On the mechanism of the phosphaturia of extracellular fluid volume expansion in the dog

On the mechanism of the phosphaturia of extracellular fluid volume expansion in the dog. This study was designed in an attempt to clarify the mechanisms involved in the phosphaturia of extracellular fluid volume expansion (ECFVE) in the dog and to determine if the time interval between the suppression or removal of the parathyroid glands and ECFVE plays a role in the phosphaturic response. Studies were performed on 20 healthy, trained, unanesthetized dogs. Fractional phosphate excretion (FE PO 4 ) was 5.4% during the control studies; after expansion with isotonic saline, FE PO 4 increased to 31.6% and fractional sodium excretion (FE Na ) was 10.9%. Plasma Ca and Mg levels decreased by approximately 1 and 0.5 mg/100 ml respectively. When these studies were repeated 4 to 6 days after thyroparathyroidectomy (T-PTX), the degree of phosphaturia had diminished, and FE PO 4 had increased from 3 to 12%. However, when the same studies were performed in dogs with intact parathyroid glands, with either Ca or Mg added to the infusate during ECFVE, no change in FE PO 4 was observed. In order to clarify the difference in the results obtained in dogs with suppression of PTH secretion by Ca or Mg with those from dogs studied several days after T-PTX, acute experiments were performed within 2 to 4 hours of T-PTX. In this setting ECFVE produced no significant change in FE PO 4 , although marked natriuresis ensued. The results suggest the existence of a regulatory system for phosphate excretion in addition to parathyroid hormone. A propos du mecanisme de la phosphaturie determinee par l'expansion du volume extra cellulaire chez le chien. Ce travail est destine a essayer de clarifier le mecanisme implique dans la phosphaturie de l'expansion du volume extracellulaire chez le chien (ECFVE) et de determiner si le temps ecoule depuis la suppression des parathyroides et l'ECFVE joue un role dans la reponse phosphaturique. Des etudes ont ete realisees chez 20 chiens bien portants, entraines, non anesthesies. L'excretion fractionnelle de phosphate (FE PO 4 ) etait de 5.4% durant les periodes de controle. Apres expansion avec du solute sale isotonique FE PO 4 a augmente jusqu'a 31.6% et l'excretion fractionnelle du sodium (FE Na ) jusqu'a 10.9%. Les concentrations plasmatiques de Ca et Mg ont diminue de 1 et 0.5 mg/100 ml respectivement. Quand ces etudes ont ete repetees 4 a 6 jours apres thyroparathyroidectomie (T-PTX), l'importance de la phosphaturie a diminue et FE PO 4 a augmente de 3 a 12%. Cependant quand le meme protocole a ete realise chez des chiens aux parathyroides intactes mais auxquels du Ca ou du Mg etait perfuse pendant l'expansion, aucun changement de FE PO 4 n'a ete observe. Afin d'eclaircir la raison de la difference dans les resultats obtenus par la suppression de la secretion de PTH par le Ca ou le Mg d'une part et ceux observes plusieurs jours apres T-PTX, des experiences aigues ont ete realisees 2 a 4 heures apres T-PTX. Dans cette situation l'expansion n'a produit aucune modification significative de FE PO 4 malgre la presence d'une natriurese importante. Ces resultats suggerent l'existence d'un systeme regulateur de l'excretion des phosphates autre que la PTH.