Modulation of Tubular pH by Acetazolamide in a Ca2+ Transport Deficient Mice Facilitates Calcium Nephrolithiasis

Proximal tubular (PT) acidosis, which alkalinizes the urinary filtrate, together with Ca2+ supersaturation in PT can induce luminal calcium phosphate (CaP) crystal formation. While such CaP crystals are known to act as a nidus for CaP/calcium oxalate (CaOx) mixed stone formation, the regulation of PT luminal Ca2+ concentration ([Ca2+]) under elevated pH and/or high [Ca2+] conditions are unknown. Since we found that transient receptor potential canonical 3 (TRPC3) knockout (KO; -/-) mice could produce mild hypercalciuria with CaP urine crystals, we alkalinized the tubular pH in TRPC3-/- mice by oral acetazolamide (0.08%) to develop mixed urinary crystals akin to clinical signs of calcium nephrolithiasis (CaNL). Our ratiometric (λ340/380) intracellular [Ca2+] measurements reveal that such alkalization not only upsurges Ca2+ influx into PT cells, but the mode of Ca2+ entry switches from receptor-operated to store-operated pathway. Electrophysiological experiments show enhanced bicarbonate related current activity in treated PT cells which may determine the stone-forming phenotypes (CaP or CaP/CaOx). Moreover, such alkalization promotes reactive oxygen species generation, and upregulation of calcification, inflammation, fibrosis, and apoptosis in PT cells, which were exacerbated in absence of TRPC3. Altogether, the pH-induced alteration of the Ca2+ signaling signature in PT cells from TRPC3 ablated mice exacerbated the pathophysiology of mixed urinary stone formation, which may aid in uncovering the downstream mechanism of CaNL.

Mechanisms of Contrast-Induced Nephropathy Reduction for Saline (NaCl) and Sodium Bicarbonate (NaHCO3)

Nephropathy following contrast media (CM) exposure is reduced by administration before, during, and after the contrast procedure of either isotonic sodium chloride solution (Saline) or isotonic sodium bicarbonate solution (IsoBicarb). The reasons for this reduction are not well established for either sodium salt; probable mechanisms are discussed in this paper. For Saline, the mechanism for the decrease in CIN is likely related primarily to the increased tubular flow rates produced by volume expansion and therefore a decreased concentration of the filtered CM during transit through the kidney tubules. Furthermore, increased tubular flow rates produce a slight increase in tubular pH resulting from a fixed acid excretion in an increased tubular volume. The mechanism for the decreased CIN associated with sodium bicarbonate includes the same mechanisms listed for Saline in addition to a renal pH effect. Increased filtered bicarbonate anion raises both tubular pH and tubular bicarbonate anion levels toward blood physiologic levels, thus providing increased buffer for reactive oxygen species (ROS) formed in the tubules as a result of exposure to CM in renal tubular fluid.

Luminal buffer transport in rat cortical tubule: relationship to potassium metabolism

Free-flow micropuncture studies were performed on superficial rat tubules and titratable acid (TA), sodium and potassium concentrations, and tubular pH were measured in control, acutely K-loaded, and chronically K-depleted animals. The proximal tubule is a key site of TA formation. Under control conditions, significant amounts of TA were lost along the loop of Henle. TA delivery (in pmol X min-1) to the late proximal tubule (LP) was 23.66 +/- 2.38 (mean +/- SE), that to the early distal tubule (ED) was 17.24 +/- 1.73, and little further loss occurred along more distally located nephron sites. In acute hyperkalemia, urinary TA excretion (in mumol X min-1) was lower than under control conditions (0.56 +/- 0.04 vs. 1.01 +/- 0.07); TA delivery to the LP (18.50 +/- 1.93) was slightly reduced compared with the controls (a change not reaching statistical significance). TA delivery to the ED was also reduced (11.54 +/- 1.04); significant amounts of TA were lost along the loop of Henle and a significant further loss occurred along the distal tubule since TA delivery to the late distal tubule (LD) was only 7.41 +/- 0.83. As luminal pH along both proximal and distal tubules in K-loaded rats was indistinguishable from that of control rats, a major factor responsible for decreased distal TA formation in K-loaded animals was diminished buffer delivery. In chronic potassium depletion urinary TA excretion was also lowered (0.30 +/- 0.06); only a moderate part of this effect was due to increased pH along the nephron. Increased buffer reabsorption along the proximal tubule (TA delivery to LP was 14.32 +/- 2.01) contributed mainly to the reduced rate of titratable acid excretion.