Selective stimulation of vascular postjunctional alpha-adrenoreceptors by (-)-amidephrine in rats and cats

The vasopressor and chronotropic responses of (-)-amidephrine and the receptor types involved were studied in pithed rats of different strains and in pithed cats. The increase in diastolic pressure of pithed rats after i.v. administration of (-)-amidephrine was not influenced by pretreatment with propranolol (1 mg/kg, i.v.), reserpine (2 x 5mg/kg in 48 h i.p.) or yohimbine (1 mg/kg, i.v.), but was strongly antagonized by prazosin (0.1 mg/kg, i.v.). In pithed cats, the pressor responses were antagonized by prazosin (1 mg/kg, i.v.) but much less so by yohimbine (1 mg/kg, i.v.). (-)-Amidephrine elicited minor positive chronotropic responses in pithed rats and pithed cats. This tachycardia was not influenced by propranolol (1 mg/kg, i.v.) but was abolished by prazosin (0.1--1.0 mg/kg). The results show that (-)-amidephrine acts as a selective agonist at vascular postjunctional alpha 1-adrenoreceptors in pithed rats and pithed cats. The positive chronotropic effects are attributable to stimulation of alpha 1-adrenoreceptors in the heart.