AJP - Endocrinology and Metabolism, Vol 252, Issue 6 E705-E711, Copyright © 1987 by American Physiological Society
Hydroosmotic responses to short pulses of vasotocin by toad bladder
P. Eggena
When toads, Bufo marinus, were mildly dehydrated, the permeability to water
of their urinary bladders increased, but blood samples placed directly on
the wall of an isolated assay bladder did not contain enough arginine
vasotocin (AVT) to induce the observed response. One explanation for this
discrepancy is that AVT is secreted intermittently and rapidly cleared from
the blood, even though the hydroosmotic response persists. The hydroosmotic
response of bladders to pulsatile secretion of AVT was simulated by dipping
isolated bladders for 1 min in Ringer's fluid with AVT followed by washout
of hormone. Bladders developed the characteristic increase in membrane
permeability to water several minutes after AVT had been removed from the
serosal bathing solution. With short periods of bladder exposure to
hormone, higher concentrations were required for triggering responses
equivalent to those induced with continuous stimulation. This requirement
was greater for vasopressin than it was for AVT. Studies with
tritium-labeled vasopressin indicated that there is a "receptor reserve"
for the hydroosmotic action of vasopressin in the toad bladder. The
magnitude of this reserve is reduced as the duration of stimulation with
hormone is diminished. It is suggested that pulsatile exposure of the
bladder to AVT increases the specificity of the hormone for hydroosmotic
receptors and minimizes the development of target organ resistance. The
large receptor reserve in this tissue may be required to capture short
bursts of AVT secreted into blood, although this remains to be
demonstrated.
