Renal function in different forms of renovascular hypertension in rats

Mechanism of FK 506/520 action on rat renal proximal tubular Na+, K+-ATPase activity

BACKGROUND

The neurotransmitter in renal sympathetic nerves, norepinephrine (NE), regulates the activity of proximal tubule (PT) Na+,K+-ATPase in a bidirectional manner via stimulation of alpha- and beta-adrenoceptors. The stimulatory alpha-adrenergic pathway is mediated by calcineurin, the target molecule for FK 506 and related compounds. We examined whether the FK 506 analogue FK 520, by interrupting the calcineurin-mediated alpha-adrenergic signaling pathway, enhance the inhibitory beta-adrenergic effect of NE on PT Na+,K+-ATPase activity.

METHODS

The effects of three days of treatment with FK 520 were examined on rat renal PT Na+,K+-ATPase activity, measured as ouabain-sensitive ATP hydrolysis in single, microdissected PT segments. Renal function studies, including glomerular filtration rate (GFR) and urinary excretion of N-acetyl-3-D-glucoseaminidase (NAG), were examined using conventional clearance techniques after three days of treatment with FK 506.

RESULTS

FK 520 treatment induced a pronounced and dose-dependent decrease in PT Na+,K+-ATPase activity. This effect was completely reversed by the competitive FK 520 antagonist, L 685 818, indicating that the effect was dependent on inhibition of calcineurin. To test whether the FK 520-induced decrease in Na+, K+-ATPase activity was mediated by enhanced beta-adrenoceptor signaling, the FK 520 effect was examined in rats pretreated with a beta-adrenoceptor antagonist (propranolol) or rats subjected to renal denervation. Both of these procedures prevented the FK 520-induced decrease in Na+,K+-ATPase activity. Thus, during FK 520 treatment, renal sympathetic nerves mediate an inhibitory effect on PT Na+,K+-ATPase activity via beta-adrenoceptors. Propranolol pretreatment also prevented FK 506-induced decreases in GFR and urinary excretion of NAG, an index of PT dysfunction.

CONCLUSIONS

The results support the hypothesis that the net effect of the neurotransmitter NE on Na+,K+-ATPase activity is dependent on the balance between the alpha- and beta-adrenergic signaling pathways and suggest that agents that interfere with these pathways may, by altering the activity of tubular Na+,K+-ATPase, also alter the function of the renal tubular epithelial cell.

Inhibition of COMT induces dopamine-dependent natriuresis and inhibition of proximal tubular Na+,K+-ATPase

The enzyme catechol-O-methyltransferase (COMT), which plays an important role for dopamine metabolism, is abundantly expressed in the kidney. To test whether the natriuretic effects of dopamine may be related to the rate of dopamine metabolism, rats were treated with nitecapone, a peripheral inhibitor of COMT. Nitecapone, given by gavage, induced a highly significant (5.6-fold) increase in sodium excretion, which was associated with an inhibition of the Na+,K+-ATPase activity in both the proximal convoluted and proximal straight tubules (PCT and PST, respectively). These effects were completely abolished if the rats were also treated with a specific dopamine 1 antagonist, SCH 23390. Furthermore, the natriuretic effect of nitecapone was also observed in rats on a high salt diet. The kidney-specific pro-drug to dopamine, glu-dopa, induced a significant, but less pronounced increase in urinary sodium excretion, associated with a dopamine-dependent inhibition of the Na+,K+-ATPase activity in the PCT but not in the PST. Nitecapone and glu-dopa had an additive natriuretic effect. It is concluded that COMT plays an important role in determining the natriuretic effects of the renal dopamine system.

Vasopressor mechanisms in acute aortic coarctation hypertension

Angiotensin II (ANG II) and vasopressin (AVP) act together with the mechanical effect of aortic constriction in the onset of acute aortic coarctation hypertension. Blockade of ANG II and AVP V1 receptors demonstrated that ANG II acts on the prompt (5 min) rise in pressure whereas AVP is responsible for the maintenance (30-45 min) of the arterial pressure elevation during aortic coarctation. Hormone assays carried out on blood collected from conscious rats submitted to aortic constriction supported a role for ANG II in the early stage and a combined role for both ANG II and AVP in the maintenance of proximal hypertension. As expected, a role for catecholamines was ruled out in this model of hypertension, presumably due to the inhibitory effect of the sinoaortic baroreceptors. The lack of afferent feedback from the kidneys for AVP release from the central nervous system in rats with previous renal denervation allowed ANG II to play the major role in the onset of the hypertensive response. Median eminence-lesioned rats exhibited a prompt increase in proximal pressure followed by a progressive decline to lower hypertensive levels, revealing a significant role for the integrity of the neuroaxis in the maintenance of the aortic coarctation hypertension through the release of AVP. In conclusion, the important issue raised by this model of hypertension is the likelihood of a link between some vascular territory-probably renal-below the coarctation triggering the release of AVP, with this vasoconstrictor hormone participating with Ang II and the mechanical effect of aortic constriction in the acute aortic coarctation hypertension.

Renal nerve activity does not contribute to the development of renovascular hypertension in rats with abdominal aortic constriction

This study examines the influence of renal nerves on the development of renovascular hypertension in proximal aortic constricted rats. The rats were studied 1 week after unilateral or bilateral denervation of the renal artery. Denervation had no effect on the increase in mean arterial pressure induced by the constriction. The glomerular filtration rate and filtration fraction in control and in proximal aortic constricted rats were not influenced by the denervation. The Na excretion was increased in the denervated kidney both in control and in proximal aortic constricted rats. Plasma angiotensin II levels were not different from controls in innervated or unilaterally denervated proximal aortic constricted rats. In bilaterally denervated proximal aortic constricted rats the plasma angiotensin II levels were significantly higher. The renovascular hypertension and the alteration in renal function in proximal aortic constricted rats are not dependent on renal nerve activity.

Renal hypertension following aortic constriction is abolished by angiotensin II converting enzyme inhibitor, but not by low-salt diet

This study evaluates the role of different sodium intakes and the role of angiotensin II in the development and the maintenance of renovascular hypertension in rats with constriction of the aorta proximal to the renal arteries. The rats were studied 3 weeks after surgery when the hypertension was well established. Glomerular filtration rate was decreased and filtration fraction was increased in rats with proximal aortic constriction. Low and high salt intakes had no effect on glomerular filtration and filtration fraction. Treatment with angiotensin II converting enzyme inhibitor increased the glomerular filtration rate and reduced the filtration fraction in rats with proximal aortic constriction to the same levels as in control rats. Serum levels of angiotensin II were about the same in rats with proximal aortic constriction as in control rats. Conclusion. The renovascular hypertension in proximal aortic constriction is influenced by locally formed angiotension II but not by alterations in salt intake.