Regional differences in alpha 1-adrenoceptor numbers and responses in canine aorta

The relationship between active force and alpha 1-adrenergic receptor binding in vascular smooth muscle was measured on strips of canine aorta. After measurement of the isometric force produced by l-phenylephrine and l-norepinephrine in vitro, [3H]prazosin was used to determine the affinity and numbers of alpha 1-receptors in homogenates prepared from the strips. Maximum active stress was 440 g/cm2 in the ascending, 317 in the descending thoracic, and 252 in the abdominal aorta. The total number of alpha 1-receptors per gram arterial wall averaged 576, 237, and 136 fmol/g, respectively. Affinity of the receptors was the same in all regions for prazosin (87 pM) and l-phenylephrine (9 microM). The relation between number of agonist-occupied receptors and response was similar in all regions but nonlinear; 50% of the maximum stress developed when only 2-11% of the receptors were occupied. Differences of contractile response within the vascular tree may thus arise from regional differences in the number of appropriate receptors present, rather than differences of affinity or occupation-response relationships.