1. The influence of urine pH on the urinary excretion of prostaglandin (PG) F and the PGE2/PGF ratio has been examined in the conscious rat.

2. The basal urinary PGF excretion rate of 3.9 pmol/h (n = 23) did not vary with urine pH. In marked contrast, PGE2 excretion increased as the urine became more alkaline. The PGE2/PGF ratio therefore progressively increased from 1.5 to 22 as the pH of the urine changed from pH 5.8 to pH 7.8.

3. The independence of PGF excretion from urine pH: (a) excludes cyclo-oxygenase as a potential site of action for the pH-dependence of urinary PGE2 excretion; (b) suggests that the urinary PGE2/PGF ratio measured in alkaline urine may be a more accurate reflection of the kidneys, ability to synthesize these two prostaglandins in vivo; (c) suggests that control of urine pH is required before the urinary PGE2/PGF ratio can be employed as an index of PGE2 9-ketoreductase (EC activity in vivo.

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