Abstract
Three distinct mammalian Na$^+$/Ca$^2+$ exchangers have been
cloned: NCX1, NCX2, and NCX3. We have undertaken a detailed functional
comparison of these three exchangers. Each exchanger was stably expressed
at high levels in the plasma membranes of BHK cells. Na$^+$/Ca$^2+$
exchange activity was assessed using three different complementary
techniques: Na$^+$ gradient-dependent 45Ca$^2+$ uptake into
intact cells, Na$^+$ gradient-dependent 45Ca$^2+$ uptake
into membrane vesicles isolated from the transfected cells, and exchange
currents measured using giant patches of excised cell membrane. Apparent
affinities for the transported ions Na$^+$ and Ca$^2+$ were
markedly similar for the three exchangers at both membrane surfaces.
Likewise, generally similar responses to changes in pH, chymotrypsin
treatment, and application of various inhibitors were obtained. Depletion
of cellular ATP inhibited NCX1 and NCX2 but did not affect the activity
of NCX3. Exchange activities of NCX1 and NCX3 were modestly increased
by agents that activate protein kinases A and C. All exchangers were
regulated by intracellular Ca$^2+$. NCX1-induced exchange currents
were especially large in excised patches and, like the native myocardial
exchanger, were stimulated by ATP. Results may be influenced by our
choice of expression system and specific splice variants, but, overall,
the three exchangers appear to have very similar properties.
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