Inhibition of the permeability response to vasopressin and oxytocin in the toad bladder: Effects of bradykinin,kallidin, eledoisin,and physalaemin |
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Authors: | Furtado M R F |
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Institution: | 1.Department of Medicine, Faculty of Medicine, University of S?o Paulo, Ribeir?o Prêto, S?o Paulo, Brazil ; |
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Abstract: | Summary It has been shown by means of Bentley'sin vitro preparation of the isolated urinary bladder of the toad,Bufo marinus paracnemis Lutz, that bradykinin reversibly inhibited the increase brought about by vasopressin on the permeability to water of the
toad bladder. The increased hydro-osmotic response of the bladder to oxytocin was also inhibited by the kinin. The effect
on water permeability was observed when bradykinin was added either to the serosal Ringer's solution or to the mucosal solution.
The addition of bradykinin alone did not alter the basal osmotic water transfer across the bladder. In this context, bradykinin
acted as a competitive antagonist of vasopressin (and oxytocin). Although lacking intrinsic activity, bradykinin exhibited
affinity for receptor sites that are also common to the neurohypophysial hormones, causing a parallel shift of the log-dose/response
curve for vasopressin without changing the maximal responses. The effects of other kinins (namely kallidin, eledoisin and
physalaemin) on the toad bladder were also tested. Each of these drugs alone did not change the basal water flux across the
bladder wall. Like bradykinin, these peptides inhibited the increase in water permeability evoked by vasopressin and oxytocin
in the bladder. In view of the importance of neurohypophysial hormones and their target tissues to the osmotic homeostasis
of amphibians, and the observation of antagonism between the kinins and the pituitary hormones coupled to the abundance of
kinins in the amphibian organism, particularly in the skin and urinary bladder, teleological reasoning predicts a physiological
role for the kinins, possibly functioning to dampen excesses and oscillations in membrane permeability that could occur in
face of a constant and variable secretion of neurohypophysial hormone, thus adding to the homeostatic response of the amphibian
organism. |
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