Dopamine receptor blockade and synthesis inhibition during exaggerated natriuresis in spontaneously hypertensive rats

Renal responses to acute volume expansion in spontaneously hypertensive rats is related to the baseline sodium excretion

Essential hypertension is associated with an exaggerated natriuresis in response to intravenous infusion of isotonic saline. We examined proximal tubular fluid output and segmental tubular handling of sodium in conscious spontaneously hypertensive rats (SHR), their normotensive counterparts Wistar-Kyoto rats (WKY), and ordinary Wistar rats using servo-controlled sodium and fluid balance and Li clearance technique. Sodium (Na) excretion rose to 2.72 ± 0.75 (by a factor of 8) and 1.73 ± 0.68 μmol/min (by a factor of 6.3) (p < 0.05) in SHR and WKY, respectively, thus confirming the presence of exaggerated natriuresis in SHR. FE(Li) rose to 34 ± 4 and 29 ± 2 (p < 0.05) and C(Na)/C(Li) rose to 3.0 ± 0.8 and 2.0 ± 0.6 (p < 0.05) in SHR and WKY, respectively, demonstrating that Na reabsorption in both the proximal and the distal nephron was involved. Additional experiments showed that giving the rats saline instead of water to drink for four days prior to the clearance measurement led to a remarkable increase in the natriuretic response to volume expansion. There was a close correlation between the peak increase in FE(Na) and the logarithmic values of the baseline FE(Na) values. In conclusion, the study confirms the presence of an exaggerated natriuresis in response to volume expansion in conscious SHR rats compared to WKY rats, and that the renal response to acute volume expansion is related to the baseline sodium excretion.

Hormonal regulation of renomedullary hyaluronan

AIM

Hyaluronan (HA) is involved in renomedullary water handling through its water-binding capacity. This study addressed the effect of hormones involved in regulating fluid-electrolyte homeostasis on renomedullary HA content in vivo and in vitro.

METHODS

The kidneys from rats treated with L-NAME, indomethacin, vasopressin (AVP) or methylprednisolone (MP) during euvolaemia or water loading were analysed for HA by RIA, ELISA and histochemical staining. HA was measured in renomedullary interstitial cells treated with AVP, angiotensin II (Ang II) or a combination of AVP and Ang II.

RESULTS

Baseline renal cortical and medullary HA content was unaffected by 2 h of intravenous treatment with L-NAME (NOS inhibitor) or indomethacin (cyclo-oxygenase inhibitor), whereas AVP reduced medullary HA by 33%. During 2 h of acute water loading, diuresis was accompanied by an increase in renomedullary HA (+45%), but cortical HA was unaffected. In both L-NAME- and indomethacin-treated animals, the water loading-induced increase in renomedullary HA was absent, indicating involvement of NO and prostaglandins. After 7 days of MP treatment, medullary HA was reduced by 40%, but the water loading-induced elevation in HA remained. In cultured renomedullary interstitial cells, AVP reduced the HA content in the supernatant by 63%, and simultaneous treatment with Ang II reduced the HA content even further (95%).

CONCLUSION

AVP reduces HA content, and NO and prostaglandins are needed for the increase in HA during water loading.

Renal dopamine and noradrenaline excretion during CNS-induced natriuresis in spontaneously hypertensive rats: influence of dietary sodium

Abnormalities in dopamine (DA) and noradrenaline (NA) activities and sodium handling may be involved in the pathogenesis of hypertension. The present study was designed to investigate whether any differences exist between normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) in urinary excretion of DA, NA and sodium after 15 weeks on a low, medium or high sodium diet and during a subsequent elevation of the cerebroventricular fluid sodium concentration (CNS-induced natriuresis). Seven features were noted: (1) Basal sodium and DA excretion after the diet regimen was correlated to the dietary sodium content in both strains, except that sodium and DA excretion in SHR showed no further increase after the high sodium diet over and above that after medium sodium diet. (2) For any given sodium diet, SHR excreted more DA and NA as compared with WKY. (3) Blood pressure in SHR, as opposed to that in WKY, was higher after medium and high sodium diet than after low sodium diet. (4) During CNS-induced natriuresis NA excretion decreased or remained unchanged in WKY, but increased in SHR. (5) The DA/NA excretion ratio during CNS-induced natriuresis increased in WKY while decreased in SHR, which would not favour a natriuretic/vasodilatory response in the latter. (6) The ability of SHR to respond with CNS-induced natriuresis was attenuated after high sodium diet. (7) The magnitude of CNS-induced natriuresis was in both strains correlated to the sodium diet; the higher the dietary sodium content, the greater the natriuretic response. In conclusion, the study shows some clear differences in the catecholamine and sodium handling between WKY and SHR which may be involved in the pathogenesis of hypertension in SHR. Furthermore, increased sodium in the diet sensitizes the brain and kidney to increase the ability to respond with natriuresis for a given sodium stimulus.