Methods L-histamine [Ring-2-'4C] dihydrochloride 59.7 mCi/mmole, and S-adenosyl-L-methionine-[14C-methyl] 57.9 mCi/mmole were purchased from Amersham Corp., Arlington Heights, Illinois. The'4C-labelled histamine was purified on cellulose plates as described [19].[(/3-side chain label)-3H] histamine was prepared from [(/3-side chain label)-3H] L-histidine 10 Ci/mmole (New England Nuclear Corp., Boston, Massachusetts) as described previously [10]. Non-labelled S-Adenosylmethionine was obtained from Boehringer, Manheim Biochemicals, Indianapolis, Indiana. Non-labelled histamine dihydrochioride, L-histidine, imidazole acetic acid, and bovine serum albumin were purchased from Sigma Chemical Co.(St. Louis, Missouri). 1-Methyl-4-(2-aminoethyl) imidazole hydrochloride (Nr-methyl-histamine) was purchased from Calbiochemical Corp.(San Diego, California). Pyrilamine maleate was a product of K and K Laboratories, Inc.,(Plainview, New York). Aminoguanidine sulphate was purchased from Eastman Organic Chemicals, Eastman Laboratory and Speciality Chemicals, Eastman Ko-dak Co., Rochester, New York. Amodiaquine was kindly supplied by Dr. SA Fusari, Park, Davis and Co.(Detroit, Michigan).

Isolation of glomeruli and tubules. Kidney tissue used for preparation of glomeruli and tubules was obtained from male Sprague-Dawley rats weighing 200 to 250 g, maintained on a standard diet,(Purina Laboratory Rat Chow, Ralston Purina Co., St. Louis, Missouri), and having free access to tap water. Rats were anesthesized and the kidneys were perfused in situ with 60 to 80 ml of modified Krebs Ringer Phosphate Buffer (KRB) until surfaces were completely blanched. The kidneys were then quickly excised, decapsulated and placed in an icecold KRB. Glomeruli and tubules were prepared from renal cortical tissue by a combination of sieving and differential centrifugation as described in our previous reports [10, 13]. The purity of each glomerular suspension (95 to 98%) was evaluated by light microscopic examination and counting of glomeruli [10, 13]. Glomeruli or tubules were then resuspended in 0, 1 M sodium phosphate buffer containing 2.7 mivi KCI, 0.8 M CaC12, and 0.1% glucose (incubation buffer) so that 8 to 12 mg of glomeruli or tubules were contained in 100 1d. Catabolism of histamine in isolated glomeruli and tubules. Freshly prepared glomeruli and tubules suspended in incubation buffer were distributed into 1.5 ml microcentrifuge tubes (Eppendorf) kept on crushed ice at 0 C. Inhibitors, labelled histamine, and the methyl donor S-Adenosylmethionine (SAM) were added in that order when appropriate. The final incubation volume was 200 pi. Tubes were mixed thoroughly and incubat-ed for 120 mm, or as specified in Results, at 37 C in a metabolic, shaking water-bath. In addition, the tubes were also shaken manually over 30 mm during the incubation. In each experiment, blank incubations were run using glomerular and tubular