Effect of divalent cation ionophore (A 23187) on renal handling of phosphorus

The in vivo and in vitro effect of calmodulin antagonists on the renal actions of 25(OH) vitamin D3 in the rat

Previous work from this laboratory has demonstrated that 25(OH) vitamin D3 [25(OH)D3] acutely suppresses the phosphaturic action of parathyroid hormone (PTH) and interferes with the PTH-induced activation of adenylate cyclase (AC). Calmodulin inhibitors block vitamin D-induced Ca2+ transport in the gut and phosphorus uptake in renal BBMV's. We have examined whether calmodulin antagonists affect the renal action of 25(OH)D3. Acute clearance experiments were performed in PTH-infused parathyroidectomized rats receiving 25(OH)D3 after pretreatment with trifluoperazine (TFP) or promethazine (P). In vitro PTH-induced activation of renal AC was also studied in membrane preparations from pretreated rats in the presence of 25(OH)D3. 25(OH)D3 reduced the PTH-stimulated increase in fractional excretion of phosphorus (CP/CIn) from 0.292 +/- 0.024 to 0.195 +/- 0.018 (p less than 0.005) and urinary cAMP from 149.3 +/- 20.3 to 78.1 +/- 10.4 pmol/min (p less than 0.01) and also blunted AC activation in vitro. TFP but not P abolished the effects of 25(OH)D3 both in vivo and in vitro. R 24571 also abolished the in vitro effect of 25(OH)D3. Thus, (1) TFP abolishes both the antiphosphaturic and the AC/cAMP-related actions of 25(OH)D3, (2) P does not have these effects, and (3) R 24571 abolishes the in vitro effect of 25(OH)D3. These results suggest that the antiphosphaturic effect of 25(OH)D3 acting via the AC/cAMP system may be calmodulin dependent.

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.

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 II. Effect of extracellular Ca2+ and application of the Ca2+ ionophore A 23187 in chronic PTX animals

Proximal inorganic phosphate (Pi) transport was evaluated using the standing droplet method with simultaneous microperfusion of the peritubular blood capillaries. In chronic parathyroidectomized (PTX) rats addition of 3 micron of the Ca2+ ionophore A 23187 to the luminal perfusate had no effect on the Pi transport, although the isotonic fluid reabsorption was reduced by 20%. When the Ca2+ concentration in the perfusates was raised from 1.5mM to 3.0mM the the reabsorption did not change significantly. But when Ca2+ was omitted from the perfusates the Pi reabsorption dropped by 19%, and when 2mM EDTA were added to the perfusates Pi transport decreased by 35%. The influx of Pi from the interstitial space and from the cell into the phosphate-free luminal perfusate did not change, when the perfusates were Ca2+ -free, but it increased by 23% in the presence of 2mM EDTA. The data indicate that 1. a rise in intracellular Ca2+ above normal is not a factor which modifies "basal" Pi transport i.e. when Pi transport is independent of the action of parathyroid hormone. 2. A reduction of extracellular Ca2+ concentration from normal toward zero reduces Pi transport without changing the paracellular leak permeability for Pi. 3. With EDTA the the paracellular leak permeability for Pi is increased, thus causing an even greater reduction in net Pi transport than with Ca2+ -free solutions alone.