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1. Calcium efflux from guinea-pig auricles followed saturation kinetics when [Ca]_o and [Na]_o were changed while the ratio [Ca]_o/[Na]_o² was kept constant. The Michaelis constant, K_m^Ca+Na = 40 mM, suggests that a hypothetical carrier system, responsible for sodium—calcium exchange, is far from saturation with the inside concentrations of these ions.

2. [Na]_i was altered in the auricles between 12·5 and 60 mM/kg fibre water while total cellular calcium concentration ([Ca]_t) at the beginning of the influx period was not significantly different in the various groups of preparations.

3. ⁴⁵Ca influx increased appreciably with increasing [Na]_i. ⁴⁵Ca influx from sodium-poor solution corresponded to an almost equal increase in [Ca]_t, while [Ca]_t did not change much in preparations loaded with ⁴⁵Ca in Tyrode solution. When the sodium-activated fraction of calcium influx was plotted against [Na]_i² the resulting curve indicated saturation with K_m^Na = 3500 (mM [Na]_i)² and maximal influx rate, J_{i, max}^Ca' = 1·35 mM/kg wet weight x 10 min.

4. When the preparations were re-equilibrated for various times in normal Tyrode solution after [Na]_i had been increased, both the sodium-activated component of calcium influx and [Na]_i² decreased with approximately the same rate constants.

5. Calcium efflux from auricles with high [Na]_i was increased when it was measured in Tyrode solution while the efflux in sodium-poor solution was inhibited.

6. Auricles with increased [Na]_i showed a positive inotropic contractile response.

7. The main conclusion reached by these experiments is that calcium influx is affected by [Na]_i in a way which is compatible with a carrier-mediated sodium—calcium exchange system.

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				R. Dipolo and L. Beauge Sodium/Calcium Exchanger: Influence of Metabolic Regulation on Ion Carrier Interactions Physiol Rev, January 1, 2006; 86(1): 155 - 203. [Abstract] [Full Text] [PDF]