[Effects of terazosin on the blood pressure responses to tilting in conscious animals: comparison with prazosin].

Inhibitory effects of terazosin on the compensatory blood pressure responses to tilting were studied in conscious rabbits and spontaneously hypertensive rats (SHR). In rabbits, doses which reduced the mean blood pressure by 15 mmHg were 330 micrograms/kg, i.v., for terazosin and 42 micrograms/kg, i.v., for prazosin, while those which depressed the blood pressure responses to tilting by 30 mmHg were 180 micrograms/kg, i.v., for terazosin and 54 micrograms/kg, i.v., for prazosin. In SHR, almost equal decreases in the mean blood pressure (about 30%) were observed by 1 mg/kg prazosin, p.o., 20 mg/kg hexamethonium, i.p., 3 mg/kg hydralazine, p.o., or 3 mg/kg nicardipine, p.o. In these conditions, prazosin and hexamethonium markedly depressed the blood pressure responses to tilting, whereas hydralazine and nicardipine showed little effect. The results with these antihypertensive drugs closely paralleled the established orthostatic profiles seen clinically. In this SHR tilting model, when the mean blood pressure was reduced by 15%, prazosin significantly depressed the tilting reflexes; however, terazosin produced no depression. Considering the dose ratio of terazosin to prazosin for antihypertensive effects and inhibitory effects on the tilting reflexes, the orthostatic liability of terazosin was about 3 times as low as that of prazosin. On the basis of these results, it is expected that terazosin causes less orthostatic hypotension than prazosin in clinical use.