Hypoxia decreases calcium influx into rat proximal tubules

Abstract

Renal ischemia results in adenosine triphosphate (ATP) depletion, particularly in cells of the proximal tubule (PT), which rely heavily on oxidative phosphorylation for energy supply. Lack of ATP leads to a disturbance in intracellular homeostasis of Na+, K+ and Cl-. Also, cytosolic Ca2+ levels in renal PTs may increase during hypoxia [1], presumably by a combination of impaired extrusion and enhanced influx [2]. However, Ca2+ influx was previously measured using radiolabeled Ca2+ and at varying partial oxygen tension [2]. We have now used to Mn2(+)-induced quenching of fura-2 fluorescence to study Ca2+ influx in individual rat PTs during normoxic and hypoxic superfusion. Normoxic Ca2+ influx was indeed reflected by the Mn2+ quenching of fura-2 fluorescence and this influx could be inhibited by the calcium entry blocker methoxyverapamil (D600; inhibition 50 +/- 2% and 35 +/- 3% for 10 and 100 mumol, respectively). La3+ completely blocked normoxic Ca2+ influx. Hypoxic superfusion or rat PTs did not induce an increase in Ca2+ influx, but reduced this influx to 79 +/- 3% of the normoxic control. We hypothesize that reducing Ca2+ influx during hypoxia provides the cell with a means to prevent cellular Ca2+ overload during ATP-depletion, where Ca2+ extrusion is limited.

Change of Genes in Calcium Transport Channels Caused by Hypoxic Stress in the Placenta, Duodenum, and Kidney of Pregnant Rats1

“…Previous studies have found that TRPV6 and CaBP-9k appear to be involved in calcium absorption [63,68,83] or reabsorption [65,84]. Similar to a previous investigation [85], we observed reduced calcium influx in the proximal tubules of the kidney after urine calcium levels were increased by hypoxia. Our results demonstrated that calcium is not reabsorbed in the kidney with hypoxic stress and is excreted in the urine.…”

ATP steal between cation pumps: a mechanism linking Na+ influx to the onset of necrotic Ca2+ overload

“…Incidentally, reduced Ca 2 þ permeability has also been reported in hypoxic kidney tubules, where it was ascribed a protective role. 26 In contrast to the permanent inactivation of the PMCA by thiol modification 27 or proteolytic cleavage, 11 the fall in PMCA activity early during stress was reversible. Resistance of the PMCA and other Ca 2 þ pumps to hydrogen peroxide has been reported earlier, 25,28 which for the PMCA is conferred by calmodulin.…”

Probenecid interferes with renal oxidative metabolism: A potential pitfall in its use as an inhibitor of drug transport

“…Probenecid has been used as a pharmacological tool in many different experimental settings next to renal transport inhibition, e.g. in measurements of intracellular pH ( Weinberg et al ., 1994 ) and free Ca 2+ ( Ruttner et al ., 1993 ; Peters et al ., 1998 ), and cell volume control ( Kanli & Terreros, 1997 ). In the first two experimental conditions probenecid was used in millimolar concentrations to retain intracellularly loaded fluorophores.…”

“…More recently, probenecid has been widely used in high concentrations (up to 10 m M ) as a pharmacological tool to retain intracellularly loaded anionic fluorophores, such as pH indicators ( Weinberg et al ., 1994 ) and calcium probes ( Ruttner et al ., 1993 ; Peters et al ., 1998 ). In addition to a direct interaction with transport, it may also be possible that the inhibition of dye ‘leakage’ by probenecid is mediated at a different level.…”