Expression and actions of HIF prolyl-4-hydroxylase in the rat kidneys

Hypoxia inducible factor (HIF) prolyl-4-hydroxylase domain-containing proteins (PHDs) promote the degradation of HIF-1α. Because HIF-1α is highly expressed in the renal medulla and HIF-1α-targeted genes such as nitric oxide synthase, cyclooxygenase, and heme oxygenase are important in the regulation of renal medullary function, we hypothesized that PHD regulates HIF-1α levels in the renal medulla and, thereby, participates in the control of renal Na+ excretion. Using real-time RT-PCR, Western blot, and immunohistochemical analyses, we have demonstrated that all three isoforms of PHD, PHD1, PHD2, and PHD3, are expressed in the kidneys and that PHD2 is the most abundant isoform. Regionally, all PHDs exhibited much higher levels in renal medulla than cortex. A furosemide-induced increase in renal medullary tissue Po2 significantly decreased PHD levels in renal medulla, whereas hypoxia significantly increased mRNA levels of PHDs in cultured renal medullary interstitial cells, indicating that O2 regulates PHDs. Functionally, the PHD inhibitor l-mimosine (l-Mim, 50 mg·kg−1·day−1 ip for 2 wk) substantially upregulated HIF-1α expression in the kidneys, especially in the renal medulla, and remarkably enhanced (by >80%) the natriuretic response to renal perfusion pressure in Sprague-Dawley rats. Inhibition of HIF transcriptional activity by renal medullary transfection of HIF-1α decoy oligodeoxynucleotides attenuated l-Mim-induced enhancement of pressure natriuresis, which confirmed that HIF-1α mediated the effect of l-Mim. These results indicate that highly expressed PHDs in the renal medulla make an important contribution to the control of renal Na+ excretion through regulation of HIF-1α and its targeted genes.

Role of renal NO production in the regulation of medullary blood flow

The unique role of nitric oxide (NO) in the regulation of renal medullary function is supported by the evidence summarized in this review. The impact of reduced production of NO within the renal medulla on the delivery of blood to the medulla and on the long-term regulation of sodium excretion and blood pressure is described. It is evident that medullary NO production serves as an important counterregulatory factor to buffer vasoconstrictor hormone-induced reduction of medullary blood flow and tissue oxygen levels. When NO synthase (NOS) activity is reduced within the renal medulla, either pharmacologically or genetically [Dahl salt-sensitive (S) rats], a super sensitivity to vasoconstrictors develops with ensuing hypertension. Reduced NO production may also result from reduced cellular uptake of l-arginine in the medullary tissue, resulting in hypertension. It is concluded that NO production in the renal medulla plays a very important role in sodium and water homeostasis and the long-term control of arterial pressure.

Angiotensin II Binding to Renomedullary Interstitial Cells Is Regulated by Osmolality

Angiotensin II (Ang II) AT1A receptors are localized to renomedullary interstitial cells (RMIC) in the inner stripe of the outer medulla but not in the inner medulla. Thus, there seems to be a correlation between decreases in AT1A receptor binding to RMIC and increases in interstitial osmolality, suggesting that osmolality is important in determining Ang II binding to RMIC. Cultured RMIC were incubated in media of differing osmolalities (330, 630, 930, and 1230 mOsm/kgH2O). 125I-[Sar1, Ile8] Ang II binding to AT1A receptors on RMIC grown in hyperosmolal media (930 mOsm/kgH2O) was reduced compared with isoosmolal (330 mOsm/kgH2O) media and was progressively reduced with further increases of osmolality. Similar studies were performed using bradykinin (BK) as a control peptide. Binding of the BK receptor ligand 125I-[HPP-Hoe 140] to B2 receptors was not affected by varying osmolality of the media. Reverse transcriptase-PCR demonstrated the presence of the mRNA expression for both AT1A and B2 receptors at each osmolality. The conclusion is that osmolality modulates Ang II binding to RMIC; in these cells, this phenomenon is restricted to Ang II as BK binding is not affected. Osmolality-induced changes in Ang II binding may modulate the actions of this peptide on RMIC and provide an important mechanism by which these cells modulate renal medullary function.