Determination of molecular size of alpha-1 and alpha-2 adrenoceptors in rat mesenteric artery by radiation inactivation

Radiation inactivation of alpha-1 and alpha-2 adrenoceptors in the purified plasma membranes of rat mesenteric artery has been performed with high energy electrons at -45 to -55°C. Alpha-1 and alpha-2 adrenoceptor inactivation was monitored with [³H]prazosin and [³H]yohimbine binding, respectively. Internal endogenous and external standards of known molecular weight were used in these studies to determine the molecular size. The average value of D₃₇ for the [³H]prazosin binding site was 6.75 ± 0.62 Mrad (n=4) with an estimated molecular size of 122,921 ± 11,329 Daltons. However, the average value of D₃₇ for the [³H]yohimbine binding site was higher (D₃₇ = 10.05 ± 0.91 Mrad) and accordingly the molecular size of this binding site was less than the [³H]prazosin binding sites (molecular weight = 82,540 ± 7478 Daltons; n=4). Irradiation did not change the dissociation constant of either radioligand, suggesting that the loss of the radioligand binding sites after radiation is due to receptor protein inactivation. These results confirm our earlier finding that [³H]prazosin and [³H]yohimbine bind to two distinct sites in the plasma membranes of rat mesenteric artery. Whether both of these sites are the subunits of a common macromolecule of alpha adrenoceptor on vascular smooth muscle in rat mesenteric artery cannot be concluded from these results. This report is the first one in the literature on the molecular size of alpha-1 and alpha-2 binding sites in vascular smooth muscle.