Lateral mobility of the phospholipase C-activating vasopressin V1-type receptor in A7r5 smooth muscle cells: A comparison with the adenylate cyclase-coupled V2-receptor

The present work examines lateral mobility of the vasopressin V₁-type receptor, representing the first determination of lateral mobility of a hormone receptor coupled to phospholipase C activation. The V₁-receptor of A7r5 smooth muscle cells was characterized for [Arg⁸] vasopressin (AVP) binding properties and affinity for the fluorescent vasopressin analogue 1-deamino[8-lysine (N⁶-tetramethylrhodamylaminothiocarbonyl)] vasopressin (TR-LVP). TR-LVP was biologically active in A7r5 cells, inducing inositol 1,4,5-trisphosphate turnover in similar fashion to AVP. TR-LVP was used to specifically label the V₁-receptor of living A7r5 cells, and lateral mobility of the V₁-receptor was measured using the technique of fluorescence microphotolysis. The apparent lateral diffusion coefficient (D) at 37°C was 5.1 × 10^-10cm²/s, falling to 2.9 × 10^-10 cm²/s at 13°C. These D values are higher than comparable values for the adenylate cyclaseactivating vasopressin V₂-receptor of LLC-PK₁, renal epithelial cells analysed with the same fluorescent ligand. In contrast to the V₂-receptor, no marked temperature dependence was observed for the V₁-receptor mobile fraction (f). From 37°C to 13°C, f was relatively low (between 0.4 and 0.5) consistent with V₁receptor immobilization through internalization, which is rapid even at room temperature in A7r5 cells. These differences between V_1- and V₂-receptor lateral mobility are discussed in terms of the implications for their respective signal transduction systems.