Uptake of l-[ propyl-2,3- 3H]dihydroalprenolol by intact HeLa cells

Abstract

This report describes the uptake of l-[ propyl-2,3- 3H]dihydroalprenolol, a β-adrenergic antagonist, by HeLa (human adenocarcinoma) cells. [ 3H]Dihydroalprenolol binds to sites of high capacity and low affinity in intact HeLa cells. The binding achieves equilibrium rapidly and is rapidly reversible. Bound [ 3H]dihydroalprenolol is displaceable by β-adrenergic antagonists in a nonstereoselective fashion, but is not displaceable by isoproterenol, an adrenergic agonist. Phentolamine, an α-adrenergic antagonist, and chloroquine, a lysosomotropic amine, also compete for [ 3H]dihydroalprenolol binding sites. [ 3H]Dihydroalprenolol binding is inhibited by metabolic inhibitors, but not by cytoskeletal blocking agents. The binding is sensitive to extracellular pH (less binding at lower pH) and is temperature-sensitive (less binding at lower temperatures). The bound radioligand is rapidly reversed following hypotonic lysis of the cells. These [ 3H]dihydroalprenolol binding sites in intact HeLa cells therefore do not have the characteristics expected for β-adrenergic receptors. Further studies showed that β-adrenergic receptors could be detected in a HeLa membrane preparation using [ 125I]iodohydroxybenzylpindolol, and that chloroquine had very low affinity for these receptors. We conclude that [ 3H]dihydroalprenolol diffuses across the plasma membrane of intact HeLa cells and accumulates in acidic intracellular compartments.