1. Renal cortical and medullary tissue and aortic wall were removed from spontaneously hypertensive rats and from age-matched Wistar-Kyoto control animals at ages 30, 60, 90 and 120 days. The tissues were incubated and the release of prostaglandins into the incubation medium was measured.

2. Compared with Wistar-Kyoto control animals, the release of prostaglandin E from renal medullary tissue in spontaneously hypertensive rats was raised at 30 days (pre-hypertensive stage) and 90 days (early hypertensive stage), but decreased later with further establishment of hypertension. No such trend was seen with renal cortical tissue. Tissue release of prostaglandin F tended to be generally high in the spontaneously hypertensive rats compared with that in the control animals, but the difference was not significant.

3. The release of prostaglandin I2, as indicated by measurements of 6-keto prostaglandin F, from aortic wall tissue in the spontaneously hypertensive rat during its pre-hypertensive and early hypertensive stages was similar to values obtained in the age-matched control animal. However, aortic wall prostaglandin I2 release in spontaneously hypertensive rats increased thereafter, and was significantly raised at 90 and 120 days. No similar trend was observed with thromboxan A2 release. Release of prostaglandin I2 and thromboxan A2 from renal tissues in spontaneously hypertensive rats did not differ significantly from that in control animals.

4. It is suggested that indomethacin-induced aggravation of hypertension in the spontaneously hypertensive rat may result from suppression of aortic wall prostaglandin I2 formation rather than from the suppression of renal prostaglandin E2 production.

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