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Departments of Physiology, Physics and Pharmacology and Therapeutics, the Middlesex Hospital Medical School, London W1P 7PN

1. The effects of acute changes in plasma Na concentration (P_Na) on renal blood flow (RBF) and glomerular filtration rate (GFR) were studied in anaesthetized greyhounds. Saline was infused at a constant rate (0·1 ml. kg^-1 min^-1) either into a renal artery or into a systemic vein. Plasma Na concentration was altered by varying the Na concentration of the infused saline from 0·154 to 0·077, 0·616 or 1·232 M.

2. Blood pressure (B.P.), packed cell volume (PCV), concentration of plasma solids (PS) and the plasma concentration of H⁺ and K (P_K) ions were measured but no attempt was made to contain their fluctuation.

3. An infusion of hypertonic saline into a renal artery usually led to an ipsilateral increase in RBF for 5-15 min, followed by a progressive fall. Over-all, mean values of RBF fell with P_Na throughout the range studied (120-190 m-mole l.^-1). Glomerular filtration rate rose with P_Na to reach maximal values at P_Na levels of 140-160 m-mole l.^-1, but fell thereafter. The combined fall in RBF and GFR, without change in filtration fraction, at P_Na values above 160 m-mole l.^-1 is consistent with an alteration in afferent arteriolar resistance. The fall in GFR despite a rise in RBF noted when P_Na was reduced below 140 m-mole l.^-1 requires an additional explanation.

4. Renal blood flow was independent of P_K; it was inversely related to [H⁺] and directly related to PS. Glomerular filtration rate was independent of PCV and P_K. It was also inversely related to [H⁺] and directly related to PS up to a value of 6 g 100 g^-1 plasma, after which the relationship was reversed. These results suggest that the renal vascular responses to acute changes in P_Na may be mediated in part, at least, by concurrent change in PS and [H⁺].

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