Abstract 52: Enhanced Phosphodiesterase 5 (PDE5) in Thick Ascending Limbs of Dahl Salt Sensitive Rats Blunts NO-induced Inhibition of Transport
In normal rats, NO inhibits thick ascending limb (TAL) NaCl reabsorption via cGMP. In Dahl Salt Sensitive (SS) rats NO fails to decrease TAL NaCl absorption. However, the mechanism for this defect is unknown. NO stimulates soluble guanylyl cyclase to produce cGMP. The intracellular concentrations of cGMP and its effect are limited by PDE5, a cGMP-specific phosphodiesterase. We hypothesize that enhanced PDE5 expression in SS TALs blunts the inhibitory effect of NO on transport by decreasing cGMP concentration. We isolated TALs from SS and Dahl Salt-Resistant (SR) rats fed normal salt and measured PDE5 expression by Western blot. PDE5 expression was 210±20% higher in SS TALs compared to SR (p<0.05). We then studied whether PDE5 inhibition with vardenafil (25 nM) enhances the inhibitory effect of NO on transport by measuring Na transport-related oxygen (O2) consumption in TALs. In SR TALs, the NO donor spermine-NONOate (NO) decreased O2 consumption by 7.9±2.3% (p<0.05). Adding vardenafil to the bath further decreased O2 consumption by 7.7±2.3% (p<0.05). In contrast, in SS TALs NO failed to decrease oxygen consumption (5±2.5% from baseline). However in the presence of a PDE5 inhibitor NO, inhibited oxygen consumption by 15.5±2.5 % (p<0.05, n=7). We then tested whether NO-stimulated cGMP production is decreased in SS TALs. To measure production, we inhibited all phosphodiesterases with IBMX. NO enhanced cGMP production in both SS and SR TALs, but production was higher in SS than SRs (delta SS: 4.0±1.0 vs SR: 1.9±0.6 fmoles/μg, p<0.05). Finally we tested whether the decreased effect of NO in SS TALs is related to enhanced cGMP degradation by PDE5. During PDE5 inhibition NO-stimulated cGMP levels were 4-fold greater in SS than SR TALs (Δ SS: 1.9±0.7 vs SR: 0.4±0.2 fmoles/μg, p<0.05). We concluded that: a) NO-induced inhibition of transport is blunted in SS TALs due to abnormally enhanced PDE5 expression and cGMP catabolism; and b) the blunted transport response to NO in SS TALs is not caused by decreased cGMP production. PDE5 inhibition in TALs could provide protection against salt-sensitive hypertension.
- © 2012 by American Heart Association, Inc.