Concerted actions of renal endothelial and macula densa NO systems in the maintenance of extracellular fluid volume

Saline-induced natriuresis and renal blood flow in conscious dogs: effects of sodium infusion rate and concentration

AIM

This study focused on static and dynamic changes in total renal blood flow (RBF) during volume expansion and tested whether a change in RBF characteristics is a necessary effector mechanism in saline-induced natriuresis.

METHODS

The aortic flow subtraction technique was used to measure RBF continuously. Identical amounts of NaCl (2.4 mmol kg(-1)) were given as slow isotonic (Iso, 120 min), slow hypertonic (Hyper, 120 min), and rapid isotonic loads (IsoRapid, 30 min).

RESULTS

During Iso and IsoRapid, arterial blood pressure increased slightly (6-7 mmHg), and during Hyper it remained unchanged. Iso and Hyper increased sodium excretion (4 +/- 1 to 57 +/- 27 and 10 +/- 4 to 79 +/- 28 micromol min(-1), respectively) and decreased plasma renin activity (by 38% and 29%), angiotensin II (by 56% and 58%) and aldosterone (by 47% and 65%), while RBF remained unchanged. IsoRapid caused a similar increase in sodium excretion (to 72 +/- 19 micromol min(-1)), a similar decrease in renin system activity, but a 15% elevation of RBF (282 +/- 22 to 324 +/- 35 mL min(-1)). Selected frequency domain parameters of RBF autoregulation did not change in response to any load.

CONCLUSIONS

In response to slow saline loading simulating daily sodium intake, the rate of sodium excretion may increase 10-20-fold without any change in mean arterial blood pressure or in RBF. Regulatory responses to changes in total body NaCl levels appears, therefore, to be mediated primarily by neurohumoral mechanisms and may occur independent of changes in arterial pressure or RBF.

Perfusate composition influences nitric oxide homeostasis in rat juxtamedullary afferent arterioles

AIM

Vascular diameters in isolated juxtamedullary nephron preparations perfused with cell-free solutions differ from those perfused with blood. In the present study, the effects of the albumin content of the perfusate on the afferent arteriolar diameter and endothelial nitric oxide were investigated in the isolated juxtamedullary nephron preparation perfused with Krebs-Ringer-bicarbonate buffer containing albumin in different concentrations.

METHODS

The endothelium was loaded with DAF-FM DA, a nitric oxide-sensitive fluoroprobe. Perfusion was maintained either with 4% (control group), 10 or 20% albumin in the perfusate or with L-NAME (10-4 m) added to the perfusate. Fluorescent images were obtained and stored for evaluation of DAF-FM fluorescence and vascular diameters (in mid-afferent arterioles) immediately before perfusate change and every 15 min thereafter, for a 2-h period.

RESULTS

Increasing the albumin concentration resulted in a decrease in fluorescence. The most rapid decline of fluorescence was obtained following L-NAME administration (relative fluorescence after 2 h: 4% albumin 92.4 +/- 5.3%; 10% albumin 79.5 +/- 4.2%; 20% albumin 66.2 +/- 2.6%; L-NAME 55.4 +/- 3.0%; mean +/- SD, n = 5). A dose-dependent constriction of the afferent arterioles was observed (normalized diameter: 4% albumin 99.8 +/- 3.0%; 10% albumin 80.3 +/- 3.3%; 20% albumin 74.3 +/- 3.2%; L-NAME 70.6 +/- 3.5%).

CONCLUSION

We propose that albumin interferes with arteriolar nitric oxide homeostasis, probably by scavenging nitric oxide intra-luminally. In this respect, albumin acts similarly to red blood cells in the circulation. The magnitude of the scavenging determines the effectiveness of autoregulation in the perfused preglomerular vessels. The scavenging properties of the perfusing fluid are important in setting operating levels of endothelial nitric oxide.

Diminished expression of constitutive nitric oxide synthases in the kidney of spontaneously hypertensive rat

In kidney, nitric oxide (NO) synthesized by nitric oxide synthase (NOS) regulates sodium and water excretion, and renal medullary blood flow. The expression of constitutive NOS, endothelial NOS (eNOS) and neuronal NOS (nNOS), were assessed in kidney of the spontaneously hypertensive rat (SHR) and the normotensive Wistar Kyoto (WKY) rat by Western blot analysis and immunocytochemistry. Neuronal NOS expression was observed in the cortex and eNOS was detected only in theinner medulla of both WKY and SHR. In SHR, expression of eNOS was attenuated to 35.1 +/- 10.8%, while expression of nNOS was only 57.5 +/- 5.7% of the levels seen in WKY rat. Immunocytochemical studies revealed decreased staining of nNOS in the macula densa, collecting ducts and in the glomerulus of SHR compared to WKY rat. Endothelial NOS immunoreactivity was restricted to vascular structures of the inner intima cells and smooth muscle cells, and was markedly reduced in the vasculature of SHR. The decreased renal blood flow observed in SHR may be linked to a diminished expression of eNOS and nNOS, underscoring the importance of these enzymes in the pathophysiology and maintenance of genetic hypertension.

Expression of endothelial nitric oxide synthase in the postnatal developing porcine kidney

The postnatal pattern of renal endothelial nitric oxide synthase (eNOS) is unknown. The purpose of this study was to characterize eNOS expression during maturation and compare this to neuronal NOS (nNOS). The experiments measured whole kidney eNOS mRNA expression by RT-PCR and protein content by Western blot, as well as cortical and medullary protein content in piglets at selected postnatal ages and in adult pigs. Whole kidney eNOS mRNA was compared with nNOS. Whole kidney eNOS expression decreased from the newborn to its lowest at 7 days, returning by 14 days to adult levels. This eNOS mRNA pattern contrasted with nNOS, which was highest at birth, and progressively decreased to its lowest level in the adult. At birth, cortical eNOS protein was greater than medullary, contrasting with the adult pattern of equivalent levels. In conclusion eNOS is developmentally regulated during early renal maturation and may critically participate in renal function during this period. The eNOS developmental pattern differs from nNOS, suggesting that these isoforms may have different regulatory factors and functional contributions in the postnatal kidney.