1985 Volume 49 Issue 9 Pages 1043-1051
To study the cellular mechanism of vascular responsiveness by vasoactive hormones, such as catecholamines and angiotensin (A), the vascular smooth muscle cells (VSMC) of two clonal cell line (A7r5 and A 10) from rat embryo, and of adult rat aorta were established in culture. Binding studies using 125I-labeled-hydroxyphenylethylaminoethyltetralone as an α-adrenergic ligand and 125I-labeled-iodocyanopindolol as a β-adrenergic ligand, revealed that cultured VSMCs contain both α- and β-adrenergic receptors ; the binding was specific, rapid, reversible, and saturable. α-Adrenergic receptors appear to be a single class of high-affinity binding sites with an apparent dissociation constant (Kd) of ∼ 2× 10-10 M and a maximal binding capacity (Bmax) of ∼300, 000-400, 000 sites/cell, and exclusively of α1-subtype that is responsible for smooth muscle contraction. On the other hand, β-adrenergic receptors show almost comparable characteristics with the apparent Kd of ∼ -0.7-1.1 10-10 M and Bmax of 50, 000-130, 000 sites/cell, and consist predominantly of 2-subtype that mediates smooth muscle relaxation. Furthermore, -adrenergic receptors are coupled to adenylate cyclase system, of which activation by -agonists induces intracellular cyclic AMP formation. In contrast, the binding of 125I-labeled-AII was demonstrated only in A7r5. The binding declined rapidly during incubation possibly due to faster degradation of AII by proteolytic enzyme (s). AII receptors appear to be a single class of high-affinity binding sites with the apparent Kd of 0.9 10-10 M and Bmax of 11, 000 sites/cell, of which affinity is higher than any of vascular AII receptors previously reported. Therefore, cultured VSMCs isolated from neurogenic and systemic influence should provide a valuable in vitro system to investigate the characteristics of specific receptors for vasoactive hormones, the cellular mechanisms involved in activation and regulation of the receptors and subsequent biochemical responses.
