[Sexual dimorphism in the pressor response to l-adrenaline in rats]

The cause of sex differences in the pressor response to l-adrenaline in adult rats (Tomori, 1986) was further examined using normal and castrated adult rats. Blood pressure was determined by the tail-cuff method. A prolonged elevation of blood pressure in male rats shown after an injection of l-adrenaline (50 micrograms/kg, s.c.) was almost completely suppressed by phentolamine (50 micrograms/kg, s.c.), and the slight l-adrenaline-induced increase in blood pressure in female rats was reversed to a decrease by phentolamine. Pretreatment with propranolol (200 micrograms/kg, s.c.) enhanced the pressor action of l-adrenaline in males and enhanced it to a greater extent in females. Castration resulted in a significant decrease in basal pressure in males and no change in females. Pressor responses to l-adrenaline in castrated male and female rats showed a minor alteration. In clonidine (20 micrograms/kg, s.c.)-treated normal rats, l-adrenaline induced an increase in blood pressure in males, but caused a decrease in females. In clonidine-treated castrated male rats, however, l-adrenaline administration induced a decrease in blood pressure. l-Adrenaline injection to yohimbine (1 mg/kg, s.c.)-treated rats induced a prolonged elevation of blood pressure in males and a transient elevation in females as shown in rats treated with l-adrenaline alone. In yohimbine-treated castrated rats, a decrease in the blood pressure was observed in males and a slight increase was observed in females after l-adrenaline administration. The present results suggest that the central pressor regulatory system is involved in the sex difference in pressor response to exogenous l-adrenaline and that androgens play a role to somehow maintain a higher pressor responsiveness to l-adrenaline in male rats.