Kinetics and osmoregulation of Na+- and Cl--dependent betaine transporter in rat renal medulla

Betaine is one of the major organic osmolytes that accumulate in the renal medulla in response to high extracellular tonicity. Recent studies in MDCK cells have shown that betaine is taken up by an Na+- and C1(-)-dependent transporter located on the basolateral membrane. We demonstrate here the presence of Na+-C1(-)-dependent betaine transporter(s) in tubule suspensions prepared from the rat outer and inner medulla. The betaine transport activity was two to three times higher in the inner medulla compared with the outer medulla. The removal of Na+ and Cl- reduced betaine uptake in the outer medullary tubules by 86% and 82%, respectively. The betaine uptake was decreased by 39% in hypotonic buffer (189 mosmol/kgH(2)O) and increased by 82% in hypertonic buffer (545 mosmol/kgH(2)O), compared with isotonic buffer (308 mosmol/kgH(2)O). Kinetic studies of Na+-dependent betaine uptake in the outer medullary tubules revealed both a low- and a high-affinity component as follows: low-affinity and high-volume component with Michaelis constant (K-m1) of 8.6 mM and maximal uptake rate (V-max1) of 112 pmol .mu g protein(-1). h(-1); and a low-volume and high-affinity component with K-m2 of 0.141 mM and V-max2 of 10 pmol .mu g protein(-1). h(-1). To investigate whether the Na+-Cl--dependent betaine transporter is regulated by tonicity in vivo, we quantitated its mRNA in rat renal cortex and outer and inner medulla using both canine and rat Na+-Cl--dependent betaine transporter cDNA probes. A single band of 3.0 kb was seen in the Northern blots prepared from both outer and inner medulla, but not in the cortex. Water deprivation for 3 days increased the abundance of this mRNA in the outer and inner medulla by 140% and 170%, respectively, but did not affect its expression in the cortex. In conclusion, Na+-Cl--dependent betaine transporter(s) is present in rat outer and inner medullary tubules, and betaine transporter mRNA abundance is regulated by the hydration state in vivo.