CHOLINE TRANSPORT AND ITS OSMOTIC REGULATION IN RENAL-CELLS DERIVED FROM THE RABBIT OUTER MEDULLARY THICK ASCENDING LIMB OF HENLE

Organic osmolytes such as betaine and glycerophosphorylcholine (GPC) a re of major importance concerning volume regulation of inner and outer medulary epithelial cells. Recently we demonstrated that the intracel lular betaine content in rabbit kidney cells derived from the outer me dullary thick ascending limb of Henle's loop (TALH) is osmotically reg ulated by betaine synthesis. In this context it was our purpose to cha racterize the uptake of choline, a precursor of betaine and GPC. We fo und TALH cells to possess a specific choline transport system with a m aximum velocity (V-max) of 71 +/- 12 pmol . mu l(-1) cell water . min( -1) and an apparent affinity (K-m) of 155 +/- 19 mu mol . l(-1). The u ptake of choline was sodium independent and not electrogenic, but it w as significantly reduced by the removement of chloride from the incuba tion medium. After long-term adaptation of TALH cells to a hyperosmoti c medium (600 mos-mol . l(-1), osmolarity adjusted with NaCl or urea) a significant higher choline uptake rate was observed (V-max: 166 +/- 9 (NaCl), 96 +/- 12 (urea) pmol . ul(-1) cell water . min(-1)). Our re sults suggest that the uptake of choline is due to higher intracellula r requirements of choline under hypertonic conditions. Finally, an inc rease in the V-max of the choline transport system may enable sufficie nt synthesis of betaine and GPC.