Functional differences between cardiac and renal isoforms of the rat Na+-Ca2+ exchanger NCX1 expressed in Xenopus oocytes.

The transcript of the Na+-Ca2+ exchanger gene NCX1 undergoes alternative splicing to produce tissue-specific isoforms. The cloned NCX1 isoforms were expressed in Xenopus oocytes and studied using a two-electrode voltage clamp method to measure Na+-Ca2+ exchanger activity. The cardiac isoform (NCX1.1) expressed in oocytes was less sensitive to depolarizing voltages and to activation by [Ca2+]i than the renal isoform (NCX1.3). The cardiac isoform of NCX1 is more sensitive to activation by protein kinase A (PKA) than the renal isoform which may be explained by preferential phosphorylation. The cardiac isoform of NCX1 is phosphorylated to a greater extent than the renal isoform. The action of PKA phosphorylation which increases the activity of the cardiac isoform of the Na+-Ca2+ exchanger in oocytes was confirmed in adult rat ventricular cardiomyocytes by measuring Na+-dependent Ca2+ flux. We conclude that alternative splicing of NCX1 confers distinct functional characteristics to tissue-specific isoforms of the Na+-Ca2+ exchanger.