Effects of alpha 1-blockade on arterial compliance in normotensive and hypertensive rats. The effects of blockade of alpha 1-adrenergic receptors on the mechanical properties of the arterial wall were studied in 10 spontaneously hypertensive rats (SHR) as compared with 10 matched normotensive Wistar-Kyoto (WKY) rats. Ascending aortic pressure and flow were recorded in open-chest anesthetized rats, and the systemic arterial compliance was calculated. Intravenous injection (1 mg/kg) of Urapidil, a selective alpha 1-adrenergic antagonist, induced a significant decrease in arterial pressure (-26%, p less than 0.01 and -37%, p less than 0.001 in WKY rats and SHR, respectively) without significant changes in cardiac output. In control conditions, systemic arterial compliance was lower in SHR (3.29 +/- 1.52 microliters/mm Hg) than in WKY rats (4.35 +/- 1.35 microliters/mm Hg, p less than 0.01). Urapidil injection induced significant increases in systemic arterial compliance values in both strains (p less than 0.001). In another set of experiments (15 WKY rats and 15 SHR), the carotid compliance (microliters/mm Hg) was determined from the arterial volume-pressure relation under control conditions, after local incubation with Urapidil, and after total abolition of the vascular smooth muscle by KCN. In WKY rats, the carotid compliance increased markedly after incubation with Urapidil at doses corresponding to 1 mg/kg (+31%, p less than 0.01). A further increase in the carotid compliance was observed after KCN poisoning (+11%, p less than 0.05). In SHR, incubation with Urapidil at doses corresponding to 2 mg/kg were necessary to induce a significant increase in compliance (+38%). At this dosage, there was no further increase in compliance after KCN poisoning. 