The adenylate cyclase-coupled vasopressin V2-receptor is highly laterally mobile in membranes of LLC-PK1 renal epithelial cells at physiological temperature

The lateral mobility of membrane-associated hormone receptors has been proposed to play an important role in signal transduction. Direct measurements, however, have shown that the receptors for insulin, epidermal growth factor and β-adrenergic antagonists exhibit low mobility at physiological temperature. The present study, which represents the first report of lateral mobility of a polypeptide hormone receptor coupled to adenylate cyclase, yielded quite different results. The lateral mobility of the vasopressin renal-type (V₂)-receptor was measured in the basal plasma membrane of cells of the LLC-PK₁ porcine epithelial line, using the technique of fluorescence microphotolysis (photobleaching) and a rhodamine-labelled analogue of vasopressin. The analogue, 1-deamino[8-lysine(N⁶-tetramethylrhodamylaminothiocarbonyl)] vasopressin (TR-LVP) was synthesized and shown to have binding properties and biological activities very similar to those of Arg⁸-vasopressin (AVP). TR-LVP could be used to label specifically the V₂-receptor of living LLC-PK₁ cells, whereby LLC-PK₁ cells incubated with TR-LVP in the presence of a 100-fold excess of AVP, or cells from the LLC-PK₁ V₂-receptor-deficient line M18 incubated with TR-LVP could be used as controls for non-specific binding. Using optical sectioning, specific receptor mobility could be measured both in the absence and presence of free TR-LVP. The V₂-receptor was found to be largely mobile at 37°C: the mobile fraction (f) was ~0.9, and the apparent lateral diffusion coefficient (D) ~3.0 x 10^-10 cm²/s. V₂-receptor mobility greatly decreased with decreasing temperature: at 10°C f was reduced to ~0.1. Lipid probe mobility demonstrated that receptor immobility at 10°C was not attributable to a temperature-induced phase transition of the plasma membrane lipid bilayer.