HYPOXIA DECREASES CALCIUM INFLUX INTO RAT PROXIMAL TUBULES

Renal ischemia results in adenosine triphosphate (ATP) depletion, part icularly in cells of the proximal tubule (PT), which rely heavily on o xidative phosphorylation for energy supply. Lack of ATP leads to a dis turbance in intracellular homeostasis of Na+, K+ and Cl-. Also, cytoso lic Ca2+ levels in renal PTs may increase during hypoxia [1], presumab ly by a combination of impaired extrusion and enhanced influx [2]. How ever, Ca2+ influx was previously measured using radiolabeled Ca2+ and at varying partial oxygen tension [2]. We have now used the Mn2+-induc ed 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 methoxyve rapamil (D600; inhibition 50 +/- 2% and 35 +/- 3% for 10 and 100 mu m respectively). La3+ completely blocked normoxic Ca2+ influx. Hypoxic s uperfusion of rat PTs did not induce an increase in Ca2+ influx, but r educed this influx to 79 +/- 3% of the normoxic control. We hypothesiz e that reducing Ca2+ influx during hypoxia provides the cell with a me ans to prevent cellular Ca2+ overload during ATP-depletion, when Ca2extrusion is limited.