The Relationship to Sodium Reabsorption

hand, calcium chloride infusion to saline-loaded, normal dogs to suppress endogenous parathyroid hormone reduced proximal TF/UF phosphate without change in TF/P inulin, while both parameters remained unchanged in saline-loaded, thyroparathyroidectomized dogs after calcium infusion. An increase in proximal TF/UF phosphate associated with unchanged TF/P inulin was also demonstrated by administration of highly purified parathyroid hormone to saline-loaded, thyroparathyroidectomized dogs. It was concluded that although proximal tubule phosphate transport is generally closely related to that of sodium, the two can dissociate under certain experimental conditions, especially under the influence of parathyroid hormone. These observations also indicate that the effect of parathyroid hormone on proximal tubule phosphate transport is not solely dependent upon its effect on sodium transport. This work was presented in part at the National Meeting of the American Federation for Clinical Research in Atlantic City, New Jersey, April, 1972, and published in abst~ract form in Clin. Res. 20: 616, 1972. Received for publication 14 February 1973 and in revised form 16 July 1973. INTRODUCTION Renal tubule handling of inorganic phosphate has been studied by clearance methods (1-8) and to a limited extent by micropuncture techniques (9-14). The available information in the literature favors the concept that filtered phosphate is reabsorbed almost exclusively by the proximal tubule and excreted in the urine with little further modification (9). This has led to the use of phosphate clearance as an indicator for proximal tubule transport (15, 16). On the other hand, evidence for distal phosphate reabsorption has been reported (11). It has also been demonstrated that a correlation exists between phosphate and sodium reabsorption since the maneuvers to inhibit sodium reabsorption, such as extracellular volume expansion or administration of diuretics, lead to phosphaturia (5-8, 17). As previous micropuncture studies have shown that parathyroid hormone, a potent phosphaturic agent, inhibits proximal tubule sodium as well as phosphate reabsorption (12, 13, 18), it has been suggested that proximal tubule phosphate transport may be dependent on reabsorption of sodium in the same segment, and the dissociation in the transport of the two ions occurs only in the distal nephron, where phosphate is presumably poorly reabsorbed (12, 19). Our present micropuncture studies were undertaken to evaluate the relationship between phosphate and sodium transport in the proximal tubule in thyroparathyroidectomized (TPTX)' dogs after various maneuvers such as infusion of parathyroid extract, saline and acetazolamide to inhibit phosphate reabsorption. Also, proximal tubule phosphate transport was studied after maneuvers l Abbreviations used in this paper: FE, fractional excretion; FR, fractional reabsorption; GFR, glomerular filtration rate; P, plasma; PAH, para-aminohippurate; PTX, parathyroidectomized; TF, tubule fluid; TPTX, thyroparathyroidectomized; UF, plasma ultrafiltrate. The Journal of Clinical Investigation Volume 53 January 1974 *143-153 143 designed to change the level of parathyroid hormone in saline-loaded animals, such as infusion of calcium chloride and administration of exogenous parathyroid hormone. These experiments were intended to examine whether a dissociation between the proximal tubule transport of the two ions could be demonstrated when sodium reabsorption by this segment was already inhibited. Our results indicate that although proximal tubule phosphate transport generally parallels that of sodium, the two processes can be dissociated under certain experimental conditions, especially under the influence of parathyroid hormone. The data also suggest that a significant fraction of filtered phosphate is reabsorbed in the segment distal to the proximal tubule in the dog nephron in the absence of parathyroid hormone.