The direct and amplifying effects of serotonin are increased with age in the isolated perfused kidney of Wistar and spontaneously hypertensive rats.

Isolated kidneys of Wistar rats and spontaneously hypertensive rats (SHR) were perfused with Tyrode's solution. In 2- and 6-month old SHR, the maximal increase in perfusion pressure caused by norepinephrine was higher than in 2- and 6-month old Wistar rats, but the sensitivity, as judged from the dose of the agonist required to reach 50% of the maximal response was the same. Both the maximal response and the sensitivity to serotonin were significantly augmented in 6-month old SHR and Wistar rats when compared to the young animals. This hypersensitivity was more pronounced in SHR than in Wistar rats. Infusion of serotonin potentiated the vasoconstriction induced by a bolus of norepinephrine. This amplification, due to activation of S2-serotonergic receptors, was more pronounced in the old animals. No amplification occurred when norepinephrine was infused instead of serotonin. Tachyphylaxis to the amplifying effect of serotonin was observed and was less pronounced in kidneys from old than from young animals. The amplifying effect of serotonin was inhibited by ketanserin at concentrations which did not, or only moderately, inhibit the response to norepinephrine.