Sodium/calcium exchange and calcium buffering in mammalian ventricular muscle

The Na/Ca exchange system was investigated in rapidly perfused small trabeculae (diameter around 250 micron) from ferret ventricle. In these preparations tension and Em were recorded, aiNa, aiK and aiMg were measured using ion-selective microelectrodes. The mean aiNa at room temperature (22-24 degrees C) was 10.9 mmol/l +/- S.D. 4.2 mmol/l. When [Na]o was reduced from 155 mmol/l to 1.5 mmol/l there was a rapid decrease in aiNa (half time 30 s), which was associated with a small increase in resting tension. The decrease in aiNa was not influenced by altering pHo, blocking the Na/K pump and was not passive, for during the decrease the driving force for Na ions was inward. The aiNa decreased if [Ca]o was increased or [Na]o was decreased or vice versa. On the basis of these findings it was concluded that the decrease was due to Na/Ca exchange. Since Ca uptake does occur under Na free perfusion and since [Ca]i hardly changes it was proposed that the mitochondria take up Ca and so prevent an excessive rise in cytoplasmic calcium. The Ca uptake mechanism of the mitochondria is inhibited by [Mg]i, so [Mg]i was measured using ion selective microelectrodes. An essential preliminary step was the determination of aiK since the Mg microelectrodes react not only to Mg but also to Na and K. The mean aiK was 104.4 mmol/l +/- S.D. 9.0 mmol/l. Based on the measured aiNa and aiK, the [Mg]i was 0.4 mmol/l, a level not high enough to markedly inhibit the Ca uptake mechanism of the mitochondria. Criteria for judging the impalement quality when using ion selective microelectrodes are discussed.