Serotonergic and adrenergic regulation of skeletal muscle metabolism in the rat. II. The use of [125I]iodolysergic acid diethylamide and [125I]iodopindolol as probes of sarcolemmal receptor function and specificity.

To evaluate serotonin receptor kinetics in skeletal muscle, we synthesized and developed 2-[125I]iodolysergic acid diethylamide [( 125I]iodoLSD) as a high affinity, high specific activity probe of serotonergic receptor function. The kinetics of binding of this probe and the profile of agonist and antagonist displacement have been compared to results obtained using [125I] iodopindolol as a probe for beta-adrenergic receptor binding. [125I]IodoLSD was prepared by chloramine-T iodination and purified by high pressure liquid chromatography. Fluorescence, ultraviolet, and nuclear magnetic resonance spectra as well as mass spectroscopy demonstrate that the iodinated compound is 2-iodoLSD. This probe bound in a concentration-dependent fashion to sarcolemma in a manner consistent with existence of a single population of specific receptors having a Kd of 1.46 nM and an abundancy of 47 fmol/mg of protein. Half-maximal binding of the probe occurred within 1.0 min and equilibrium binding was observed at 8.0 min. The apparent t 1/2 for the probe-receptor complex was 40 s; the Kd calculated from kinetic data was 1.69 nM. The IC50 for displacement of iodoLSD was 330 +/- 185 nM for methysergide, 757 +/- 309 nM for cyproheptadine, and 2,570 +/- 1,390 nM for serotonin. Adrenergic antagonists such as l-propranolol and oxprenolol also displaced [125I]iodoLSD, but did so with IC50 values of 17,800 +/- 5,100 and 23,300 +/- 5,500 nM, respectively. Using [125I]iodopindolol as a receptor probe, we found the order of potency for adrenergic antagonists to be l-propranolol greater than oxprenolol much much greater than practolol. Serotonergic antagonists, although effective in displacing [125I]iodopindolol, did so only at very high concentrations. These results are consistent with the existence of separate and discrete D-serotonergic and beta 2-adrenergic receptors in skeletal muscle and the finding that adrenergic agonists and antagonists may interact with low affinities with the serotonergic receptor, but that serotonergic agonists and antagonists interact poorly, if at all, with the beta 2-adrenergic receptor.