Phosphate transport in the proximal convolution of the rat kidney II. Effect of extracellular Ca2+ and application of the Ca2+ ionophore A 23187 in chronic PTX animals

Mechanisms of phosphate transport

Over the past 25 years, successive cloning of SLC34A1, SLC34A2 and SLC34A3, which encode the sodium-dependent inorganic phosphate (P_i) cotransport proteins 2a-2c, has facilitated the identification of molecular mechanisms that underlie the regulation of renal and intestinal P_i transport. P_i and various hormones, including parathyroid hormone and phosphatonins, such as fibroblast growth factor 23, regulate the activity of these P_i transporters through transcriptional, translational and post-translational mechanisms involving interactions with PDZ domain-containing proteins, lipid microdomains and acute trafficking of the transporters via endocytosis and exocytosis. In humans and rodents, mutations in any of the three transporters lead to dysregulation of epithelial P_i transport with effects on serum P_i levels and can cause cardiovascular and musculoskeletal damage, illustrating the importance of these transporters in the maintenance of local and systemic P_i homeostasis. Functional and structural studies have provided insights into the mechanism by which these proteins transport P_i, whereas in vivo and ex vivo cell culture studies have identified several small molecules that can modify their transport function. These small molecules represent potential new drugs to help maintain P_i homeostasis in patients with chronic kidney disease - a condition that is associated with hyperphosphataemia and severe cardiovascular and skeletal consequences.

Modulation of phosphate absorption by calcium in the rabbit proximal convoluted tubule

Proximal convoluted (S2) and straight (S3) renal tubule segments were studied to determine the effect of Ca on lumen-to-bath phosphate flux (JlbPO4). Increasing bath and perfusate Ca from 1.8 to 3.6 mM enhanced JlbPO4 from 3.3 +/- 0.7 to 6.6 +/- 0.6 pmol/mm per min in S2 segments (P less than 0.001) but had no effect in S3 segments. Decreasing bath and perfusate Ca from 1.8 to 0.2 mM reduced JlbPO4 from 3.7 +/- 0.6 to 2.2 +/- 0.6 in S2 segments. These effects were unrelated to changes in fluid absorption and transepithelial potential difference. Increasing cytosolic Ca with a Ca ionophore, inhibiting the Ca-calmodulin complex with trifluoperazine, or applying the Ca channel blocker nifedipine had no effect on JlBPO4 in S2 segments. Increasing only bath Ca from 1.8 to 3.6 mM did not significantly affect JlbPO4. However, increasing only perfusate Ca enhanced JlbPO4 from 3.4 +/- 0.7 to 6.1 +/- 0.7 pmol/mm per min (P less than 0.005). Inhibition of hydrogen ion secretion, by using a low bicarbonate, low pH perfusate, both depressed base-line JlbPO4 and abolished the stimulatory effect of raising perfusate Ca. Net phosphate efflux (JnetPO4) also increased after ambient calcium levels were raised, ruling out a significant increase in PO4 backflux. When net sodium transport was abolished by reducing the bath temperature to 24 degrees C, JnetPO4 at normal ambient calcium was reduced and increasing ambient calcium failed to increase it, ruling out a simple physicochemical reaction wherein phosphate precipitates out of solution with calcium. The present studies provide direct evidence for a stimulatory effect of Ca on sodium-dependent PO4 absorption in the proximal convoluted tubule, exerted at the luminal membrane. It is postulated that Ca modulates the affinity of the PO4 transporter for the anion.

Glomerular filtration rate and plasma pH as determinants of phosphate reabsorption

Glomerular filtration rate (GFR) was altered by varying renal perfusion pressure in volume-expanded, anesthetized dogs infused with ethacrynic acid. Phosphate reabsorption varied linearly with GFR (r greater than 0.9), 0.83 of the increase in filtered load being reabsorbed. Phosphate reabsorption at comparable filtered loads was not significantly changed by raising plasma bicarbonate concentration from 30 to 55 mM and adjusting PCO2 to keep plasma pH constant. Plasma pH was altered by inducing hyper- and hypocapnia or infusing bicarbonate. Plasma phosphate concentration varied with plasma pH before phosphate infusion and was kept constant at 3.4 +/- 0.1 mM in intact and thyroparathyroidectomized dogs; some of which were also examined during hyperchloremic acidosis. At comparable GFR, phosphate and bicarbonate reabsorption correlated (r greater than 0.9), except during acidosis when the filtered load of bicarbonate became inadequate. In all experiments phosphate reabsorption and plasma pH correlated (r greater than 0.85). Compared with control values at plasma pH 7.4, phosphate reabsorption increased by about 40% during acidosis (pH 7.1) and decreased by about 50% during alkalosis (pH 7.8) both in intact and thyroparathyroidectomized dogs. We propose that net hydrogen ion secretion is the common determinant of phosphate and bicarbonate reabsorption.

Sodium-coupled taurocholate transport in the proximal convolution of the rat kidney in vivo and in vitro

Using the standing droplet technique in the renal proximal convolution and simultaneous microperfusion of the peritubular capillaries, the zero net flux transtubular concentration difference of taurocholate (DeltaC(TC-)) at 45 s was determined as a measure of active bile acid reabsorption in vivo. Starting with 0.1 mmol/liter taurocholate in both perfusates the control DeltaC(TC-) of 0.042 mmol/liter fell to 0.006 mmol/liter (P < 0.001) when the Na(+) concentration in the perfusates was reduced to zero. Removal of bicarbonate from the perfusates to alter pH had no influence on DeltaC(TC-). When glycocholate was added to the perfusates DeltaC(TC-) was decreased, while probenecid increased DeltaC(TC-). These observations were extended by studies performed with brush border membrane vesicles derived from renal cortex. The initial (20 s) uptake of 0.01 mmol/liter taurocholate in the presence of a Na(o) (+) > Na(i) (+) gradient was stimulated twofold compared with its uptake in the absence of a Na(+) gradient. Uptake of taurocholate was osmotically and temperature sensitive. Membranes preloaded with unlabeled glycocholate showed accelerated entry of labeled taurocholate (trans-stimulation) only in the presence of Na(+). Replacement of Na(+) in the media with K(+), Li(+), and choline(+) decreased initial taurocholate uptake by 49, 53, and 62%, respectively. Stimulation of taurocholate transport by cation gradient diffusion potentials was unlikely inasmuch as the addition of valinomycin under K(+) gradient conditions had no effect. A transmembrane pH gradient (pH(o) < pH(i)) did not influence initial uptake of taurocholate. Finally, in the presence of Na(+) taurocholate transport showed cis-inhibition with unlabeled bile acids and saturation kinetics with respect to increasing taurocholate concentrations. The micropuncture and vesicle data indicate that the net transport of taurocholate in the proximal tubule is the result of an electroneutral Na(+)-taurocholate cotransport across the brush border membrane.

Bidirectional active transport of thiosulfate in the proximal convolution of the rat kidney

Using the standing droplet method in the late proximal convolution and simultaneous microperfusion of the peritubular capillaries, the zero net flux transtubular concentration difference of thiosulfate at 45 s was determined, the latter being taken as a measure of active thiosulfate transport. Under control conditions, in the presence of Na+, near zero delta c values were observed. When 1 mmol/l carinamide or paraaminohippurate (PAH) were added to the perfusates significant reabsorptive delta c arose. However, when 7.5 mmol/l sulfate was added to the Na+ containing perfusates or when the perfusates were Na+-free secretory delta c values were observed. Tested under Na+-free conditions, the secretory delta c was not influenced by simultaneously present 5 mmol/l of SO2-4 but was diminished by 50 mmol/l SO2-4. PAH (1 mmol/l), carinamide (0.2 mmol/l) and probenecid (1 mmol/l) decreased the secretory delta c by 48, 65 and 48%, respectively. The PAH secretion was not influenced, when thiosulfate or sulfate up to 50 mmol/l was added to both perfusates. Under Na+-free conditions the delta c of thiosulfate in early loops of the proximal convolution is higher than in late loops, while for PAH this pattern is reversed. Taken together with the previously published inhibition of sulfate reabsorption by thiosulfate the data indicate 1. thiosulfate is reabsorved by the Na+-dependent sulfate transport system and 2. thiosulfate is simultaneously secreted by a carinamide-, probenecid- and PAH-sensitive secretory system. The secretory system might also be shared by sulfate. The thiosulfate net flux is the result of the difference in the activity of the counteracting transporters, located at the luminal and contraluminal cell side. Is it possible that the higher activity of the transporter at one cell side leads to a reversal of the flux through the transporter at the other cell side.

Active sulfate reabsorption in the proximal convolution of the rat kidney: specificity, Na+ and HCO3- dependence

Using the standing droplet technique in the proximal convolution and simultaneous microperfusion of the peritubular capillaries, the decrease in luminal sulfate concentration with time and the zero net flux transtubular concentration difference of sulfate (delta CSO42-) at 45 s was determined - the latter being taken as a measure of the rate of active sulfate reabsorption. Starting with 0.5 mmol/l sulfate in both perfusates the delta CSO42- value of 0.35 mmol/l was approached exponentially with a half value time of 4.3 s. The delta CSO42- values in the early proximal and late proximal convolution did not deviate from each other. If the Na+ concentration in the perfusates was reduced, the delta CSO42- approached zero and extrapolated to a slightly negative value (Ci greater than Co). When 1 mmol/l ouabain was added to the perfusates delta CSO42- decreased by 66% (the latter experiments were performed in the golden hamster which is more sensitive to ouabain than the rat). 1 mmol/l thiosulfate diminished delta CSO42- by 68% and 1 mmol/l molybdate by 24%. Omitting or replacing bicarbonate by HEPES or glycodiazine reduced the sulfate reabsorption significantly, while acetazolamide (0.1 mmol/l) and increasing the CO2-pressure from 4.66 to 14.0 kPa (i.e. 5-15% CO2) had no effect. SITS 1 mmol/l had no effect on sulfate reabsorption. The data indicate that the sulfate reabsorption is driven by a Na+ gradient and inhibited by thiosulfate and molybdate, i.e. molecules which have a similar tetrahedral molecule structure. The sulfate reabsorption depends in an undefined manner on the presence of bicarbonate ions.

Influence of calcium and ionophore 23187 on tubular phosphate reabsorption

In previous studies it has been demonstrated that a decline of plasma calcium concentration accounts for the decrease of phosphate reabsorption in thyroparathyroidectomized (TPTX) rats undergoing phosphate loading. Microinfusion studies were performed in TPTX rats in order to discriminate between a systemic effect of calcium an a direct renal effect. Thyroparathyroidectomized animals were infused with a phosphate solution continuously. When plasma calcium concentration fell below 1.30 mmol/l, proximal convoluted tubules were microinfused with a phosphate tracer solution for 42 min. After 18 min a calcium chloride-containing solution was applied superficially (superfused) to the area of the microinfused tubule. This elevation of peritubular calcium concentration led to an immediate increase of phosphate reabsorption up to 12% of the microinfused phosphate load within 24 min. In another series of experiments, the calcium specific ionophore A23187--a substance which is known to increase intracellular calcium--was superfused on the microinfused tubule. This resulted again in an increase of fractional phosphate reabsorption of about 15% after 24 min. In contrast, when calcium chloride-free as well as ionophore-free solutions were superfused fractional phosphate reabsorption decreased (7%). From these data we conclude that 1. calcium has a direct renal effect on phosphate reabsorption in the absence of parathyroid hormone and 2. intracellular calcium appears to be a major parameter in the regulation of renal phosphate transport under these conditions.

Phosphate transport in the proximal convolution of the rat kidney. III. Effect of extracellular and intracellular pH

Inorganic phosphate (Pi) transport was evaluated using the standing droplet method with simultaneous microperfusion of the peritubular capillaries. To evaluate rather small differences in Pi transport and to eliminate the influence of tubular heterogeneity, the technique of crossed paired samples was applied. 1. In chronic PTX rats changing the luminal or both luminal and peritubular pH by varying the HCO-3-concentration between 4 and 50 mmol/l at constant 5% CO2 had no influence on Pi transport. 2. If, however, bicarbonate was omitted from the perfusate and 2 mmol/l phosphate (pH 7.4) was the only buffer, Pi transport was decreased from the control. It was, however, further reduced when the perfusates were gased with 5% CO2 i.e. the starting pH was 5.6. 3. When the solutions contained HEPES buffer (25 mmol/l), Pi transport at pH 8 was much larger than at pH 6.0. 4. Raising the CO2 pressure from 35 to 70 mm Hg did not change the Pi transport when both perfusates had a HCO-3-concentration of 25 mmol/l. It reduced, however, the Pi transport, when the luminal perfusate had only 4 mmol/l bicarbonate. 5. Lowering the CO2 pressure from 38 to 7.6 mm Hg did hardly change the Pi transport when the luminal perfusate contained 4 mmol/l bicarbonate. It lowered, however, the Pi transport significantly when the luminal perfusate had 2k mmol/l bicarbonate. 6. Acetazolamide, 10-4M, lowered the Pi transport when the luminal perfusate contained 4 or 25 mmol/l bicarbonate. At 4 mmol/l luminal HCO-3, raising the pCO2 to 228 mmol/l depressed Pi transport even more. At 25 mmol/l luminal bicarbonate, raising the pCO2 from 38 to 114 mm Hg reversed the acetazolamide inhibition of the Pi transport almost completely. The data indicate that luminal acidosis and intracellular alkalosis inhibits the transtubular Pi transport. A shift of the intracellular pH to a more alkaline value seems to be responsible for the inhibition of Pi transport by acetazolamide, while omission of buffer from the perfusate inhibits Pi transport by effecting an acidic luminal pH.