Summary—Postjunctional renal alpha‐adrenoceptors were studied (1)in vivo, on the renal vasculature of the anaesthetized rat and compared with those in the femoral vasculature, and (2)in vitro, on the renal vascular bed of isolated perfused rat kidney.In vivo, renal and iliac blood flows were measured with an electromagnetic flow meter. The i.v. injection of (‐)‐phenylephrine (1–16 μ/kg) and B‐HT 920 (0.6–600 μ/kg) induced an increase in both renal and iliac vascular resistance, inhibited respectively with prazosin (300 μ/kg) or yohimbine (300 μ/kg). In the kidney, maximum response to B‐HT 920 was equivalent to 64 of that to (‐)‐phenylephrine; on the iliac vasculature, vasoconstrictor responses to both drugs were identical, but only corresponded to 50 of the maximum renal response to (‐)‐phenylephrine. This indicates the predominance of alpha1‐over alpha2‐adrenoceptors in the renal vascular bed.In vitro, on the isolated perfused rat kidney, vasoconstriction was induced by the preferential alpha1‐adrenoceptor agonists (‐)‐phenylephrine, cirazoline and methoxamine and the preferential alpha2‐adrenoceptor agonists (alpha‐methylnoradrenaline, dopamine and clonidine) at concentrations at which they lose their selectivity for the alpha2‐adrenoceptors; all responses were antagonised by prazosin but not by yohimbine. B‐HT 920, the selective alpha2‐adrenoceptor agonist, only induced renal vasoconstrictionin v
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