Dynamics of the late Na(+) current during cardiac action potential and its contribution to afterdepolarizations

The objective of this work is to examine the contribution of late Na(+) current (I(Na,L)) to the cardiac action potential (AP) and arrhythmogenic activities. In spite of the rapidly growing interest toward this current, there is no publication available on experimental recording of the dynamic I(Na,L) current as it flows during AP with Ca(2+) cycling. Also unknown is how the current profile changes when the Ca(2+)-calmodulin dependent protein kinase II (CaMKII) signaling is altered, and how the current contributes to the development of arrhythmias. In this study we use an innovative AP-clamp Sequential Dissection technique to directly record the I(Na,L) current during the AP with Ca(2+) cycling in the guinea pig ventricular myocytes. First, we found that the magnitude of I(Na,L) measured under AP-clamp is substantially larger than earlier studies indicated. CaMKII inhibition using KN-93 significantly reduced the current. Second, we recorded I(Na,L) together with I(Ks), I(Kr), and I(K1) in the same cell to understand how these currents counterbalance to shape the AP morphology. We found that the amplitude and the total charge carried by I(Na,L) exceed that of I(Ks). Third, facilitation of I(Na,L) by Anemone toxin II prolonged APD and induced Ca(2+) oscillations that led to early and delayed afterdepolarizations and triggered APs; these arrhythmogenic activities were eliminated by buffering Ca(2+) with BAPTA. In conclusion, I(Na,L) contributes a significantly large inward current that prolongs APD and unbalances the Ca(2+) homeostasis to cause arrhythmogenic APs