(Hypertension. 1999;34:1101-1105.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
From the Institut für Physiologie (C.W., A.K.) and Klinik und Poliklinik II für Innere Medizin (B.K.K.) der Universität Regensburg, and HoffmannLa Roche AG Grenzach (M.H.) (Germany).
Correspondence to Charlotte Wagner, PhD, Institut für Physiologie, Universität Regensburg, D-93040 Regensburg, Germany. E-mail charlotte.schmid{at}vkl.uni-regensburg.de
| Abstract |
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Key Words: renal artery denervation mibefradil blood pressure renin RNA
| Introduction |
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Another parameter of possible relevance in this context is
renal innervation,14 15 which stimulates renin secretion
and renin mRNA expression through
ß-adrenoreceptors1 and consequently
through cAMP.16 The role of renal nerves in the
stimulation of renin secretion after an acute fall in renal perfusion
pressure has been examined in numerous studies.17 18 19 20 21 22 23 24 25 26 27 28 The
results of the studies that focused on changes of plasma renin activity
(PRA) immediately after onset of reduced renal perfusion pressure, have
not, however, produced unequivocal results. Thus, some studies have
reported complete abrogation of the pressure droprelated renin
response in denervated kidneys, while others have found no effect of
denervation.17 18 26 Still others have suggested an
attenuation of renin secretion,21 22 24 25 with the effect
vanishing with prolonged reduced renal arterial
pressure.23 For conscious dogs it has been reported that
an activation of renal nerve activity causes a rightward shift of the
perfusion pressure/renin secretion relationship.27 28 This
rightward shift was prevented by
-adrenoreceptor
blockade but not by ß-adrenoreceptor blockade. The
latter, however, attenuated the gain of renin secretion in response to
a fall of renal perfusion pressure.27 28 Taken together,
it would appear that the immediate response of renin secretion to an
acute fall of perfusion pressure is more complex, rendering the role of
renal innervation rather difficult to assess. It thus appeared of
interest to us to establish the relevance of renal nerves for renin
secretion and renin synthesis in response to reduced renal
arterial pressure in the days after acute renal artery
stenosis. To this end we examined PRA and renal renin mRNA
levels 1, 2, and 4 days after renal artery clipping in rats with
innervated kidneys and in rats in which the clipped kidney
was denervated 4 days before clipping. We found that the stimulation of
the renin system by renal artery clipping was attenuated markedly in
the denervated kidneys at all days of examination. This finding
suggests that renal nerves provide an important stimulation of the
renin system that is normally suppressed by the ambient perfusion
pressure, but which is unmasked during low renal perfusion pressure and
thus is an important determinant of the gain of renin stimulation
during reduced renal perfusion pressure.
| Methods |
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Rats With Denervated Left Kidneys (With or Without a Unilateral
Left Renal Artery Clip)
The left kidney was denervated by a combination of mechanical
and chemical methods, as described previously.16 On the
fourth day after denervation, left renal arteries were clipped for 1,
2, or 4 days, as described previously.10
Rats With Innervated Left Kidneys (Sham-Denervated
Controls)
For sham denervation, the left renal artery was exposed, but
mechanical and chemical treatments were omitted. Four days after sham
denervation, left renal arteries were clipped for 1, 2, or 4 days. One
group of rats remained unclipped and served as sham-denervated
controls.
Rats With Denervated and Clipped Left Kidneys Receiving
Mibefradil
Rats with denervated left kidneys received a left renal artery
clip for 4 days. Concomitantly they were treated with the T-type
calcium channel blocker mibefradil (15 mg ·
kg-1 · d-1). The
drug was administered for 4 days in the drinking water.
At the end of the experiments the animals were killed by decapitation. Blood was collected from the carotid arteries, EDTA was added to the blood, and PRA was determined. The kidneys were removed rapidly, weighed, cut into halves, and frozen rapidly in liquid nitrogen. The organs were stored at -80°C until isolation of total RNA, which was extracted from 1 of the frozen kidney halves, as described by Chomczynski and Sacchi.29
Determination of Preprorenin mRNA and Cytosolic
ß-Actin mRNA
Renin mRNA and ß-actin mRNA were measured by specific RNase
protection assays, as described previously.10 ß-Actin
mRNA was used as a standard RNA for controlling the quality of the RNA
preparation.
Determination of PRA
PRA was determined by measurement of the generated
angiotensin I with the use of a commercially available
radioimmunoassay kit for angiotensin I (Sorin
Biomedica).
Measurement of Blood Pressure
Systolic blood pressure was measured in conscious rats
at 8 AM and 4 PM on each experimental day by
the tail-cuff method and an appropriate recorder (TSE System), as
described previously.30
Statistical Analysis
For intraindividual and interindividual comparisons between 2
groups, Students paired and unpaired t tests were used,
respectively. Multiple comparisons were performed by 2-way ANOVA and
Fishers exact test. P<0.05 was considered
significant.
| Results |
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Despite the marked differences in PRA and renin mRNA levels in the
clipped kidneys between sham-denervated and denervated rats, the renin
mRNA levels in the contralateral intact (right) kidneys fell to the
same levels, ie,
40% of the value found for normal
innervated kidneys, regardless of whether the clipped
kidney was innervated or not (Figure 2A). We
therefore asked whether the contralateral suppression of renin
expression in unilaterally clipped rats might not be related directly
to circulating angiotensin II, as is commonly thought.
Another candidate mediating the suppression of renin expression could
be the increase in blood pressure that follows unilateral renal artery
clipping.
In fact, blood pressures increased rather similarly in rats with sham-denervated clipped kidneys and in animals with denervated clipped kidneys (Figure 3). To establish whether the increase of blood pressure mediates the contralateral suppression of renin mRNA in the rats with renal denervation, another group of rats with denervated clipped kidneys was treated with the antihypertensive drug mibefradil at 15 mg · kg-1 · d-1, a dose that exerts no direct stimulatory effect on the renin system.30 As shown in Figure 4A, mibefradil prevented the rise of blood pressure in response to clipping of the denervated kidneys. Preventing the hypertension, however, did not change the moderate increases of PRA (Figure 4B) and renin mRNA levels (Figure 4C) in the clipped denervated kidneys. Interestingly, the suppression of renin expression in the contralateral intact kidney also was not prevented by the antihypertensive treatment (Figure 4C).
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| Discussion |
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- and
ß-adrenoreceptors for acute, pressure-dependent renin
secretion.28 29 In any case, our findings suggest that
renal nerve activity plays an important role for continued stimulation
of renin synthesis and renin secretion during states of reduced kidney
perfusion pressure. Renal nerve activity thus appears to contribute
importantly to background stimulation of renin secretion and renin
synthesis. This stimulation is normally masked by the
inhibitory effect of ambient blood pressure. We have shown
previously that inhibition of prostaglandin
formation10 or of nitric oxide formation11
also partly inhibits the stimulation of renin secretion and renin mRNA
expression after renal artery clipping. In view of the present
findings, it appears likely that a combination of renal nerve activity,
renal prostaglandins, and renal nitric oxide accounts for
the basal background stimulation of the renin system, which is unmasked
when the inhibitory effect of perfusion pressure is
removed. These factors could act in concert such that renal nerves via
ß-adrenoreceptors1 and also
prostaglandins12 stimulate cAMP formation in
juxtaglomerular cells, while nitric oxide retards cAMP
degradation by inhibiting cAMP phosphodiesterases.13 How the normal magnitude of renal perfusion pressure inhibits the stimulatory action of cAMP on renin secretion and renin synthesis so effectively remains to be clarified. Evidence suggests that the inhibitory effect of pressure on the renin system is calcium dependent.3 It should be noted in this context that the requirement of normal renal nerve activity for stimulation of the renin system appears to be more specific for reduced renal perfusion pressure, because previously we found that the stimulation of the renin system by angiotensin II antagonists is rather insensitive to renal denervation.31 The suppression of renin synthesis and renin secretion induced in the intact contralateral kidney by renal arterial stenosis, as seen in this study, is a well-known phenomenon.6 10 11 Since angiotensin II is a potent inhibitor of renin synthesis and secretion,31 it is assumed that the enhanced release of renin from stenosed kidneys is causally involved in the contralateral suppression of the renin system.32 Given that PRA mirrors circulating angiotensin II levels, our data would suggest that systemic angiotensin II formation is lower in rats with denervated clipped kidneys than in those with innervated clipped kidneys. Notably, the renin system in the contralateral intact kidneys was suppressed to the same degree in rats with denervated or innervated clipped kidneys. This might indicate that the increased renin secretion rate from the denervated stenosed kidney, although attenuated compared with the increase from the innervated stenosed kidney, is still sufficient to suppress the renin system in the contralateral kidney effectively. Another explanation could be that the suppression of the renin system in the contralateral kidney is causally related to the increase of systemic blood pressure. In fact, the blood pressure increased with very similar temporal patterns in rats with innervated or denervated clipped kidneys, a finding that was, at first glance, unexpected in view of the marked differences of renin synthesis and renin secretion between the 2 groups of animals. However, preventing the increase in blood pressure by the T-type channel blocker mibefradil did not attenuate suppression of the renin system in the contralateral kidneys. This is in good agreement with our previous observations in bilaterally innervated 2-kidney, 1 clip rats.30 We therefore infer that the suppression of the renin system in the kidney contralateral to the denervated, stenosed kidneys is not causally related to the increase of blood pressure. Therefore, one may assume that the attenuated increase in renin secretion rate from the denervated clipped kidney is already sufficient to block the renin system in the contralateral kidney. A more provocative but yet hypothetical speculation, however, is that the contralateral suppression of the renin system involves factors other than renin released from hypoperfused kidneys.
Taken together, our findings indicate that renal nerve activity plays a major role in the prolonged stimulation of renin secretion and renin synthesis in response to reduced renal arterial pressure by producing a background stimulation of renin secretion and renin mRNA expression that is normally masked by the inhibitory effect of the blood pressure on the renin system. Renal nerve activity is therefore an important determinant of the gain of renin stimulation during reduced renal perfusion pressure.
| Acknowledgments |
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Received April 6, 1999; first decision May 3, 1999; accepted July 9, 1999.
| References |
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