Site of renal phosphate reabsorption

Using modified microinfusion and free flow micropuncture techniques in the same intact and acutely thyroparathyroidectomized (TPTX) Munich-Wistar rats the nephron sites for phosphate reabsorption were reinvestigated. In intact animals, 62% of filtered phosphate was reabsorbed in the proximal tubule but none in the loop of Henle, here defined as the nephron segment between the last accessible proximal and the first distal convolution. Delivery of phosphate to the superficial distal tubule significantly exceeded urinary phosphate excretion but no phosphate reabsorption could be detected in the terminal nephron by distal microinfusions of radioactively labelled phosphate (32P). In TPTX rats, proximal phosphate reabsorption was enhanced and there was marked phosphate reabsorption in the loop of Henle. Similarly, 32P microinfused in the late proximal tubule was almost completely reabsorbed. Again, no phosphate tracer outflux was detected after distal microinfusion. It is concluded that phosphate reabsorption is confined to the proximal tubule and the loop of Henle.     

Effect of parathyroid hormone secretion on sodium reabsorption by the proximal tubule.

To determine if an increase in the endogenous secretion of parathyroid hormone could decrease sodium reabsorption by the proximal tubule, the ionized calcium concentration of blood perfusing the parathyroid gland of eight unilaterally thyroid parathyroidectomized dogs (TPTX) was reduced by infusion of an isotonic sodium citrate plus sodium chloride solution into the blood supply of the parathyroid gland. The fractional clearance of phosphate increased significantly (+9.3 +/- 2.8 ml/min per 100 ml GFR), while fractional sodium reabsorption by the proximal tubule decreased (-.06 +/- .02; P less than .025). In seven normal control dogs that received isotonic sodium chloride infusion, neither fractional sodium reabsorption by the proximal tubule nor the fractional clearance of phosphate was significantly altered. In five bilaterally TPTX dogs that received a sodium citrate plus sodium chloride infusion, sodium reabsorption by the proximal tubule was not significantly altered. There were no significant changes in glomerular filtration rate or renal plasma flow in any of these groups. The data demonstrate that alterations in endogenous parathyroid hormone secretion can play a significant role in the regulation of sodium reabsorption by the proximal tubule.     

Absence of estimated glomerular pressure autoregulation during interrupted distal delivery.

Micropuncture studies in dogs have suggested that a distal tubule-to-afferent arteriole feedback system may participate in the autoregulation mechanism at the single-nephron level. To evaluate the effect of interrupted distal delivery on glomerular capillary pressure (GP) and its autoregulation, the proximal tubule was blocked with oil and maximal stop-flow pressure was measured with a micropressure servo-null system. The GP was estimated from the sum of stop-flow pressure and the plasma colloid osmotic pressure (membrane oncometer). In 18 dogs given a mild mannitol load, average +/- SD control arterial pressure was 118 +/- 16 mmHg, proximal tubule pressure was 24 +/- 5 mmHg, and estimated GP averaged 70 +/- 10 mmHg. There was a highly significant relationship between estimated GP and arterial blood pressure. Similar results were obtained in hydropenic dogs. In response to decreases in renal arterial pressure in individual dogs, stop-flow pressure and estimated GP failed to exhibit autoregulation although autoregulation of renal blood flow, GFR, and proximal tubule pressure was observed over an arterial pressure range of 150-95 mmHg. These results indicate that interruption of normal distal delivery by proximal tubule blockage interferes with the ability of the nephron to autoregulate glomerular pressure. They provide further evidence in support of the concept that a distal tubular feedback mechanism participates, at least in part, in the autoregulatory control of glomerular pressure.     

Effects of parathyroid hormone on electrolyte transport in the hamster nephron.

Recollection micropuncture and clearance studies were carried out on thyroparathyroidectomized hamsters to clarify the localization of the effects of parathyroid hormone (PTH) on renal electrolyte transport. The clearance data confirmed that PTH inhibits phosphate and enhances calcium and magnesium reabsorption. These effects appeared to result from actions of the hormone in several parts of the nephron. In the proximal tubule PTH did not affect H2O reabsorption but inhibited phosphate reabsorption ((TF/P)PO4 increased from 0.46 +/- 0.04 to 0.57 +/- 0.03, P less than 0.02) and appeared to enhance calcium and magnesium reabsorption ((TF/UF)Ca decreased from 1.41 +/- 0.07 to 1.25 +/- 0.06, P less than 0.001, and (TF/UF)Mg from 1.66 +/- 0.10 to 1.51 +/- 0.08, P less than 0.05; in control animals (TF/UF)Ca increased from 1.51 +/- 0.10 to 1.65 +/- 0.11, P less than 0.01). PTH further inhibited phosphate reabsorption and enhanced calcium and magnesium reabsorption between the late proximal and early distal sites of puncture. Comparison of fractional deliveries of calcium and magnesium from the late distal tubule with their fractional excretions suggests an additional effect beyond the distal puncture site. The phosphaturic, but not the calcium- and magnesium-retaining, effects of PTH were abolished by a 16-h fast.     

Chronic thyroparathyroidectomy and tubular handling of phosphate

The fractional reabsorption (FR) of inorganic phosphate (Pi) along the proximal tubule depends upon both the filtered load of Pi (FLPi) and the tubular reabsorptive capacity of the Pi transporting system. To assess the actual effect of parathyroid hormone on the reabsorptive capacity only, the influence of Pi load has to be eliminated. In this study FRPi was determined by free-flow micropuncture along superficial nephrons of chronically (48 h) thyroparathyroidectomized (TPTX) and pair-fed sham-operated (SHAM) rats at identical FLPi [TPTX 3.07±0.14 (n=26) and SHAM 3.07±0.11 (n=26) mol/min±SEM]. The micropuncture results indicate that in the ranges of tubular fluid over plasma inulin concentration [TF/P)_In] 1.00–1.49 and 1.50–1.99, no difference in FRPi between TPTX and SHAM could be detected. It is only between a TF/P_In of 2.0 and 2.49 that chronic TPTX resulted in a significant increase in FRPi. Accordingly the present study indicates that chronic TPTX increases FRPi in late but not in early portions of proximal tubule. Thus in the early proximal tubule the tubular reabsorptive capacity of the Pi transporting system appears to be unaffected by chronic removal of the parathyroid glands. From this result it can be inferred that the increased plasma concentration of Pi which follows the removal of the parathyroid glands, particularly in the chronic stage, will lead to an apparently paradoxical decrease in FRPi in early proximal tubule as a mere consequence of the increased filtered load of Pi.     

Distal site of calcium reabsorption in the rat nephron

The sites of outflux of⁴⁵Ca along the nephron were investigated using microinfusion technique in acutely thyroparathyroidectomized (TPTX), intact and TPTX Wistar rats substituted with parathyroid hormone (PTH). In all three groups⁴⁵Ca outflux occurred along the proximal tubule, the loop of Henle and along the distal tubule. After microinfusion into late distal tubules⁴⁵Ca recovery in ipsilateral urine was essentially complete for the TPTX group but was only 83 and 65% for the intact and the PTH substituted animals. Increases in microinfusion flow rate from 2–20 nl/min into early and middle distal tubules resulted in increased urinary recovery of⁴⁵Ca for all three groups. Similarily, increases in microinfusate Ca concentration from 0.2–2.0 and 5.0 mmol/l resulted in increased fractional urinary recovery of⁴⁵Ca both in the presence and absence of PTH. When⁴⁵Ca and³H inulin containing solutions were continuously microinfused into early distal tubules for one hour periods an anticalciuric effect of PTH could be demonstrated. It is concluded that Ca, in addition to its outflux in the proximal tubule and in the loop of Henle, is reabsorbed in the distal tubule accessible to micropuncture. PTH acts anticalciuric at this latter site.     

Maleic acid-induced reabsorptive dysfunction in the proximal and distal nephron

Micropuncture and clearance studies were performed to assess reabsorptive function in the proximal and distal nephron of rats with experimental Fanconi's syndrome induced by maleic acid. Anesthetized rats were studied by free-flow micropuncture of the late proximal tubule 90-120 min after continuous intravenous administration of maleic acid, 100 mg X kg-1 X h-1. Compared with control rats, the reabsorption of sodium and phosphate was significantly reduced (P less than 0.001 and less than 0.02, respectively). Tubular fluid-to-ultrafiltrate (TF/UF) chloride concentration ratio was 1.00 +/- 0.02 compared with 1.16 +/- 0.03 (P less than 0.01) in controls, suggesting a nearly total inhibition of proximal bicarbonate reabsorption. Whole kidney fractional excretions of sodium and chloride were increased significantly (P less than 0.02) but could not be explained by enhanced delivery of these solutes out of the late proximal tubule. To assess whether distal nephron reabsorption of sodium and chloride were inhibited by maleic acid, clearance studies were performed during water diuresis in awake rats. During maleic acid administration, 200 mg X kg-1 X h-1, urine flow rate (P less than 0.02) and the fractional excretions of sodium and chloride (P less than 0.001) increased significantly, but fractional free water clearance decreased from 7.16 +/- 0.42 to 4.03 +/- 0.68% (P less than 0.001). In acetazolamide-treated control rats but not in maleic acid-treated rats with similar bicarbonaturia, the magnitude of fractional free water clearance closely approximated the simultaneously measured fractional distal delivery of chloride. These studies suggest that maleic acid inhibits reabsorption at a distal nephron site or sites as well as in the proximal tubule.     

Effect of volume expansion on renal glucose transport in normal and uremic dogs.

Proximal and distal tubule micropuncture studies were performed in normal and uremic remnant-kidney dogs to examine the tubule mechanism of glucose reabsorption before and after 10% extracellular volume expansion. In normal dogs volume expansion markedly inhibited glucose reabsorption in the proximal convoluted tubule, but the ensuing increase in further distal glucose delivery was nearly completely reabsorbed in the intermediate segment (between late proximal tubule and distal tubule). In the uremic, remnant-kidney dogs, glomerulotubular balance for glucose was well maintained in the proximal convoluted tubule despite an adaptive increase in nephron filtration rate. Volume expansion markedly increased glucose delivery out of the proximal convoluted tubule and an incomplete glucose reabsorption in the intermediate segment led to glycosuria. When glucose delivery to the intermediate segment was increased to a comparable degree by subthreshold glucose loading in hydropenic normal dogs, glucose reabsorption in this segment was virtually complete, suggesting that in the volume-expanded uremic dogs glucose reabsorptive capacity in the intermediate segment was reduced. Thus, the intermediate segment appears to play a significant role in the fine regulation of urinary glucose excretion.     

Phosphate transport along the inner medullary collecting duct of the rat.

The question of phosphate transport along the collecting duct remains controversial inasmuch as no data from direct in vivo evaluation of this nephron segment have been reported. We measured net phosphate transport along the inner medullary collecting duct (IMCD) using the collecting duct microcatheterization technique in five groups of rats. In control rats no net phosphate transport was found and 9.4% of the fraction of filtered phosphate (FFP) entered the IMCD and was excreted. After acute thyroparathyroidectomy (TPTX) there was a striking reduction in the FFP entering the IMCD, 1.8%, and significant reabsorption occurred, 0.5% being excreted. With acute TPTX and parathormone infusion, delivery increased to 33% without significant change along the IMCD. With acute TPTX and phosphate infusion, delivery was increased to control levels but no change was found in net phosphate transport. In rats studied 5-7 days after uninephrectomy alone, phosphate delivery was greater than in control, 25%, and no net phosphate transport was found. These studies demonstrate that phosphate absorption occurs along the IMCD in acutely TPTX rats when the delivery of phosphate to the IMCD is markedly reduced. The increase in phosphaturia which occurs after a reduction in renal mass cannot be accounted for by changes in net phosphate transport along the IMCD.     

Interrelationship of chlorothiazide and parathyroid hormone: a micropuncture study.

Sodium and calcium are normally reabsorbed in parallel in the renal tubule. Both parathyroid hormone (PTH) and thiazide diuretics may influence this relationship. This study was designed to show whether the dissociation of Na from Ca transport produced by thiazides is dependent upon the presence of PTH. Hydropenic thyroparathyroidectomized (TPTX) dogs were given chlorothiazide alone and together with PTH. Chlorothiazide alone significantly increased fractional excretion of sodium (0.5 +/- 0.3-5.6 +/- 0.3%) and calcium (0.74 +/- 0.18-1.4 +/- 0.24%). However, the Ca/Na excretion ratio fell markedly from 1.57 to 0.24%. Micropuncture revealed this dissociation to occur at the distal tubule. Proximal reabsorption of water, sodium, and calcium were inhibited to an equal extent. However, distal fractional sodium reabsorption fell 10% whereas calcium reabsorption remained unchanged following chlorothiazide administration in TPTX animals. When phosphaturic doses of PTH were administered with chlorothiazide, no significant changes were observed in calcium or sodium reabsorption. It is concluded that PTH plays no role in the dissociation of sodium from calcium reabsorption resulting from acute chlorothiazide administration.     

A micropuncture study of urate excretion byCebus monkeys employing high performance liquid chromatography with amperometric detection of urate

Net renal reabsorption of endogenous urate was studied by the micropuncture technique inCebus monkeys in the absence of osmotic diuresis. Most of filtered urate (more than 70%) was reabsorbed in the proximal convoluted tubules. Samples from early distal tubules contained 9% of filtered urate; approximately 18% being reabsorbed between the late proximal and early distal segments. There was no detectable reabsorption along the distal tubule. Fractional delivery of urate to late distal tubules was greater than fractional excretion, implying reabsorption of some 4% of filtered urate in the collecting system. However, we cannot exclude nephron heterogeneity as the cause of the difference.The foregoing results were obtained using the method of Pachla and Kissinger for the determination of urate. Urate is separated by high performance liquid chromatography and detected by an amperometric technique. We found the method to be sufficiently sensitive, precise and specific for renal micropuncture samples.     

Effects of antidiuretic hormone on electrolyte reabsorption and secretion in distal tubules of rat kidney

The effects of (1-desamino-8-d-arginine) vasopressin (dDAVP) on water and electrolyte transport in the distal tubule were investigated by micropuncture. Since, in addition to antidiuretic hormone, parathyroid hormone, calcitonin and glucagon stimulate the adenylate-cyclase system in this nephron segment, experiments were performed on hormone-deprived rats, i.e. homozygous DI Brattleboro rats with reduced levels of endogenous parathyroid hormone, calcitonin and glucagon. Along the distal tubule, dDAVP enhanced water, Cl, Na and Ca reabsorption and sharply increased net K secretion. Phosphate transport was left unchanged and Mg reabsorption was not significantly altered by dDAVP between the early and late distal tubule. Antidiuretic hormone also slightly increased water filtration rate in the superficial nephron, which rose in proportion to whole kidney glomerular filtration rate. It is concluded that, in rats: 1) antidiuretic hormone stimulates water, NaCl and Ca absorption and enhances K secretion along the distal tubule and 2) the tubular effects of dDAVP on electrolyte transport in the loop and distal tubule are responsible for decreasing Mg and Ca urinary excretion.     

Amino acid reabsorption in the rat nephron

The concentration of nine endogenous free L-alpha-amino acids (ALA, LEU, ILE, PHE, TYR, LYS, GLU, PRO, GLY) and of taurine were determined simultaneously along the nephron of the rat kidney using free-flow micropuncture techniques without altering plasma amino acid concentration or kidney function. The amount of each amino acid was determined after dansylation (14C-labelled dansyl-chloride) in the micropuncture sample followed by thinlayer chromatography. The main site of reabsorption is the proximal tubule. After 15-20% of the proximal tubule length the bulk of reabsorption has taken place (18.9 plus or minus 3.4% S.E. of the filtered load remaining). Net reabsorption continues to a small but significant extent along the distal nephron (disal tubule and collecting duct). Reabsorption of taurine is less rapid (% remaining of filtered load at the early proximal tubule 37.0 plus or minus 4.6%). The transtubular concentration ratio of all amino acids except taurine follows a homogeneous course. Under the experimental conditions of this study no distction with respect to different systems of reabsorption "neutral", "basic", "acidic", "imino-glycine") could be made.     

Influence of extracellular fluid volume expansion on magnesium,calcium and phosphate handling along the rat nephron

Summary Renal tubular handling of P, Ca, Mg and Na was studied in the rat both before and during mild hypertonic NaCl loading (ECVE), using micropuncture and clearance techniques and electron microprobe analysis. Micropuncture was performed at the late proximal and early distal tubule sites. ECVE significantly increased the urinary output of all four elements. In the case of Mg, the increase was relatively small and dependend of reabsorption all along the entire length of the nephron. For Ca, it depended on the inhibition of proximal reabsorption, partially compensated by increased reabsorption along the loop. For P, it depended on proximal inhibition, no important net phosphate movement occuring in the loop during both periods. Ca reabsorption was highly correlated to that of sodium along the proximal tubule and Henle's loop. This was not the case for Mg and P. In the loop, Ca and Mg reabsorption were closely related to the load delivered at the beginning of the structure. These observations are compatible with the view that tubular reabsorption of Ca and Mg is concentration rather than Tm limited, and that reabsorption of Ca, unlike that of Mg, is linked to the movements of sodium. Following ECVE, the difference between early distal and urinary deliveries increased significantly for Ca and P, but not for Mg. For phosphate, this difference accounted for by 45% of the delivery at the early distal tubule site, at variance with microinjection data obtained in the rat under similar salt loading conditions, which indicated that 17% only of the phosphate distal delivery were reabsorbed along the terminal segments. This discrepancy is discussed in terms of nephron functional heterogeneity.Collaborateur temporaire de thèse C.E.A.     

Effects of parathyroid hormone on renal tubular calcium and phosphate handling

Central to the maintenance of calcium homeostasis is the regulated reabsorption of calcium along the nephron. To this end, parathyroid hormone (PTH) is released from the parathyroid gland in response to lowered plasma calcium levels. This hormone acts through the PTH 1 receptor along the nephron to increase urinary phosphate excretion and decrease urinary calcium excretion. In the proximal tubule, PTH inhibits phosphate reabsorption by reducing the abundance of sodium phosphate cotransporters in the apical membrane. PTH likely decreases calcium reabsorption from the proximal tubule, by reducing the reabsorption of sodium, an event necessary for the paracellular movement of calcium across this segment. In the thick ascending limb (TAL), PTH increases calcium permeability and may increase the electrical driving force thereby increasing calcium reabsorption in the TAL. Finally, in the distal convolution, PTH acts to increase transcellular calcium reabsorption by increasing the activity and abundance of the apically expressed calcium channel TRPV5.     

Calcium transport in the nephron.

Ionized and complexed calcium are filtered at the glomerulus and more than 95% of the filtered load is reabsorbed along the length of the nephron. In the proximal convoluted tubule calcium is absorbed in proportion to sodium and water, suggesting a passive mechanism. The high permeability of this segment is compatible with passive transport, but evidence for active transport has been advanced. A role for Ca2+-ATPase and/or for a Ca2+/Na+ antiport has also been proposed. The straight portion of the proximal tubule appears to transport calcium actively but little is known about the mechanism and regulation of calcium absorption in this segment. Both passive and active transport of calcium in the thick ascending limb have been demonstrated, and heterogeneity in the function of medullary and cortical segments has been proposed. Definite evidence has been advanced for avid active calcium absorption in the distal convoluted tubule. Both chlorothiazide and parathyroid hormone enhance the transport of calcium in this segment. The granular portion of the collecting tubule resembles in its properties and function the distal convoluted tubule. The light portion, however, is incapable of transporting calcium. The distal tubule and collecting tubule may be the final regulators of urinary excretion of calcium but much more data are required before this view can be adopted.     

Stationary microperfusion study of phosphate reabsorption in proximal and distal nephron segments

Summary Micropuncture studies demonstrate phosphate reabsorption in proximal tubules and between the late proximal and early distal convoluted tubule accessible to micropuncture. To further define the sites of phosphate reabsorption, the stationary microperfusion technique was applied to proximal and distal nephron segments. Phosphate reabsorption was evaluated in superficial loops of proximal tubules, descending segments beyond late proximal tubules accessible to micropuncture, ascending segments up to the point of micropuncture in the distal tubule, and superficial loops of distal tubules of thyroparathyroidectomized rats. Microperfusates of 1.3 or 2.6 nl (100 mmol/l mannitol, 100 mmol/l NaCl,³²P-phosphate and³H-inulin) were injected and then withdrawn after contact times of 2–108 s. Phosphate recovery relative to that of inulin was determined. A steep exponential decline of phosphate recovery (R) with increasing contact time (t) was observed in the superficial proximal tubule and descending segments. The slopes of the logarithmic regressions (¹⁰logR)/t, ±SEM) were: –1.68±0.33 and –1.21±0.24 min^–1 in superficial proximal tubules and descending segments respectively. In contrast, no significant decline in phosphate recoveries (–0.02±0.04 and +0.11±0.10 min^–1) was apparent in the ascending segments and distal tubule. It is concluded that phosphate is reabsorbed in the proximal convoluted tubule and adjacent descending segments of the superficial nephron and that there is no significant phosphate reabsorption in distal convoluted tubules and adjacent ascending segments.Study carried out during a one year fellowship at the Mayo Clinic. Present address: Institut für Physiologie, Fritz-Pregl-Strasse 3, A-6020 Innsbruck, Austria     

Maturation of diluting capacity in loop of Henle of rat superficial nephrons

The postnatal development of renal diluting capacity was studied by free-flow micropuncture and by microdissection of single superficial loops of Henle in rats kept on a high salt diet. Total renal filtration rate, sodium absorption, total solute excretion, and systemic arterial pressure were monitored during single nephron evaluation at two maturational stages (12-15 days and 27-35 days postnatal). Nephron filtration rates were identical when measured in early distal and late proximal segments of the same nephron. Absolute fluid absorption between these sites increased by a factor of 2.5, whereas loop fractional absorption of the volume load changed from 38.1 +/- 6.9 to 49.8 +/- 4.8% (SD), while proximal volume delivery to the loop increased from 4.33 +/- 1.12 to 7.14 +/- 0.65 nl/min. Simultaneously, the osmolarity of early distal fluid (8.8% of distal length) decreased significantly from 284 + 19.8 to 180.9 +/- 18.2 mosmol/liter during maturation. This study suggests that the ability of the loop of Henle to generate hypotonic fluid is attained only gradually during ontogeny. Volume absorption in the loop of Henle increases disproportionately to the loop volume load.     

Activation of the tubuloglomerular feedback mechanism in dehydrated rats

To study the influence of the tubuloglomerular feedback control (TGF) on the regulation of glomerular filtration rate (GFR) during dehydration, micropuncture experiments were performed on surface nephrons of dehydrated rats. Dehydration was achieved by withdrawal of food and water for 24 h. The urine flow rate decreased to 1.5 μl/min (controls 2.9 μl/min) and GFR decreased in these rats to 0.80 ml/min (controls 1.22). TGF was studied by two different micropuncture procedures. With the first technique the changes in proximal stop-flow pressure in response to changes of the late proximal microperfusion rate were measured. With this technique the perfusion rate necessary to induce a half maximal stop-flow pressure response, the turning point, was also determined. An increased TGF sensitivity was found in dehydrated rats, as indicated by increased stop-flow pressure responses (35 versus 26%) and decreased turning points (16 versus 21 nl/min). With the second micropuncture technique the single nephron GFR (SNGFR) was measured at distal and proximal tubular sites, in the same nephron. Distal SNGFR was decreased during dehydration to 32.2 nl/min, versus 42.7 nl/min in controls. A significant difference between paired SNGFR measurements in the same nephron was observed during dehydration, the proximal value being 5.3 nl/min higher than the distal, whereas this difference was not seen in control rats. This finding indicates that activation of the feedback mechanism takes place to reduce SNGFR. It is concluded that the decrease in whole kidney GFR is partly caused by the observed increase in feedback activity. The present results are also in agreement with our earlier hypothesis that the hydrostatic and oncotic pressure conditions within the interstitial space surrounding the macula densa cells modulate the sensitivity of the tubuloglomerular feedback mechanism.     

Lithium administration and phosphate excretion.

The phosphaturic effect of parathyroid hormone (PTH), cyclic adenosine monophosphate (cAMP), acetazolamide (Az), and HCO3 loading was studied in normal, thyroparathyroidectomized (TPTX), and Li-treated dogs. PTH administration to normal animals markedly increased fractional excretion (F) of PO4 but had a blunted effect on FPO4 in the Li-treated animals. Cyclic AMP likewise markedly increased FPO4 in the normal animals but had a markedly blunted effect in the Li-treated animals. Az led to a significant increase in FNa, FHCO3, and FPO4 in the normal animals. In the Li-treated dogs, Az induced a significant natriuresis and bicarbonaturia but failed to increase phosphaturia. HCO3 loading in normal dogs caused a significant phosphaturia while having little effect on FPO4 in Li-treated dogs. HCO3 loading to TPTX dogs was associated with a lower FPO4 as compared to normal HCO3-loaded animals. These data suggest that Li administration not only blocks the adenyl cyclase-cAMP system in the renal cortex, but it may also interfere with a step distal to the formation of cAMP, since the phosphaturic effect of both PTH and cAMP was markedly diminished in Li-treated animals.