Changes of Vascular β-Adrenoceptors in Spontaneously Hypertensive Rats: Characterization and Technical Problems

Derangement of Ca handling in vascular smooth muscle (VSM) and accelerated VSM cell growth have been related to the etiology of spontaneous hypertension. Alteration of the β-adrenoceptor has also been implicated in the functional (impaired relaxation) as well as structural (increased proliferation) changes of VSM cells in hypertension. In this study, we utilized subcellular membranes isolated from aortic muscle and cultured aortic smooth muscle cells (CASMC) from SHR and WKY to investigate the properties of vascular β-adrenoceptors by [125I]-ICYP binding. We observed (a) ICYP binding sites (Bmax) are located primarily on the plasma membranes of VSM and occur more frequently in small arteries than in large arteries; (b) ICYP binding to crude tissue homogenate often yielded a nonlinear Scatchard plot, whereas ICYP binding to relatively purified membranes yielded a linear Scatchard plot (nonlinearity was also observed in microsomal fractions isolated from smaller branches of mesenteric arteries); (c) microsomes from SHR aorta failed to show reduced β-adrenoceptor density compared with those from WKY or normotensive Wistar rats; (d) cultured aortic muscle cells from SHR showed substantially higher Bmax for ICYP binding than those from WKY without a change in Kd. We conclude that the use of a purified membrane fraction is advantageous in receptor binding studies. In a comparative study of the receptor density, knowledge of the relative purity of the membranes for the control and test groups is essential. Increased Bmax of ICYP binding to SHR aortic cells may be related to the increased cell size grown in culture and, if present in vivo, may reflect impaired relaxation.