Does varying intracellular sodium concentration and NCX activity modulate the action potential duration in normal and failing canine ventricular myocytes?
NCX plays an important role in shaping the action potential of canine ventricular myocytes, helping repolarization at high intracellular sodium levels but contributing a potentially arrhythmogenic depolarizing influence at low levels.
Increased Na+-Ca2+ exchange (NCX) activity in heart failure and hypertrophy may compensate for depressed sarcoplasmic reticular Ca2+ uptake, provide inotropic support through reverse-mode Ca2+ entry, and/or deplete intracellular Ca2+ stores. NCX is electrogenic and depends on Na+ and Ca2+ transmembrane gradients, making it difficult to predict its effect on the action potential (AP). Here, we examine the effect of Na+i on the AP in myocytes from normal and pacing-induced failing canine hearts and estimate the direction of the NCX driving force using simultaneously recorded APs and Ca2+ transients. AP duration shortened with increasing Na+i and was correlated with a shift in the reversal point of the NCX driving force. At Na+i > or =10 mmol/L, outward NCX current during the plateau facilitated repolarization, whereas at 5 mmol/L Na+i, NCX had a depolarizing effect, confirmed by partially inhibiting NCX with exchange inhibitory peptide. Exchange inhibitory peptide shortened the AP duration at 5 mmol/L Na+i and prolonged it at Na+i > or =10 mmol/L. With K+ currents blocked, total membrane current was outward during the late plateau of an AP clamp at 10 mmol/L Na+i and became inward close to the predicted reversal point for the NCX driving force. The results were reproduced using a computer model. These results indicate that NCX plays an important role in shaping the AP of the canine myocyte, helping it to repolarize at high Na+i, especially in the failing heart, but contributing a depolarizing, potentially arrhythmogenic, influence at low Na+i.
Armoundas et al. (2003) studied this question.