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Pieces of dog carotid artery were studied with respect to water and sodium content. Total sodium content averaged 113 ± 1.2 mm/kg fresh tissue; total water 73.6 ± 0.3% and inulin space 36.2 ± 0.5% of tissue wet wt. A total of 94.8 ± 1.3% of sodium exchanged within 6–12 min with Na22, and 97.4 ± 0.7% of the stable sodium was extracted in sodium-free solution (choline replacement). The curve of efflux of Na22 at 37 C could be decomposed into three simple exponentials with half-times of 42.5 ± 2.3 sec ( phase 1), 5.0 ± 0.3 min ( phase 2), and 71.0 ± 7.3 min ( phase 3). Exchange of sodium of phase 1 (extracellular) fitted well with a theoretical diffusion curve, showed low temperature coefficient and no potassium dependency, and was not influenced by ouabain. Exchange of sodium of phase 2 (cellular) showed high temperature coefficient and potassium dependency and was sensitive to ouabain. Calculations based on the compartmental analysis indicate a too-great intracellular sodium concentration. Binding of sodium to polyanions in the extracellular space is suggested as a possible explanation of the results.

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1. Exchangeable potassium, sulphate space and intracellular fluid have been determined in eight normal males and in seven uraemic male patients on a regime of twice weekly 8 h dialysis.2. Intracellular water was more closely correlated with exchangeable potassium than were body weight or lean body mass.3. There was no correlation between exchangeable potassium and plasma or erythrocyte potassium. 4. Uraemic patients had a 20% potassium depletion that may have been due to a restricted oral intake (40 mmol/day) and too little potassium (0-1 mmol/l) in the dialysis fluid.

SUMMARY Changes in inulin space, plasma and blood volume, exchangeable and "noninulin" sodium were studied during the prehypertensive, early and late hypertensive stages of deoxycorticosterone (DOC)-saJt administration in the rat. The effect of an acute water load in previously nephrectomized animals was also studied. Hypertension developed after 1 to 2 weeks of the DOC-salt regimen and was always preceded by enlargement of the inulin space and increased plasma and blood volume. Expansion of extracellular fluids receded when blood pressure started to rise but reappeared after 4 to 6 weeks of treatment. Plasma sodium was high only in the hypertensive groups. An acute water load increased blood pressure of normal rats and decreased blood pressure of DOC-salt early hypertensive rats. These findings suggest that extracellular volume expansion inhibits a vasopressor mechanism that involves vasopressin and could be stimulated by hypernatremia. (Hypertension 4: 620-624, 1982) KEY WORDS • body fluids • deoxycorticosterone • hypertension H YPERTENSION produced by deoxycorticosterone (DOC) and salt is usually included in the "volume-dependent" variety because it is regularly associated with the expansion of the extracellular volume 1 -2 at least at some stage of the disease. However, we have recently found that an acute extracellular expansion lowers rather than raises the blood pressure of early, nonexpanded DOC-salt hypertensive rats.3 This latter effect was tentatively ascribed to the inhibition of some vasopressor volume-dependent mechanism possibly involving vasopressin.The aim of this paper was to try to reconcile these apparently contradictory findings and to gain further insight into the problem by following the time course of changes in exchangeable sodium (Na) and extracellular body fluids during the prehypertensive, early, and late stages of hypertension. In addition we have studied the effect of an acute water load, which is the most powerful physiological inhibitor of vasopressin release on blood pressure. 4 Methods General ProcedureWe studied 105 male Wistar rats weighing approximately 250 g. We gave 71 rats a drinking fluid of 1% (w/v) saline supplemented with 0.2% KG in water, and 25 mg of DOC enantate (Schering, Argentina) in castor oil intramuscularly twice a week during 1 to 6 weeks. As controls we used 34 male rats of similar breed, who drank tap water. All animals were given normal rat chow. Systolic blood pressure was measured in the unrestrained awake animals at 30°C by the plethysmographic tail method before starting treatment and then at weekly intervals until the time of sacrifice. Reproducibility between successive readings was better than 10 mm Hg. Therefore, rats were considered hypertensive when differences between readings performed at different days exceeded this upper limit. Two series of experiments were conducted: in the first one the time course of changes in blood pressure, exchangeable Na, and body fluids were followed at different stages of the disease. The second series was aime...