Mechanisms of the reversal of blood pressure by ephedrine (Report 2)

In rats anesthetized with urethane, the reversal of blood pressure by l-isomer (10 mg/kg i.v.) after injection of the l-isomer (10 mg/kg i.v.) disappeared by the pretreatment of phentolamine (5 mg/kg i.v.), phenoxybenzamine (2.5 mg/kg i.v.), chlorpromazine (2.5 mg/kg i.v.), sulpiride (12.5 mg/kg i.v.) and LSD-25 (200 μg/kg i.v.), respectively. The reversal of blood pressure did not disappear with vagotomy and cocaine (10 mg/kg i.v.) and tubocurarine (30 μg/kg i.v.). The reversal was evident in spinal rats and rats administered pchlorphenylalanine (300 mg/kg i.p. for 3 days). The depressor effect of d-isomer was potentiated in rats administered tryptophane (1000 mg/kg p.o. before 4 hr). In rats administered iproniazid (300 mg/kg i.p. before 8 hr) in addition to tryptophane, the decrease in blood pressure was much greater than that seen in the rats treated with tryptophane alone. In rats administered Li2CO3 (2 mEq/kg p.o. twice daily X5), the decrease in blood pressure was increased by twice the value. On the other hand, the level of 5-HT in blood obtained from rat carotid artery was increased with the reversal of blood pressure by d-isomer. Release of 5-HT from rat platelets occurred with incubation (37°C, 20 min) with both d- and l-isomer (3 × 10-3 g/ml). These data indicate that the reversal of blood pressure by d-isomer is due to the effect of 5-HT. Regarding the mechanisms of the reversal of blood pressure, the following results were obtained; 1) The first injection of 1-isomer caused a pressor effect and masked a-action, therefore, the pressor effect of d-isomer disappeared with the second injection. 2) The depressor effect of endogenous 5-HT appeared. 3) Endogeneous 5-HT was released from platelets by the d-isomer. 4) The depressor effect was partially due to dilation of the peripheral blood vessels by the direct and indirect action of the d-isomer.