Frequency-dependent regulation of cardiac Na(+)/Ca(2+) exchanger

The activity of the cardiac Na(+)/Ca(2+) exchanger (NCX1.1) undergoes continuous modulation during the contraction-relaxation cycle because of the accompanying changes in the electrochemical gradients for Na(+) and Ca(2+). In addition, NCX1.1 activity is also modulated via secondary, ionic regulatory mechanisms mediated by Na(+) and Ca(2+). In an effort to evaluate how ionic regulation influences exchange activity under pulsatile conditions, we studied the behavior of the cloned NCX1.1 during frequency-controlled changes in intracellular Na(+) and Ca(+) (Na(i)(+) and Ca(i)(2+)). Na(+)/Ca(2+) exchange activity was measured by the giant excised patch-clamp technique with conditions chosen to maximize the extent of Na(+)- and Ca(2+)-dependent ionic regulation so that the effects of variables such as pulse frequency and duration could be optimally discerned. We demonstrate that increasing the frequency or duration of solution pulses leads to a progressive decline in pure outward, but not pure inward, Na(+)/Ca(2+) exchange current. However, when the exchanger is permitted to alternate between inward and outward transport modes, both current modes exhibit substantial levels of inactivation. Changes in regulatory Ca(2+), or exposure of patches to limited proteolysis by alpha-chymotrypsin, reveal that this "coupling" is due to Na(+)-dependent inactivation originating from the outward current mode. Under physiological ionic conditions, however, evidence for modulation of exchange currents by Na(i)(+)-dependent inactivation was not apparent. The current approach provides a novel means for assessment of Na(+)/Ca(2+) exchange ionic regulation that may ultimately prove useful in understanding its role under physiological and pathophysiological conditions.