Regulation kinetics of Na + ‐Ca 2+ exchange current in guinea‐pig ventricular myocytes

1 To investigate the regulation of native cardiac Na + ‐Ca 2+ exchange by cytoplasmic Na + (Na + i ) and Ca 2+ (Ca 2+ i ), we recorded the Na + ‐Ca 2+ exchange current ( I Na‐Ca ) from inside‐out ‘macro patches’ excised from intact guinea‐pig ventricular cells. 2 The half‐maximal concentration ( K h ) of Ca 2+ i required to induce an inward I Na‐Ca was 7 μM. The K h of Na + i required to induce an outward I Na‐Ca was 21 mM, and tended to decrease at the steady state of Na + ‐dependent inactivation. 3 The time constant (τ) of Na + ‐dependent inactivation was ≈1.5 s at 100 mM Na + i and 1 μM Ca 2+ i . The K h for Na + i was 14 mM. 4 Ca 2+ i augmented the peak outward I Na‐Ca ( K h = 0.2 μM) and attenuated Na + ‐dependent inactivation ( K h = 2.2 μM). The outward I Na‐Ca was activated by 5 μM Ca 2+ i with a half‐time to reach steady state ( t ½ ) of ≈0.4 s. This activation was composed of two exponential processes. Deactivation of the current upon Ca 2+ i removal also consisted of two exponential processes and had a t ½ of ≈0.5 s. 5 A Na + ‐Ca 2+ exchange model, consisting of one consecutive 4Na + :1Ca 2+ exchange cycle and two inactive states, well mimicked the experimental data with regard to ion dependencies and regulation kinetics. 6 These data provide detailed information on the kinetics of the Na + i ‐ and Ca 2+ i ‐dependent regulation of native Na + ‐Ca 2+ exchange. They also indicate that the regulation kinetics operate faster in macro patches than in the giant membrane patch from cardiac ‘blebs’, or in Xenopus oocytes expressing a cloned exchanger (NCX1.1).