Renal expression of constitutive NOS and DDAH: separate effects of salt intake and angiotensin.

BACKGROUND

Nitric oxide (NO) is generated from NO synthase (NOS) isoforms. These enzymes can be inhibited by asymmetric dimethylarginine, which is inactivated by N(G)-N(G)-dimethylarginine dimethylaminohydrolase (DDAH). The neuroneal (nNOS) type I and endothelial (eNOS) type III constitutive NOS isoforms are expressed predominantly in the macula densa and microvascular endothelium of the renal cortex, respectively. DDAH is expressed at sites of NOS expression. Since NO may coordinate the renal responses to angiotensin II (Ang II) and changes in salt intake, we tested the hypothesis that salt intake regulates the expression of nNOS, eNOS and DDAH by Ang II acting on type 1 (AT(1)) receptors.

METHODS

Groups (N = 6) of rats were adapted to low-salt (LS) or high-salt (HS) intakes for 10 days. Other groups of LS and HS rats received the AT(1) receptor antagonist losartan for six days (to test the effects of salt independent of AT(1) receptors). A further group of HS rats received an infusion of Ang II for six days (to test the effect of Ang II independent of salt intake).

RESULTS

Compared with HS rats, there was a significant (P < 0.05) increase in LS rats of nNOS protein in kidney and immunohistochemical expression in the macula densa, and of eNOS protein expression and immunohistochemical expression in the microvascular endothelium, and of DDAH protein expression. Losartan prevented these effects of salt on the expression of eNOS or DDAH, both of which were also increased by Ang II infusions in HS rats. In contrast, losartan did not prevent the effects of salt on nNOS expression, which was unresponsive to Ang II infusion. The generation of NO(2)(-) released by slices of renal cortex, in the presence of saturating concentrations of L-arginine, was increased by LS, compared to HS, independent of losartan and by Ang II during HS.

CONCLUSION

The expressions of eNOS in cortical microvascular endothelium and DDAH in kidney are enhanced by Ang II acting on AT(1) receptors. The expression of nNOS in the macula densa is enhanced by salt restriction independent of Ang II or AT(1) receptors.