Internalization of fluorescent vasotocin-receptor agonist and antagonist in the toad bladder

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Abstract
This study compares hydrosmotic action, receptor binding, and fluorescent uptake of an agonist, d9phe(flu)AVT, and an antagonist, d4lys(flu)AVT, in the toad bladder. D9phe(flu)AVT increased osmotic water flow across the bladder with a 50% effective dose of 2 nM, whereas d4lys(flu)AVT inhibited water flow with a 50% effective dose of 0.1 microM. D9phe(flu)aVT displaced 10 nM [3H]arginine vasopressin (AVP) from plasma membranes by 50% (IC50) with 10 nM, whereas d4lys(flu)AVT had an IC50 of 3 microM. The fluorescent agonist induced a persistent increase in membrane permeability to water after removal from the serosal bathing solution. This residual response was diminished by preincubation with an agonist (AVP), but not with an antagonist [d4lys(N3)AVT]. The agonist, d9phe(flu)AVT, was internalized into toad bladder epithelial cells, as seen by epifluorescence microscopy, and this uptake was blocked by d4lys(N3)AVT. The antagonist, by contrast, was not internalized but remained at the cell surface. After stimulation with forskolin, however, the fluorescent antagonist was also internalized. These experiments suggest that agonists, but not antagonists, form functional complexes with receptors that, via formation of cAMP, trigger not only an increase in membrane permeability to water but also facilitate the clearance of hormone from the cell surface by endocytic uptake.