SITES OF ARGININE SYNTHESIS AND UREA PRODUCTION ALONG THE NEPHRON OF A DESERT RODENT SPECIES, MERIONES SHAWI

The distribution of arginine synthase and arginase activities along th e successive nephron segments of Meriones kidney was measured in vitro with single tubule enzymatic microtechniques making use of either L-[ ureido-C-14] Citrulline (0.108 mM) or L-[guanidino-C-14] arginine (0.2 mM) as the respective substrates. Arginase activity (fmol urea formed per min per mm of tubule) was very low (5-25 fmol . min(-1). mm(-1)) in most nephron segments including the early portions of proximal conv oluted tubules (early PCT). It increased progressively after 3 mm of t he PCT to reach a value of 200 fmol . min(-1). mm(-1) in the cortical portion of the straight proximal tubule (CPST), with a further increas e, along the pars recta, of up to 250 fmol . min(-1). mm(-1) in the ou ter medullary portion (OSPST). In addition, arginase activity in OSPST and the adjacent descending thin limb (DTL) was higher in juxtamedull ary nephrons (JN) than in the corresponding portions of superficial ne phrons (SN). Arginine synthase activity (fmol arginine formed per mm o f tubule per min) was present in proximal tubules exclusively, with a gradient decreasing along the PCT (about 600 fmol . min(-1). mm(-1) in the 1st mm, 65 fmol . min(-1). mm(-1) in CPST and 30 fmol . min(1)(-) . mm(-1) in OSPST). It has been checked that CPST and OSPST (where th e two enzyme systems are present) are able to convert citrulline direc tly into urea with a yield of 65%. It is suggested that: (1) in early PCT cells, arginine synthase activity permits the conversion of the re absorbed citrulline into arginine (which then diffuses towards blood v essels); and (2) in pars recta cells, arginase activity results in a n et entry of arginine across the basolateral membranes and in a net exi t of the formed urea into the tubular fluid, if the permeability