alpha 1-Receptor localization in rat heart and kidney using autoradiography

To study the distribution of alpha 1-adrenergic receptors in rat heart, kidney, and skeletal muscle, we used light-microscopic autoradiography of [3H]prazosin. Scintillation spectrometry of frozen sections demonstrated rapid binding, saturability, stereospecificity, and agonist and antagonist binding characteristic of an alpha-receptor. For autoradiography, sections were incubated, processed, and grain density quantified using a computer-based image analyzer. Specific alpha 1-receptor binding was found over cardiac myocytes in the left and right ventricles but not over skeletal muscle. Scatchard analyses of specific grain densities over cardiac myocytes gave a dissociation constant (Kd) of 0.55 +/- 0.18 nM (SD, n = 4 rats) and a maximum number of binding sites (Bmax) of 448 +/- 90 grains/10(-2) mm2. Renal arterioles had a higher specific grain density than myocardial arterioles at all concentrations of [3H]prazosin (P less than 0.001). Scatchard analyses showed that renal arterioles had a Kd of 0.27 nM and a Bmax of 1,259 grains/10(-2) mm2, whereas myocardial arterioles had a Kd of 1.64 nM and a Bmax of 183 grains/10(-2) mm2. Arterioles in the flexor carpi radialis muscle were not labeled. Renal cortex tubules had the highest grain density of any structure studied, i.e., higher than grain density over glomeruli or tubules in the renal medulla. These observations indicate that significant differences exist in the distribution and affinity of alpha 1-adrenergic receptors in various vascular beds and parenchymal tissues.