Adrenergic regulation of vascular smooth muscle tone in calf digital artery

Radioligand binding studies and functional assays on isolated smooth muscle preparations were performed in order to obtain a biochemical and functional characterization of the b-adrenoceptor (b-AR) subtypes involved in regulation of the smooth muscle relaxation of the calf’s common digital artery. The results indicate that the common digital artery possesses two b-AR populations (40% b1 and 60% b2) and the b2-subtype appears to predominate as far as function is concerned. Only the b2-AR agonists clenbuterol and fenoterol caused doserelated relaxant effects, antagonized by propranolol, when tested in preparations precontracted both with PGF2a (1.4 • 10)5 M) and noradrenaline (1.2 • 10)6 M). In noradrenaline precontracted preparations the b1-AR selective agonists dobutamine and xamoterol caused vasodilation which was not antagonized by (±)propranolol. While the functional relaxant effects of dobutamine may be attributed to its potent competitive a-AR blocking activity, further investigations are required to explain the effect of xamoterol. The vasodilator effect of (±)isoproterenol was irregular. The recorded contractile effects, mainly at dosages greater than 10)6 M, suggest the loss of drug selectivity for b-AR and a-AR activation. Indirect evidence indicates that the aadrenoceptor (a-AR) population in this tissue which produces a strong contraction is functionally dominant over the b-AR, suggesting limited therapeutic benefit for b-AR drugs to control blood flow disorders in the calf’s distal limb.