EXPRESSION, FUNCTIONAL-CHARACTERIZATION AND TISSUE DISTRIBUTION OF A NA+ H+ EXCHANGER CLONED FROM XENOPUS-LAEVIS OOCYTES [XL-NHE)/

We examined the functional properties of a Na+/H+ exchanger cloned fro m Xenopus laevis oocytes (XL-NHE) upon stable transfection into PS120 fibroblasts which lack endogenous Na+/H+ exchange. In contrast to untr ansfected cells, XL-NHE-transfected cells displayed Na+-dependent alka linization upon acidification with nigericin. XL-NHE activity was inhi bited by amiloride, ethylisopropylamiloride, HOE694 [(3-methylsulphony l-4-piperidinobenzoyl)-guanidine methanesulphonate] and HOE642 [4-isop ropyl-3-methylsulphonylbenzoyl)-guanidine methanesulphonate], K-i valu es being calculated at 5 mu mol/l, 25 nmol/l, 300 nmol/l and 180 nmol/ l, respectively. The Na+ dependence of pH(i) recovery was compatible w ith simple Michaelis-Menten kinetics, the K-m for Na+ being 22.0+/-3.2 mmol/l and the Hill coefficient for Na+ being approximately 1. XL-NHE was activated by phorbol ester, whereas forskolin exerted no effect, suggesting the involvement of phospholipase C/protein kinase C signall ing pathways rather than protein kinase A signalling pathways in XL-NH E stimulation, Using reverse transcription polymerase chain reaction, XL-NHE message could he detected in various Xenopus tissues including heart, brain, skeletal muscle, reticulocytes, A6-kidney cells and oocy tes.