Kinetics and osmoregulation of na+, cl--dependent betaine
transporter in rat renal medulla.
Moeckel, Gilbert W., Li-Wen Lai, Walter G. Guder, H. Moo Kwon, and
Yeong-Hau H. Lien.
Department of Medicine, Section of Renal Disease, the Department of
Pediatrics, Section of Medical and Molecular Genetics, University of
Arizona Health Sciences Center, Tucson, Arizona 85724, Institute of
Clinical Chemistry, Bogenhausen Hospital, Munich 81925, Germany, and
Division of Nephrology, Department of Medicine, The Johns Hopkins
University School of Medicine, Baltimore, Maryland 21205
APStracts 3:0202F, 1996.
Betaine is one of the major organic osmolytes which accumulate in the
renal medulla in response to high extracellular tonicity. Recent
studies in MDCK cells have shown that betaine is taken up by a
Na+/Cl--dependent transporter located on the basolateral membrane. We
demonstrate here the presence of Na+/Cl--dependent betaine
transporter(s) in tubule suspensions prepared from the rat outer and
inner medulla. The betaine transport activity was 2-3 times higher in
the inner medulla when 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 mOsm), and increased by 82% in hypertonic
buffer (545 mOsm), when compared with isotonic buffer (308 mOsm).
Kinetic studies of Na+-dependent betaine uptake in the outer
medullary tubules revealed a low and a high affinity component, with
Km1 of 8.6 mM, Vmax1 of 112 pmol/[mu]g protein/hour, Km2 of 0.141 mM
and Vmax2 of 10 pmol/[mu]g protein/hour, respectively. To investigate
whether the Na+/Cl--dependent betaine transporter is regulated by
tonicity in vivo, its mRNA was quantitated in rat renal cortex, 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 medulla tubules, and betaine transporter mRNA
abundance is regulated by the hydration state in vivo.
Received 7 February 1996; accepted in final form 24 October 1996.
APS Manuscript Number F43-6.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 13 November 1996