Effect of renal denervation on the antinatriuresis of caval constriction

Norepinephrine stimulates the epithelial Na+ channel in cortical collecting duct cells via α2-adrenoceptors

There is good evidence for a causal link between excessive sympathetic drive to the kidney and hypertension. We hypothesized that sympathetic regulation of tubular Na(+) absorption may occur in the aldosterone-sensitive distal nephron, where the fine tuning of renal Na(+) excretion takes place. Here, the appropriate regulation of transepithelial Na(+) transport, mediated by the amiloride-sensitive epithelial Na(+) channel (ENaC), is critical for blood pressure control. To explore a possible effect of the sympathetic transmitter norepinephrine on ENaC-mediated Na(+) transport, we performed short-circuit current (Isc) measurements on confluent mCCDcl1 murine cortical collecting duct cells. Norepinephrine caused a complex Isc response with a sustained increase of amiloride-sensitive Isc by ∼44%. This effect was concentration dependent and mediated via basolateral α2-adrenoceptors. In cells pretreated with aldosterone, the stimulatory effect of norepinephrine was reduced. Finally, we demonstrated that noradrenergic nerve fibers are present in close proximity to ENaC-expressing cells in murine kidney slices. We conclude that the sustained stimulatory effect of locally elevated norepinephrine on ENaC-mediated Na(+) absorption may contribute to the hypertensive effect of increased renal sympathetic activity.

Kidney and liver function in rats during the edema following constriction of thoracic inferior vena cava with and without adrenalectomy or hypophysectomy

In several animal species, constriction of the thoracic inferior vena cava (TIVCC) is known to increase proximal sodium reabsorption and inhibit natriuresis following saline loading, leading to edema. To eluicidate the role of adrenal and hypophyseal hormones in the development of edema, kidney and liver functions after TIVCC were compared in adrenalectomised or hypophysectomised rats, and in intact controls. It was found that edema (body weight increase) and kidney and liver affliction were much less pronounced after the operations. The roles of aldosterone and ADH deficiency in renal sodium and water excretion are discussed. It is concluded that adrenal and hypophyseal hormones do not initiate edema but modulate its extent. The absence of edematous changes in the liver of hypophysectomised and adrenalectomised rats deserves further attention.

Expression of multiple alpha-adrenoceptor isoforms in rat CCD

In the rat cortical collecting duct (CCD), epinephrine inhibits vasopressin (AVP)-dependent water permeability and Na+ reabsorption. Although inhibition is reversed by the alpha2-adrenoceptor (AR) antagonist yohimbine, suggesting the epinephrine effect is primarily mediated by an alpha2-AR [C. T. Hawk, L. H. Kudo, A. J. Rouch, and J. A. Schafer. Am. J. Physiol. 265 (Renal Fluid Electrolyte Physiol. 34): F449-F460, 1993], there are also suggestions of an effect at an additional receptor, perhaps an alpha1-AR. For the present experiments, we used RT-PCR of total RNA extracted from 1 to 5 mm of microdissected CCDs from rat kidney to identify the alpha-AR isoforms expressed. Specific primers for the alpha2-ARs amplifying from the 6th transmembrane (TM) to the 3'-untranslated regions, revealed the presence of alpha2A and alpha2B. Western blot analysis also indicated the presence of alpha2B-AR at the protein level. Degenerate alpha1-AR primers that amplify from conserved regions of TM-1 to TM-5, as well as specific primers that amplify either the same region (alpha1B), the carboxy terminus (alpha1A), or within the third cytoplasmic loop (alpha1D), indicated the presence of all three alpha1-ARs. Measurement of transepithelial voltage in isolated perfused renal tubules indicated a small inhibitory effect mediated by alpha1-ARs. Although the functional effects of epinephrine on AVP-dependent transport processes appear to be mediated predominantly by an alpha2-AR, a small contribution to the overall alpha-AR effect may be due to simultaneous activation of an alpha1-AR.

Clonidine inhibits fluid absorption in the rabbit proximal convoluted renal tubule

Previous studies have shown that norepinephrine (NE) and the beta-adrenoceptor agonist, isoproterenol (I), enhance fluid absorption (JV) in isolated, perfused proximal convoluted tubule segments (PCT). Pretreatment of PCT with the beta-adrenoceptor antagonist, propranolol, inhibited the action of NE and produced a significant decline in JV, suggesting modulation of JV by both alpha- and beta-adrenoceptors. The present studies further characterize the alpha-adrenoceptor control of JV in isolated perfused PCT using specific agonists and antagonists. Basal JV declined significantly with the addition of the alpha 2-adrenoceptor agonist, clonidine (10(-4) M), to the bath; however, it was unchanged with the addition of the alpha 1-adrenoceptor agonist, methoxamine (10(-6) or 10(-4) M). With the addition of 10(-6) M isoproterenol JV increased significantly, and returned to control values with the subsequent addition of clonidine (10(-6) or 10(-4) M). Pretreatment of PCT with the alpha 2-adrenoceptor antagonist, yohimbine (10(-5) M), or with pertussis toxin (100 ng/ml) did not interfere with the stimulation of JV by isoproterenol, but abolished the inhibition of isoproterenol-stimulated JV by clonidine. Thus, clonidine inhibits JV in PCT via an alpha 2-adrenoceptor. This effect is mediated by a pertussis toxin inhibitable GTP-binding protein, but not one that is coupled to adenylyl cyclase.

Comparison of resting and stimulus-evoked catecholamine release from the femoral and renal vascular beds of the dog

Plasma levels of dopamine (DA) and noradrenaline (NA) were measured in arterial and in femoral and renal venous blood of chloralose-anaesthetised dogs at rest, and during electrical stimulation of the femoral and renal sympathetic nerve supplies. In the femoral bed, sympathetic nerve stimulation elevated venous efflux of NA, but did not reproducibly elevate DA efflux: when this was increased, it comprised less than 1% of the stimulus-evoked catecholamine efflux. By contrast, renal nerve stimulation liberated both NA and DA from the kidney, and DA comprised about 8% of the total stimulus-evoked efflux. Comparison of efflux from intact and denervated kidneys indicated substantial neurogenic release of both NA and DA at rest, with DA comprising 20% of this efflux. The results extend previous evidence for dopaminergic sympathetic innervation of the dog kidney, and suggest that both dopaminergic and noradrenergic renal nerves are tonically active in anaesthetised animals.

Renal overflow of noradrenaline and dopamine to plasma during hindquarter compression and thoracic inferior vena cava obstruction in the dog

Efferent renal nerve activity was assessed by measurements of the overflow of endogenous noradrenaline (NA) and dopamine (DA) to plasma in the kidney. To obtain correct estimates of the renal contribution to the renal venous outflow of NA and DA, corrections for the renal extraction of catecholamines in arterial plasma were performed by use of tracer amounts of [3H]NA. Hindquarter compression (previously known to cause a neurogenically mediated blood pressure elevation) increased the concentrations of NA, adrenaline and DA in arterial and renal venous plasma. The renal overflow of NA increased from 83.7 +/- 32.0 to 361.3 +/- 119.4 pmol min-1 (P less than 0.05) during hindquarter compression. When compared to the renal NA overflow during electrical renal nerve stimulation, this corresponds to an increase in average renal nerve impulse activity from approximately 0.4 to 1.6 Hz. Hindquarter compression also increased the renal overflow of DA to plasma. When venous return to the heart was reduced by obstruction of the thoracic inferior vena cava, the mean arterial blood pressure fell and all catecholamines in plasma increased gradually during the first 10 min of obstruction. The renal overflow of NA was only slightly increased, indicating a minor increase in renal nerve activity. The overflow of DA to plasma was not altered by obstruction of the thoracic inferior vena cava. Neither maneuver substantially altered the DA/NA ratio for renal overflow rates or for renal venous plasma concentrations indicating that there was no preferential activation of either noradrenergic or putative dopaminergic nerve fibres.

Role of renal sympathetic nerves in mediating hypoperfusion of renal cortical microcirculation in experimental congestive heart failure and acute extracellular fluid volume depletion

To evaluate the pathophysiologic importance of renal nerves in regulating the renal vasomotor tone, we measured several parameters of renal cortical microcirculation before and after acute renal denervation (DNx) in the following three groups of anesthetized Munich-Wistar rats: (group 1) congestive heart failure after surgically induced myocardial infarction (n = 10), (group 2) acute extracellular fluid volume depletion after deprivation of drinking water for 48 h (n = 8), and (group 3) sham or nontreated controls (n = 6). In the myocardial-infarcted rats, DNx led to a uniform increase in glomerular plasma flow rate of, on average, 36%. Single nephron glomerular filtration rate of myocardial-infarcted rats also increased despite a reduction in glomerular capillary hydraulic pressure. These changes were associated with a fall in arteriolar resistances, particularly in the efferent arteriole. The glomerular capillary ultrafiltration coefficient rose in all but one myocardial-infarcted animal. A similar hemodynamic pattern was seen after DNx in water-deprived animals. In every water-deprived animal, glomerular plasma flow rate and single nephron GFR increased on average by 28 and 14%, respectively. Again, afferent and efferent arteriolar resistances decreased significantly. Furthermore, the ultrafiltration coefficient increased uniformly and substantially with DNx. To ascertain the potential importance of the interaction between the renal nerves and angiotensin II in these circumstances, we compared the renal cortical hemodynamics in additional groups of water-deprived rats (group 4) after DNx (n = 15), (group 5) during inhibition of angiotensin II with saralasin (n = 15), and (group 6) during treatment with both saralasin and DNx (n = 15). No appreciable difference was detected between group 4 vs. 6. In contrast, substantial differences were noted between group 5 vs. 6: on average, the glomerular plasma flow rate was 26% higher and the afferent and efferent arteriolar resistances 25% and 27% lower, respectively, in group 6. These observations provide direct evidence to indicate pathophysiologic importance of renal nerves in the profound intrarenal circulatory adjustments in prerenal circulatory impairment. The vasoconstrictive effects of renal nerves appear to be mediated in part by their stimulatory influence on angiotensin II release and their direct constrictor actions on pre- and post-glomerular vessels as well.

The role of the adrenergic nervous system in sodium and water excretion

There is considerable evidence that the renal nerves contribute to the regulation of salt and water excretion by a direct effect on tubular reabsorption, independent of changes in renal hemodynamics. Whereas the effect of the adrenergic nervous system on sodium reabsorption appears to be established in anesthetised animals, it has been suggested that the basal activity of the renal sympathetic nerves in conscious dogs is too low to have a significant effect on sodium reabsorption by the proximal tubules. However, denervation natriuresis and diuresis has recently been demonstrated in conscious euvolemic and conscious volume expanded rats. The effects of renal nerve stimulation on the handling of sodium and water by the proximal tubule can be mimicked by infusion of the alpha-adrenergic agonist norepinephrine and prevented by infusion of an alpha-adrenergic antagonist. This confirms that it is mediated by alpha-receptors. The adrenergic nervous system may have an independent role in the control of sodium excretion or may be complementary to other systems such as the renin-angiotensin-aldosterone system.

Regulation of sodium and water excretion by catecholamines

There is considerable evidence that the renal nerves contribute to the regulation of salt and water excretion by a direct effect on tubular reabsorption, independent of changes in renal hemodynamics. Whereas the effect of the adrenergic nervous system on sodium reabsorption appears to be established in anesthetized animals, it has been suggested that the basal activity of the renal sympathetic nerves in conscious dogs is too low to have a significant effect on sodium reabsorption by the proximal tubules. However, denervation natriuresis and diuresis have recently been demonstrated in conscious euvolemic and conscious volume-expanded rats. The effects of renal nerve stimulation on the handling of sodium and water by the proximal tubule can be mimicked by infusion of the alpha-adrenergic agonist norepinephrine and prevented by infusion of an alpha-adrenergic antagonist. This confirms that they are mediated by alpha-receptors. The adrenergic nervous system may have an independent role in the control of sodium excretion or may be complementary to other systems such as the renin-angiotensin-aldosterone system.

Angiotensin receptors in glomeruli differ from those in renal arterioles

Angiotensin receptors in afferent and efferent arterioles and in the glomerulus are strategically located to influence renal perfusion and glomerular function. With the size of isolated glomeruli as the index, we have demonstrated identical dose-response relationships for graded concentrations (10(-13) to 10(-3) g/liter) of angiotensin II (AII) and angiotensin III (AIII). An octapeptide analogue (saralasin 10(-6) to 10(-2) g/liter) was equally effective at blocking glomerular responses to both AII and AIII, but two heptapeptide analogues (des-asp, 8-ile AII and des-asp, 8-gly AII; 10(-6) to 10(-2) g/liter) failed to block responses to either agonist. The relative influence of octapeptide and heptapeptide analogues on GFR was examined in anesthetized dogs with partial occlusion of the thoracic inferior vena cava. In 18 dogs, caval occlusion reduced renal blood flow (35%), GFR (29%), and arterial pressure (13%). Saralasin (300 to 3000 ng/kg/min, i.v.) and des-asp, 8-ile AII (100 to 3000 ng/kg/min, i.v.) increased renal blood flow by 0.41 +/- 0.11 and 0.62 +/- 0.11 ml/g/min, respectively, but only the octapeptide induced a concomitant increase in GFR (octapeptide: delta GFR = 0.11 +/- 0.03 ml/g/min; heptapeptide: delta GFR = -0.08 +/- 0.07 ml/g/min; P less than 0.025). As octapeptide and heptapeptide analogues were equally effective on renal blood flow in this and in previous studies, but only the octapeptide was effective in isolated glomeruli and in increasing GFR in the intact animal, we conclude that renal vascular and glomerular receptors differ. Furthermore, the glomerular receptor may be the more important in modulating the glomerular functional response to angiotensin.

Effects of renal and hepatic venous congestion on renal function in the presence of low and normal cardiac output in dogs

We investigated the effect of acute renal vein and hepatic vein hypertension induced by partial balloon-occlusion of the abdominal inferior vena cava (AIVC-O) and the thoracic inferior vena cava (TIVC-O) on systemic and renal hemodynamics and renal function in 13 dogs anesthetized with pentobarbital. When a renal vein pressure of 13 cm H2O was induced by AIVC-O, cardiac output, stroke volume, central venous pressure, renal blood flow, and renal function (GFR, free water clearance, osmolar clearance, urine output, urinary sodium excretion, fractional sodium excretion) decreased significantly. When systemic hemodynamics were restored to control values by transfusion of autologous blood (mean of 9 ml/kg body weight) while renal vein pressure was kept elevated, renal function also was restored. A hepatic venous pressure of 13 cm H2O then was induced by TIVC-O. The effects on systemic hemodynamics and renal function were very similar to those observed during AIVC-O. When systemic hemodynamics were restored to control values by transfusion (mean of 9 ml/kg), while hepatic venous hypertension was maintained by TIVC-O, renal function also was restored. Despite significant changes in natriuresis and diuresis, intrarenal blood flow distribution, as determined by the radioactive microsphere technique, remained essentially unchanged throughout. We conclude that renal and hepatic congestion induced by partial AIVC-O and TIVC-O do not, per se, alter renal function significantly.

Natriuretic hormone--its possible role in fluid and electrolyte disturbances in chronic liver disease

Besides intrarenal physical factors and aldosterone, a natriuretic hormone has been postulated to modulate renal tubular sodium resorption in order to maintain body fluid homeostasis. To investigate the possible role of a natriuretic activity in sodium retention of chronic liver disease, the effects of plasma and plasma fraction IV from patients with cirrhosis of the liver and ascites on sodium transport of the isolated frog skin and on renal sodium excretion in the rat were compared to the antinatriferic and natriuretic effects of plasma from healthy subjects. While plasma from healthy individuals obtained following acute expansion of the extracellular fluid volume (ECV) significantly inhibited potential difference (PD) by -43·8 ± 5·5% and short circuit current (SCC) by -41·3 ± 1·7% when applied to the inner skin surface, control plasma and plasma from patients with liver cirrhosis and ascites affected PD by -3·8 ± 4·7% and -5·2 ± 3·7% and SCC by -7·3 ± 4·6% and -11·7 ± 2·5% respectively.

Similar effects on PD and SCC were observed with plasma fractions IV.

In contrast to fraction IV from ECV-expanded individuals, which caused marked diuresis and natriuresis when injected in the rat, fraction IV of plasma from cirrhotic patients failed to affect urinary flow rate, free-water clearance or renal sodium excretion. The results suggest that at least some patients with cirrhosis of the liver and sodium retention may lack an adequate humoral natriuretic activity sufficiently to promote renal sodium excretion.

Failure to demonstrate a humoral mechanism in the antinatriuresis of acute caval constriction

Previous studies reported from this laboratory provided support for the hypothesis that the natriuresis of volume expansion is mediated in part by a humoral mechanism. In the present study we examined whether suppression of this factor participates in the antinatriuresis of acute constriction of the thoracic inferior vena cava. An isolated kidney was perfused by a second dog pretreated with deoxycorticosterone acetate. Expansion of the perfusion dog with equilibrated blood from a reservoir resulted in an increase in sodium excretion from 102+/-30 to 259+/-65 muEq/min, P < 0.001. Fractional sodium excretion increased from 2.3+/-0.6 to 6.2+/-1.2%, P < 0.01. Inulin clearance, plasma protein concentration, and packed cell volume remained constant; renal perfusion pressure and renal blood flow decreased. After the natriuresis was established, the thoracic inferior vena cava was constricted to decrease systemic arterial pressure in the perfusion dog 50 mm Hg. This maneuver suppressed urine output in the dog but did not significantly alter sodium excretion in the isolated kidney. During the period of caval constriction absolute sodium excretion in the isolated kidney measured 198+/-42 muEq/min and fractional sodium excretion measured 5.7+/-1.1%. Neither value is significantly different from that measured during volume expansion alone. The data suggest that the antinatriuresis of acute caval constriction probably does not require suppression of a humoral natriuretic factor and that other more rapidly acting mechanisms, presumably hemodynamic and neural, may be involved.