AJP - Endocrinology and Metabolism, Vol 255, Issue 5 E613-E616, Copyright © 1988 by American Physiological Society
Calcitonin is a competitive inhibitor of the hydrosmotic effect of oxytocin in toad bladder
M. Parisi, C. Ibarra, M. Ladizesky and C. Mautalen
Departamento de Fisiologia, Hospital de Clinicas, Facultad de Medicina, Universidad de Buenos Aires, Argentina.
The effects of calcitonin (CT) on the water transfer in the toad (Bufo
arenarum) urinary bladder, an epithelial barrier commonly employed as a
model of the mammalian nephron, were studied. The net transmembrane water
flux was measured at minute intervals, while the endogenous adenosine
3',5'-cyclic monophosphate (cAMP) levels were determined in isolated
epithelial cells. It was observed that 1) CT, up to 10(-6) M, did not have
any effect on water permeability. 2) Preincubation with CT, between 10(-7)
and 10(-8) M, inhibited the hydrosmotic response to a supramaximal dose of
oxytocin (OXT; 2 x 10(-8) M), used here as an antidiuretic hormone (ADH)
analogue. This inhibition was reversible and concentration related.
Nevertheless, although the magnitude of the response was reduced, its time
course of evolution did not change. 3) When CT was added on the previously
developed response to OXT, inhibition was also dose dependent with a time
course not distinguishable from hormonal washout. 4) CT, up to 10(-6) M,
did not modify the hydrosmotic response to 8-bromo cAMP, a potent analogue
of the ADH second messenger. 5) CT and OXT increased the intracellular cAMP
levels, but both effects were not cumulative. The increase induced by CT
plus OXT was significantly lower than the one elicited by OXT alone. It is
concluded that CT is a competitive inhibitor to the hydrosmotic effect of
OXT in toad urinary bladder. Its action must be located prior to cAMP
formation.
