AJP - Cell Physiology, Vol 259, Issue 3 C462-C470, Copyright © 1990 by American Physiological Society
Internalization of fluorescent vasotocin-receptor agonist and antagonist in the toad bladder
P. Eggena, M. Lu and A. Buku
Department of Physiology and Biophysics, Mount Sinai Medical School, City University of New York, New York 10029.
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.
