control responses); diminished in the presence of 50 μM 2-4 Dichlorbenzamil (DCB) or 5 mM nickel, two relatively specific inhibitors of the Na⁺/Ca²⁺ exchange mechanism. Conversely, when the reversal potential of the Na⁺/Ca²⁺ exchange was shifted to negative potentials by lowering [NA⁺]₀ or by increasing [Na⁺]_i by treatment with 20 μM monensin, the amplitude of these Ca²⁺ transients increased. Ca²⁺ transients elicited by membrane depolarization and largely mediated by reverse operation of Na⁺Ca²⁺ exchange could be recorded in the presence of ryanodine, verapamil and monensin. These findings suggest that in intact guinea pig cardiac cells, Ca²⁺ influx through the Na⁺/Ca²⁺ exchange mechanism activated by a membrane depolarization in the physiological range can be sufficient to play a significant role in excitation-contraction coupling.