Decreased phosphate reabsorption after renal transplantation: Evidence for a mechanism independent of calcium and parathyroid hormone

Decreased phosphate reabsorption after renal transplantation: Evidence for a mechanism independent of calcium and parathyroid hormone. We studied a group of 73 transplant recipients who had had stable renal function for more than 6 months after their transplant surgery. Of these 73 patients, 23 had fasting plasma phosphate concentrations less than 2.5 mg/dl (normal, 3 to 5 mg/dl). The concentration of plasma phosphates was 2.1 ± 0.1 mg/dl in the hypophosphatemic transplant-recipient group compared with 3.0 ± 0.1 mg/dl in the total transplant-recipient population. The hypophosphatemic patients had similar fractional phosphate reabsorption when compared with the total transplant-recipient population, and their renal phosphate threshold (Tm PO 4 /GFR) was reduced from 3.0 ± 0.1 mg/dl in the total population to 1.4 ± 0.2 mg/dl. Although PTH levels correlated inversely with the Tm PO 4 /GFR in the total transplant-recipient group, the hypophosphatemic patients appeared to exhibit lower renal phosphate thresholds than did normophosphatemic transplant-recipients with similar PTH levels. To further evaluate the relationship between PTH and renal phosphate handling, we performed calcium infusion and phosphate titration studies on 11 hypophosphatemic transplant-recipients. The data revealed that 5 of the 11 patients with only mildly abnormal PTH levels failed to decrease their rate of phosphate excretion, their phosphate clearances, and their fractional excretion of phosphate during calcium infusion. The 5 nonresponders to calcium infusion also exhibited smaller increments in their plasma phosphate concentration and Tm PO 4 /GFR than did the 6 patients who responded to calcium infusion with reductions in phosphate excretion rates. Before calcium infusion, cyclic 3′5′-adenosine monophosphate excretion rates were similar in both responders and nonresponders and were decreased to the same degree by the calcium infusions. These data indicate the presence of either a defect in phosphate reabsorption that is independent of the action of PTH and calcium or a heightened sensitivity to PTH with a decreased suppressibility of response to PTH. Diminution de la reabsorption de phosphate apres transplantation renale: Preuve de l'existence d'un mecanisme independant du calcium et de l'hormone parathyroidienne. Dans un groupe de 73 receveurs de reins ayant une fonction renale stable, 23 avaient une concentration plasmatique de phosphate a jeun inferieure a 2,5 mg/dl (normale, 3 a 5 mg/dl). Les phosphatemies etaient de 2,1 ±0,1 mg/dl dans le groupe hypophosphatemique et de 3,0 ± 0,1 mg/dl dans l'ensemble de la population des sujets transplantes. Les sujets hypophosphatemiques avaient la meme reabsorption fractionnelle de phosphate que l'ensemble de la population et leur seuil de phosphate (Tm PO4 /GFR) etait reduit de 3,0 ± 0,1 mg/dl dans l'ensemble de la population a 1,4 ± 0,2 mg/dl. Bien que les concentrations de PTH soient inversement correlees avec le Tm PO4 /GFR dans l'ensemble du groupe des transplantes, les receveurs hypophosphatemiques paraissent avoir des seuils renaux de phosphate inferieurs a ceux des receveurs normophosphatemiques pour des niveaux semblables de PTH. Afin de mieux evaluer la relation entre PTH et comportement renal du phosphate, 11 receveurs hypophosphatemiques ont subi une perfusion de calcium et une titration de phosphate. Les resultats montrent que 5 parmi 11 malades dont les valeurs de PTH etaient seulement moderement elevees n'ont pas diminue leur debit d'excretion de phosphate, leur clearance de phosphate et leur excretion fractionnelle de phosphate au cours de la perfusion de calcium. Les 5 sujets qui n'ont pas repondu a la perfusion de calcium avaient aussi des augmentations plus faibles de la phosphatemie et de Tm PO4 /GFR que celles des 6 malades qui respondaient a la perfusion de calcium par une diminution du debit d'excretion du phosphate. Les valeurs d'excretion du 3′5′-cAMP etaient semblables dans les deux groupes de sujets et etaient abaissees de la meme facon par les perfusions de calcium. Ces resultats indiquent l'existence d'un deficit de la reabsorption de phosphate independant de la PTH et du calcium ou bien d'une augmentation de la sensibilite a la PTH avec une diminution de la possibilite de supprimer la reponse a la PTH.