Expression of renal Na+-Ca2+ exchange activity in Xenopus laevis oocytes
The expression of a renal Na(+)-Ca2+ exchanger by Xenopus oocytes has been investigated. Each oocyte was injected with 50 ng of poly(A)+ RNA from either rat or rabbit kidney or with an equivalent volume of water. Na(+)-Ca2+ exchange was determined 3 days after injection, by measuring Ca2+ uptake by oocytes in the presence or absence of an outwardly directed Na+ concentration gradient. To manipulate Na+ concentration gradients, oocytes were first loaded with Na+ in Ca(2+)-free medium containing 90 mM Na+ and nystatin. They were then exposed to medium containing 45Ca and either 90 mM or 0 Na+. Na(+)-free media contained (in mM) either 90 K+, 90 choline or 85 choline plus 5 K+. Oocytes injected with rat kidney poly(A)+ RNA showed a Na+ gradient-dependent Ca2+ uptake of 6.6 +/- 0.8 (SE, n = 5) pmol.oocyte-1.30 min-1. This is significantly higher than the value of 3.4 +/- 0.5 (SE, n = 5) pmol.oocyte-1.30 min-1 obtained in water-injected oocytes (P less than 0.001). Similar results were obtained using poly(A)+ RNA from rabbit kidney cortex. Neither 10 microM nifedipine nor 0.5 mM D 600 significantly affected this Ca2+ uptake. However, 90% of the Ca2+ uptake was inhibited in the presence of 0.1 mM La3+. The poly(A)+ RNA-induced Na(+)-Ca2+ exchange activity was stimulated by the presence of 5 mM K+ in the extracellular choline solution compared with choline alone. Fractionation experiments indicate that the rat kidney Na(+)-Ca2+ exchanger was encoded by poly(A)+ RNA of 3-4 kb.