Renin responses to water restriction and rehydration.

1. The effect of restricted water intake and rapid rehydration was studied in three conscious sheep with respect to plasma renin concentration (PRC), blood corticosteroid levels, plasma protein and electrolyte concentrations, and the renal and faecal excretion of sodium and potassium.2. During water restriction the plasma concentrations of renin, protein and sodium rose while aldosterone levels were low or undetectable. Plasma potassium levels were unchanged. External sodium and potassium balance appeared to be unaffected.3. During rehydration the sheep drank more than their estimated water deficit in 3-4 min with the following effects: PRC rose three- to fourfold during the ensuing 12 hr. Aldosterone levels too rose, while plasma protein, sodium and potassium concentrations fell. Urinary sodium excretion virtually ceased for 24 hr, and urine flow rate increased only little during this period.4. If there was a single stimulus to renin release during water restriction and rehydration, it was not an alteration in vascular or extravascular volume, total body sodium, systemic B.P. or plasma sodium concentration.5. It is concluded that the rise in PRC in these experiments is compatible with the theory that altered sodium transport at the macula densa was the stimulus for renin release.