METHODS

Experiments were performed on Sprague-Dawley rats weighing 165-325 g (maximum weight range within experiments was 50 g) in individual metabolic cages withscreens to separate feces from urine, the latter being collected under mineral oil and preserved with thymol. During the studies, rats were fed a fixed daily intake (10 or 12 g) of a synthetic, high-calorie, low-residue diet, which was eaten by all experimental or control rats daily, including the 24-hr period of hydropenia except for the instances noted. The rats were induced to void at the end of each collection period by sniffing ether. At the end of the experiments rats were sacrificed by exsanguination after pentobarbital aneghesia(5 mg/100 g rat); blood urea nitrogen was measured. The basic diet used was a low-chloride, normal sodium diet (Nutritional Biochemicals Corporation, Cleveland). Each batch of diet was analyzed and different batches contained 18-30 PEq chloride, 350-550 PEq sodium (as bicarbonate), and 1,350-1,600 PEq potassium (as monopotassium phosphate and potassium bicarbonate) per 10 g diet, but multiple analyses for each batch were consistent. The same batch of diet was used throughout each individual experiment so that daily electrolyte intake was constant. The diet was modified in different experiments by omission of the sodium salt (low-sodium chloride diet containing 24 PEq sodium/l0 g diet), or adding 0.5 mEq KC1 to 10 g of low-salt diet (normal chloride, low-sodium diet), or adding 1.5 mEq NaCl and 0.5 mEq KC1 to 10 g of the low-chloride diet (high-sodium chloride diet). Sodium and potassium in urine, serum, and renal tissue were measured by a flame photometer with an internal lithium standard, chloride by the Cotlove chloridometer, osmolality by an Advanced Osmometer, and urinary and blood urea nitrogen by the AutoAnalyzer or by the method of Cracker (5); urinary creatinine was measured by the method of Kennedy et al.(15). Renal tissue electrolytes were measured by the method of Mannitus and co-workers (21) except that 0.75 N nitric acid was used for tissue extraction. Each sample taken for measurement of papillary electrolytes consisted of whole papillas from three rats (six kidneys).