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Regulation kinetics of Na⁺-Ca²⁺ exchange current in guinea-pig ventricular myocytes

Yasutada Fujioka*, Koh Hiroe and Satoshi Matsuoka

1. To investigate the regulation of native cardiac Na⁺-Ca²⁺ exchange by cytoplasmic Na⁺ (Na⁺_i) and Ca²⁺ (Ca²⁺_i), we recorded the Na⁺-Ca²⁺ 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 Ca²⁺_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 () of Na⁺-dependent inactivation was ~1·5 s at 100 mM Na⁺_i and 1 µM Ca²⁺_i. The K_h for Na⁺_i was 14 mM.

4. Ca²⁺_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 Ca²⁺_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 Ca²⁺_i removal also consisted of two exponential processes and had a t_½ of ~0·5 s.

5. A Na⁺-Ca²⁺ exchange model, consisting of one consecutive 4Na⁺:1Ca²⁺ 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 Ca²⁺_i-dependent regulation of native Na⁺-Ca²⁺ 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).

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