Age and the antihypertensive effect of aspirin in rats.

1. We previously showed that chronic exposure to aspirin (100 mg kg-1 daily, by mouth) is effective in preventing the onset of hypertension in young (28-84 day old) spontaneously hypertensive rats (SHRs). This is contrary to what others have reported using older SHRs. 2. Renal prostaglandin F2 alpha was also reduced in young SHRs and Wistar-Okamoto strain rats (WKYs) exposed to aspirin. 3. In the present study we extended the period of aspirin treatment in young rats to beyond 84 days of age. We found that aspirin lost its antihypertensive effect in SHR and WKY rats at 110 +/- 7 days of age regardless of whether the exposure to aspirin had begun at age 28, 49 or 87 days. 4. We conclude that the loss of antihypertensive effect of aspirin in the SHR and in older WKY rats, is determined by some factor(s) probably not related to prostaglandin F2 alpha, which reaches full expression in the 110 +/- 7 day old rat, or is fully dissipated at this age. 5. The anti-PGF2 alpha activity of aspirin in the SHR and WKY rat was short-lived and apparently unrelated in time to the antihypertensive effect of aspirin.