1. In normally hydrated rats prostaglandin F (PGF) in doses of 5 μg/100 g body weight given subcutaneously every 2 h (three times) induced a significant increase in urinary kallikrein activity, and in sodium, potassium and water excretion for 8 h after the first injection. In moderately hyperhydrated rats loaded 2·5% of body wt. with 0·5% NaCl solution, PGF produced similar changes in kallikrein activity and electrolyte excretion.

2. In normally hydrated rats prostaglandin E2 (PGE2) in the same conditions and doses as in 1 had no effect on kallikrein activity, showing a tendency to decrease potassium and water excretion.

3. PGE2 in doses of 5, 12·5 and 25 μg/100 g body wt. in overhydrated rats given 2·5% and 0·5% NaCl and 5% of tap water/100 g body wt. 1 h later, significantly increased kallikrein activity in the urine collected for 120 min after the injections. A significant decrease in potassium and water excretion was observed with the highest dose.

4. PGF, had no effect on kallikrein activity in overhydrated rats, but an increase in sodium and a decrease in potassium excretion was seen at the highest dose.

5. The different actions of PGE2 and PGF may be part of a regulatory mechanism associated with the kallikrein—kinin system which contributes maintainance of extracellular fluid homeostasis.

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