Expression of the Na-Ca exchanger in diverse tissues: a study using the cloned human cardiac Na-Ca exchanger.

In many cells including cardiac myocytes, cytoplasmic Ca is importantly controlled by the plasmalemmal Na-Ca exchanger (3, 8). The tissue diversity and differences in cellular environment raise the question whether the same exchanger is found in all tissues. Recent experiments using rod cells have demonstrated that at least two forms of Na-dependent Ca transport exist. We have examined this issue in various rat and human tissues using the cloned human cardiac Na-Ca exchanger cDNA. Northern blot analysis in these two species show that the major transcript of the Na-Ca exchanger is 7.2 kilobases in heart, brain, kidney, liver, pancreas, skeletal muscle, placenta, and lung. Furthermore, ribonuclease protection analysis in rats shows conservation of the 348-base pair segment tested in heart, brain, kidney, skeletal muscle, and liver. Additionally, Southern blot analysis suggests that a single gene encodes this Na-Ca exchanger. Finally, we show that the clone used to generate our probes encodes a completely functional Na-Ca exchanger. With the use of COS cells and 293 cells transfected with the cloned human cardiac Na-Ca exchanger, we tested the Ca transport properties of the Na-Ca exchanger, the voltage dependence of the Na-Ca exchanger, as well as the Na dependence of the transport function of the Na-Ca exchanger. We conclude that the cardiac form of the Na-Ca exchanger is completely functional when the cDNA is expressed in mammalian cell lines, and, furthermore, this "cardiac" form of the Na-Ca exchanger is naturally expressed in all human and rat tissues tested (but at varying levels).