Receptor-independent sequestration of beta-adrenergic ligands by alveolar type II cells

Numerous studies indicate that synthesis and secretion of surfactant by type II pneumocytes are modulated by the interaction of beta-adrenergic agonists with specific cell surface receptors. Two 125I-labeled beta-adrenergic ligands, l-iodopindolol (IPIN) and l-iodocyanopindolol (ICYP), were thus employed to investigate the properties of type II cell beta-receptors. Saturable, high-affinity, stereospecific binding to crude membrane fractions from whole rat lungs was exhibited by both ligands (IPIN, KD 283 pM, Bmax 508 fmol/mg protein; ICYP, KD 18 pM, Bmax 404). Type II cell membranes obtained by N2 cavitation also revealed stereospecific, saturable, high-affinity binding. In intact cells (37 degrees C) however, rapid, highly concentrative (cell/media greater than 1000), nonspecific ligand uptake compromised estimates of specific binding (specific/total binding less than 0.1). Total ligand uptake was inhibited at 4 degrees C, by decreasing pH within the physiological range (7-8) and by the lysosomotropic compound chloroquine (50-200 microM), without a detectable change in specific binding. Other basic drugs were also inhibitory at similar concentrations; acidic drugs had no effect. Even at 4 degrees C, specific binding remained low, as IPIN and ICYP were displaced less than 30% by l-alprenolol (1 microM). Physicochemical properties of IPIN and ICYP considered with the above studies suggest that passive ligand entry into intact pneumocytes and subsequent trapping of the protonated species in a cellular compartment of low pH may account for high nonspecific ligand uptake.