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 Kwere monitored with confocal laser scanned microscopy in intact, guine a pig cardiac myocytes loaded with the calcium indicator fluo-3. Singl e wavelength ratioing of fluorescence images obtained after prolonged integration times revealed nonuniformities of intracellular Ca2+ chang es across the cell, suggesting the presence of significant spatial Ca2 + gradients. Treatment with 20 mu M ryanodine, an inhibitor of Ca2+ re lease from the SR, and 10 mu M verapamil, a calcium channel blocker, r educed by 42% and 76% respectively the changes in [Ca2+](i) elicited b y membrane depolarization. The overall spatial distribution of [Ca2+]( i) changes appeared unchanged. Ca2+ transients recorded in the presenc e of verapamil and ryanodine (about 20% of the size of control respons es); diminished in the presence of 50 mu M 2-4 Dichlorbenzamil (DCB) o r 5 mM nickel, two relatively specific inhibitors of the Na+/Ca2+ exch ange 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 mu M monensin, the ampl itude of these Ca2+ transients increased. Ca2+ transients elicited by membrane depolarization and largely mediated by reverse operation of N a+-Ca2+ exchange could be recorded in the presence of ryanodine, verap amil and monensin. These findings suggest that in intact guinea pig ca rdiac cells, Ca2+ influx through the Na+/Ca2+ exchange mechanism activ ated by a membrane depolarization in the physiological range can be su fficient to play a significant role in excitation-contraction coupling .