Sodium Current-Induced Release of Calcium from the Sarcoplasmic Reticulum in Rabbit Ventricular Myocytes

Abstract

The hypothesis that Na current (INa) can induce release of Ca from the sarcoplasmic reticulum (SR) by activating reverse Na-Ca exchange (NCX) has been debated since 1990. We tested this hypothesis with epi-fluorescence imaging of adult rabbit ventricular myocytes loaded with the Ca indicator fluo-4. Ca release was triggered with an action potential clamp with and without an initial voltage ramp from −80 to −40 mV, for a duration of 1.5s. We confirmed that this protocol selectively blocked INa without altering Ca influx through L-type Ca channels (LCCs) and SR Ca load. With 0 mM Na in the pipette (to reduce intracellular Na), inactivating INa reduced SR Ca release flux by 27%±4% (n=9). With 5 mM Na in the pipette, the Ca release upon inactivation of INa was reduced by 33%±5% (n=4). We suggest that increased activation of reverse NCX by increased intracellular Na concentration mainly produced by INa explains these findings. These conclusions are in agreement with studies on normal and NCX knockout mice, which show that INa affects SR Ca release only in normal, but not in NCX knockout mice. In similar experiments, we applied 100 nM TTX to selectively block brain isoforms of Na channels. In the presence of TTX, the SR Ca release flux was reduced by 35%±3% (n=6). This effect of INa on Ca release can be explained by early reverse NCX, activated by TTX sensitive INa, which could prime the dyadic cleft with Ca. Furthermore, the results can be explained if INa activation of NCX, and subsequent priming of the dyadic cleft with Ca, increases the coupling fidelity between LCCs and ryanodine receptors within a couplon. Thus the presence of INa increases the likelihood that couplons are activated.