Cortical tubules of the mouse increase nitric oxide production when stressed with sodium ions

Abstract

Renal cortex tubules contain eNOS that may be used to generate nitric oxide (NO) when cells are challenged. We propose sodium stress could lead to increased NO production through a calcium linked mechanism. NO was measured with microelectrodes at 400 or 600 mOsm NaCl in the tissue bath over 150–200 um slices of mouse kidney. Resting NO of cortical tubules was 350–400 nM compared to 75–120 nM for papilla tubules. 400 mOsm NaCl caused a transient increase in NO of 39% and a larger transient increase of 124% in NO during isotonic recovery. 600 mOsm NaCl caused a sustained increase in NO of 265% with a transient overshoot of 308% during isotonic recovery. Tubules of the papilla responded minimally to hypertonic NaCl. To separate the effect of hyperosmolarity from sodium ion stress, the cortical tubules were exposed to media made hypertonic to 400 and 600 mOsm with mannitol. The NO response to mannitol was about one-fourth that of NaCl hyperosmolarity. To determine the mechanism linking sodium stress to increased NO production, we investigated if intracellular sodium used in osmotic equilibration was exchanged for calcium ions using the sodium/calcium exchanger blocker KB-R7943. After blockade, the basal NO was reduced to 57% of control and the NO response to 600 mOsm NaCl was about 20% of normal. The data indicate sodium accumulation by cortical tubules can initiate a large increase in NO generation as sodium ions are exchanged for calcium ions. With 400 mOsm NaCl, the NO response is transient as the cell adapts to hyperosmolarity. As isotonicity is restored, a large transient increase in reflects removal of sodium ions used for osmotic equilibration for a transient increase in intracellular calcium that activates NO production. (Supported by NIH Grant HL-20605 and American Heart Association Grant-In-Aid)