Nitric oxide and superoxide transport in a cross section of the rat outer medulla. II. Reciprocal interactions and tubulovascular cross talk

Abstract

In a companion study (Edwards A and Layton AT. Am J Physiol Renal Physiol. doi:10.1152/ajprenal.00680.2009), we developed a mathematical model of nitric oxide (NO), superoxide (O(2)(-)), and total peroxynitrite (ONOO) transport in mid-outer stripe and mid-inner stripe cross sections of the rat outer medulla (OM). We examined how the three-dimensional architecture of the rat OM, together with low medullary oxygen tension (Po(2)), affects the distribution of NO, O(2)(-), and ONOO in the rat OM. In the current study, we sought to determine generation rate and permeability values that are compatible with measurements of medullary NO concentrations and to assess the importance of tubulovascular cross talk and NO-O(2)(-) interactions under physiological conditions. Our results suggest that the main determinants of NO concentrations in the rat OM are the rate of vascular and tubular NO synthesis under hypoxic conditions, and the red blood cell (RBC) permeability to NO (P(NO)(RBC)). The lower the P(NO)(RBC), the lower the amount of NO that is scavenged by hemoglobin species, and the higher the extra-erythrocyte NO concentrations. In addition, our results indicate that basal endothelial NO production acts to significantly limit NaCl reabsorption across medullary thick ascending limbs and to sustain medullary perfusion, whereas basal epithelial NO production has a smaller impact on NaCl transport and a negligible effect on vascular tone. Our model also predicts that O(2)(-) consumption by NO significantly reduces medullary O(2)(-) concentrations, but that O(2)(-) , when present at subnanomolar concentrations, has a small impact on medullary NO bioavailability.

Fig. 1.

A: baseline nitric oxide (NO) concentrations (C_NO) in the interstitium, vasa recta, capillaries, and tubules in each region (R1–R4), at the mid-inner stripe (IS). Results are shown for case C; C_NO values in cases A, B, and C are nearly indistinguishable. Each tubule or vasa recta is assigned to the region with which it is in contact for 50% or more in the IS. The term “cellular layer” denotes the endothelium in vasa recta and capillaries, and the epithelium in tubules. C_NO in red blood cells (RBC) are <1 nM and are not shown. LDV, long descending vasa recta; SDV, short descending vasa recta; LAVa and LVAb, 2 populations of long ascending vasa recta; SAVa and SAVb, 2 populations of short ascending vasa recta; LDL, long descending limb of Henle's loop; SDL, short descending limb; LAL; long ascending limb; SAL, short ascending limb; CD, collecting duct; cRBC, capillary red blood cells. B: baseline net generation (production minus consumption) rate of NO (×10⁻³ nmol·cm⁻¹·s⁻¹) in the interstitium, vasa recta, capillaries, and tubules at the mid-IS. The rates for a given type of tubule or vessel take into account the number of such tubules or vessels.

Fig. 2.

A: baseline O₂⁻ concentrations (C_O2−) in the interstitium, vasa recta, capillaries, and tubules in each region, at the mid-IS. Results are shown for case C. In case B, C_O2− is 33% lower everywhere. In case A, C_O2− is ∼50% lower in R1, and 70–85% lower in R2–R4. B: base-case O₂⁻ net generation rates (×10⁻³ nmol·cm⁻¹·s⁻¹) in the interstitium, vasa recta, capillaries, and tubules at the mid-IS (case C).

Fig. 3.

A: baseline peroxynitrite (ONOO) concentrations (C_ONOO) in the interstitium, vasa recta, capillaries, and tubules in each region, at the mid-IS. Results are shown for case C. In case B, C_ONOO is 33% lower everywhere. In case A, C_ONOO is ∼50% lower in R1, and 70–85% lower in R2–R4. B: base-case ONOO net generation rates (×10⁻³ nmol·cm⁻¹·s⁻¹) in the interstitium, vasa recta, capillaries, and tubules at the mid-IS (case C).

Fig. 4.

Effect of varying NO permeability (P_NO) on interstitial C_NO (A), C_O2− (B), and C_ONOO (C) in each region. RBC, vascular, and tubular P_NO are taken to vary together. P_NO is set to its base-case value, decreased by a factor of 5, and increased by a factor of 5. Results are shown for case C. Relative concentration increases or decreases are almost identical in cases A, B, and C.

Fig. 5.

Effect of varying the diffusivity of O₂⁻ (D_O2−) and ONOO (D_ONOO) on interstitial C_O2− (A) and C_ONOO (B). DO₂₋ and D_ONOO are set to their base-case values and decreased by a factor of 10. Results are shown for case C. Relative concentration increases or decreases are almost identical in cases A, B, and C. O₂₋ permeability reduction preserves more O₂⁻ in the epithelia and endothelia where it is produced; hence the reduction in interstitial C_O2−. ONOO permeability reduction reduces ONOO transport to RBC and its subsequent rapid scavenging by HbO₂ and peroxiredoxin 2 (Prx2), thereby raising C_ONOO elsewhere.

Fig. 6.

Effect of NO synthesis inhibition on C_NO in the interstitium, vasa recta endothelium, capillary endothelium, and tubular epithelium in each region. Results are displayed for case C; C_NO values in cases A, B, and C are nearly indistinguishable. Shown are concentrations under baseline conditions (black bars), assuming inhibition of tubular epithelial NO synthesis only (white bars, “zero epi”), and assuming inhibition of vascular endothelial NO synthesis only (grey bars, “zero endo”). Results suggest that vascular NO synthesis acts to limit Na⁺ reabsorption across mTAL, whereas tubular NO synthesis does not significantly affect vascular tone.

Fig. 7.

Effect of O₂⁻ synthesis inhibition on C_O2− in the interstitium, vasa recta endothelium, capillary endothelium, and tubular epithelium in each region. Results are displayed for case C. Relative C_O2− changes are similar in cases A, B, and C. Shown are concentrations under baseline conditions (black bars), assuming inhibition of tubular epithelial O₂⁻ synthesis only (white bars, “zero epi”), and assuming inhibition of vascular endothelial O₂⁻ synthesis only (grey bars, “zero endo”).

Fig. 8.

Effect of NO-O₂⁻ interactions on interstitial C_NO (A) and C_O2− (B) in each region. Concentrations are given for baseline conditions (black bars), in the absence of reaction between NO and O₂⁻ (white bars, “no reaction”), or if O₂⁻ generation were 10 times higher (grey bars, “O₂⁻ production × 10”). In the latter case, C_O2− would be 10 times greater than in the base case. Results correspond to case C; as described in the text, in cases A and B the relative changes would be smaller for C_NO, and identical for C_O2−. The model predicts that O₂⁻ consumption by NO significantly reduces medullary C_O2−, but that O₂⁻ has a small impact on medullary NO bioavailability under physiological conditions.

Fig. 9.

Effect of increasing rates of NO and O₂⁻ synthesis on C_NO in long and short DVR. Results are shown for baseline conditions, assuming that the NO and O₂⁻ production rates by ascending limb epithelia simultaneously increase by a factor of 2, 5, and 10 (so as to mimic the effects of ANG II), and assuming that the NO generation rate (G_NO) by tubular (i.e., descending limb, ascending limb, and collecting duct) epithelia separately increases 10-fold. Concentration values correspond to case C, and are similar in cases A and B. Results suggest that a significant, isolated elevation in epithelial G_NO affects the distribution of blood flow between the inner and outer medulla.