Flow‐Stimulated NO Production is Altered in Thick Ascending Limbs of Dahl Salt‐Sensitive Rats Fed Either normal or High‐Salt Diets

Abstract

Thick ascending limbs (TALs) of the loop of Henle reabsorb 30% of the filtered NaCl, regulating extracellular fluid volume and electrolyte excretion, thereby affecting blood pressure. NO produced by NO synthase 3 (NOS3) inhibits NaCl transport by TALs and luminal flow increases NO production. Reductions in TAL NO are associated with salt‐sensitive hypertension. In TALs of Sprague‐Dawley rats, dietary salt increases NOS3 expression by day 7, and animals don't become hypertensive. We hypothesized that Dahl salt‐sensitive rat (SS) TALs produce less NO in response to luminal flow than those from Dahl‐salt resistant rats (SR) due to altered NOS3 expression. We used SS or SR fed either normal or high‐salt diets for 7 days. We measured flow‐induced NO in isolated perfused TALs and performed Western blots of renal medullary lysates. On normal‐salt, flow‐induced NO was 32 ± 2 AFU/min in SR TALs and 12 ± 2 AFU/min in SS TALs (Δ 267 ± 63 %; p<0.03; n=6). In the presence of the superoxide (O2‐)scavenger TEMPOL, flow‐induced NO was 35 ± 5 AFU/min in SR TALs and 19 ± 2 AFU/min (p<0.05; n=9) in SS TALs, indicating that NO scavenging by O2‐ plays a minor role in this difference. NOS3 expression was not different between strains fed normal salt. When animals were given a high‐salt diet, flow‐induced NO was 44 ± 10 AFU/min (n = 11) in SR TALs and 9 ± 2 AFU/min (n=8) in SS TALs (Δ 389 ± 110 %; p<0.01). NOS3 expression was 69 ± 15 % (p<0.02; n=5) higher in TALs from SR compared to SS. We conclude that on a normal‐salt diet SR TALs produce more NO in response to luminal flow than those from SS, and that factors other than NOS3 expression or increased O2‐ are involved. On a high‐salt diet NO production is further affected in SS TALs by differences in NOS3 expression.