Effect of beta adrenergic blockade on renin response to renal nerve stimulation

Renin release in relation to plasma noradrenaline during supine exercise in cardiac patients

Plasma renin activity in the brachial artery and in the renal vein together with arterial noradrenaline concentration were determined repeatedly during and after 10 min of dynamic exercise in eight cardiac patients. Arterial renin increased slightly during exercise and gradually returned to control level after exercise. Renal vein renin increased markedly during exercise and returned to the resting value immediately after exercise, similarly to the changes in plasma noradrenaline and heart rate. A close temporal relationship between the changes in the renal veno-arterial renin difference, plasma noradrenaline and heart rate strongly suggests that the sympathetic nervous system is a major determinant of renin release in man during exercise.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Renin-angiotensin system inhibition: how much is too much of a good thing?

Inhibitors of the renin-angiotensin system (RAS) are valuable therapeutic agents for a wide range of clinical conditions. Increasingly, consideration is being given to the combination of angiotensin-converting enzyme (ACE) inhibitors and angiotensin-II receptor (AR) antagonists to obtain more complete inhibition of the RAS than can be achieved by either agent alone. Beta-blockers also inhibit the RAS by inhibiting renin secretion. Whereas the combination of an ACE inhibitor and AR antagonist represents dual RAS inhibition, the combination of both of these agents with beta-blocker therapy represents triple RAS inhibition. Animal studies indicate that complete blockade of the RAS produces adverse effects. Moreover, post-hoc analysis of the recent Valsartan Heart Failure Trial study suggests that the combination of ACE inhibitor and AR antagonist therapies may have an adverse effect in heart failure when combined with beta-blocker therapy. There is therefore a need for caution in the combination of ACE inhibition and AR antagonism, particularly in patients receiving beta-blockers, until the impact of this strategy is evaluated.

Role of neuropeptide Y in the development of two-kidney, one-clip renovascular hypertension in the rat

OBJECTIVE

Along with the renin-angiotensin system, sympathetic stimulation may contribute to renovascular hypertension. The vasoactive peptide neuropeptide Y (NPY) is co-released with and potentiates the pressor effects of norepinephrine through the Y-1 receptor. NPY, by exaggerating sympathetic activity, may contribute to renovascular hypertension, possibly by augmenting adrenergic-mediated renin release. This was studied by determining the effect of continuous Y-1 blockade on the development of two-kidney, one-clip renovascular hypertension and the effect of NPY on in vitro renin release.

METHODS

Mean arterial pressure and renal blood flow responses to NPY (10 microg/kg, administered intravenously) were measured in five anesthetized Sprague-Dawley rats before and after BIBO3304TF administration to test the Y-1 antagonist BIBO3304TF. In hypertension studies, 28 rats underwent left renal artery clipping. Of these, 13 were implanted with a mini-osmotic pump for continuous BIBO3304TF infusion (0.3 microg/h, administered intravenously); the other 15 underwent sham implantation. Systolic blood pressure was then monitored for 4 weeks. Finally, in vitro renin release was measured from renal cortical slices (n = 6-12) incubated with NPY (10(-8) to 10(-6) mol/L) or NPY plus the adrenergic agonist isoproterenol (10(-4) mol/L).

RESULTS

BIBO3304TF attenuated the NPY-induced increase in mean arterial pressure by 54% (P <.02) and the NPY-induced decrease in renal blood flow by 38% (P <.05). In 4-week hypertension studies, systolic blood pressure in clipped controls increased from 130 +/- 3 mm Hg to 167 +/- 6 mm Hg (P <.01), whereas BIBO3304TF-treated rats had no significant increase (125 +/- 3 mm Hg to 141 +/- 8 mm Hg). Final systolic blood pressure was 26 mm Hg lower in BIBO3304TF-treated rats than in controls (P <.01). In renal cortical slices, no NPY effect was observed in basal or isoproterenol-stimulated renin release.

CONCLUSIONS

The Y-1 receptor antagonist BIBO3304TF attenuated acute pressor responses to NPY and blunted the development of two-kidney, one-clip renovascular hypertension in rats. NPY may contribute to the hypertensive response in this renovascular hypertension model. Our in vitro data do not suggest that this is due to NPY enhancement of renin release.

High- versus low-dose ACE inhibition in chronic heart failure: a double-blind, placebo-controlled study of imidapril

OBJECTIVES

To determine dose-related clinical and neurohumoral effects of angiotensin-converting enzyme (ACE) inhibitors in patients with chronic heart failure (CHF), we conducted a double-blind, placebo-controlled, randomized study of three doses (2.5 mg, 5 mg and 10 mg) of the long-acting ACE inhibitor imidapril.

BACKGROUND

The ACE inhibitors have become a cornerstone in the treatment of CHF, but whether high doses are more effective than low doses has not been fully elucidated, nor have the mechanisms involved in such a dose-related effect.

METHODS

In a parallel group comparison, the effects of three doses of imidapril were examined. We studied 244 patients with mild to moderate CHF (New York Heart Association class II-III: +/-80%/20%), who were stable on digoxin and diuretics. Patients were treated for 12 weeks, and the main end points were exercise capacity and plasma neurohormones.

RESULTS

At baseline, the four treatment groups were well-matched for demographic variables. Of the 244 patients, 25 dropped out: 3 patients died, and 9 developed progressive CHF (3/182 patients on imidapril vs. 6/62 patients on placebo, p < 0.05). Exercise time increased 45 s in the 10-mg group (p = 0.02 vs. placebo), but it did not significantly change in the 5-mg (+16 s), and 2.5-mg (+11 s) imidapril group, compared to placebo (+3 s). Physical working capacity also increased in a dose-related manner. Plasma brain and atrial natriuretic peptide decreased (p < 0.05 for linear trend), while (nor)epinephrine, aldosterone and endothelin were not significantly affected. Renin increased in a dose-related manner, but plasma ACE activity was suppressed similarly (+/-60%) on all three doses.

CONCLUSIONS

Already within 3 months after treatment initiation, high-dose ACE inhibition (with imidapril) is superior to low-dose. This is reflected by a more pronounced effect on exercise capacity and some of the neurohormones, but it does not appear to be related to the extent of suppression of plasma ACE.

Serotonin and stress-induced increases in renin secretion are not blocked by sympathectomy/adrenal medullectomy but are blocked by beta antagonists

The present study examined the role of the sympathetic nervous system as a mediator of the message from the CNS to the kidneys to stimulate the secretion of renin. Two procedures that increase the secretion of renin were tested: administration of the serotonin releaser fenfluramine, which increases renin release without altering blood pressure [53], and subjecting the rats to the 'psychological' stressor of conditioned emotional response (CER) stress. Pretreatment of rats with either the beta antagonist sotalol or the beta 1-selective antagonist atenolol completely prevented the increase in plasma renin activity and concentration caused by fenfluramine (5 mg/kg i.p.) injection. However, chemical sympathectomy with 6-hydroxydopamine (6-OHDA) combined with surgical adrenal medullectomy did not prevent the increase in plasma renin activity and concentration following fenfluramine injection. Since beta-antagonists have been previously shown to prevent the renin response to CER stress, we also tested whether the sympathetic nervous system mediates the renin response to CER stress. Chemical sympathectomy combined with adrenal medullectomy did not prevent the effect of CER stress on renin release. The completeness of the sympathectomy/adrenal medullectomy was verified biochemically by measuring plasma epinephrine and both plasma and renal norepinephrine concentrations. Plasma epinephrine and renal norepinephrine levels were reduced to below 1% of control while plasma norepinephrine was reduced to below 8% of control values. In conclusion, our data support previous reports suggesting that activation of CNS pathways increases the secretion of renin. However, the message from the brain to release renin from the kidneys does not exclusively involve either the sympathetic innervation of the kidneys or adrenal epinephrine. Although beta 1 receptors are involved in mediating this phenomenon, their location or mechanism remains unknown and will be discussed.

Hormonal, renal, and autonomic nerve factors involved in the excretion of sodium and water during dynamic salt and water loading in hypoxaemic chronic obstructive pulmonary disease

BACKGROUND

Some patients with hypoxaemic chronic obstructive pulmonary disease (COPD) develop sodium and water retention and a subclinical autonomic neuropathy. The possibility that these might be associated has been investigated.

METHODS

The ability of 24 patients with COPD to excrete a 6 ml/kg 2.7% intravenous saline or 15 ml/kg oral water load was studied and changes in plasma electrolyte levels, osmolality, plasma aldosterone and vasopressin levels, urinary volume and sodium content, glomerular filtration rate, renal blood flow, and cardiovascular autonomic nerve function were measured. Patients were divided into groups of eight: those in group A (controls) had mild COPD with a Pa02 of > 9 kPa and no oedema, patients in group B were more hypoxaemic but had never been oedematous, whilst those in group C were hypoxaemic and mildly oedematous at the time of the study.

RESULTS

Patients in groups B and C excreted less sodium and water during saline loading and a lesser proportion of the water load. Patients in group C had a reduction in renal blood flow and glomerular filtration rate and all had a subclinical autonomic neuropathy, which was also found in three patients in group B. Their plasma aldosterone level was raised but did suppress appropriately on saline loading. Vasopressin levels were abnormally raised for the osmolality in patients in group C and in those with autonomic dysfunction throughout the water load and at 240 minutes after the salt load. Sodium and urine excretion was highly correlated with autonomic dysfunction, aldosterone levels at time zero, and renal blood flow. The 11 patients with autonomic dysfunction were more likely to be oedematous, more hypoxaemic, excreted much less urine and sodium, had lower glomerular filtration rate and renal blood flow, and higher aldosterone and vasopressin levels than the remaining patients.

CONCLUSIONS

In patients with COPD the inability to excrete sodium and water is multifactorial. This is the first study to show that autonomic dysfunction is at least associated and might play an important part in the impaired sodium and water homeostasis seen in patients with severe COPD.

Spatial association of renin-containing cells and nerve fibers in developing rat kidney

The development of renin-containing cells and nerve fibers was studied in Sprague-Dawley rat kidneys during the last third of gestation and the first 15 days of postnatal life. Kidney tissue sections were stained for nerve fibers or double stained employing an anti-rat renin polyclonal antibody and a monoclonal antibody (TUJ1) directed against a neuron-specific class III beta-tubulin isotype. Renin-containing cells and nerve fibers were detected at 17 days of gestation, in close spatial relationship along the main branches of the renal artery. During fetal life, renin-containing cells and nerve fibers were spatially associated along arcuate and interlobular arteries, renin-containing cells being also present throughout the entire length of afferent arterioles supplying juxtamedullary glomeruli. During postnatal life the distribution of renin-containing cells progressively shifted to a restricted juxtaglomerular position in afferent arterioles. Simultaneously, density and organization of nerve fibers increased with age along the arterial vascular tree. Our results suggest that innervation of renin-containing cells is present in fetal life and follows the centrifugal pattern of renin distribution and nephrovascular development.

Osmoregulation of thirst and vasopressin release in severe chronic renal failure

Subjects with severe chronic renal failure (CRF) have higher plasma concentrations of arginine vasopressin (AVP) than normal subjects, and some develop severe thirst. Eight patients with CRF and seven matched controls underwent hypertonic saline infusion to explore the relationship of thirst and plasma AVP with plasma osmolality. Differences in urea concentration between the two groups were controlled for by correcting measured osmolality to a urea of zero. Linear regression analysis of the relationships between plasma AVP and thirst with plasma osmolality (corrected for urea) was performed. Mean results were: control, pAVP = 0.26 (pOsmc - 283.7) versus CRF, pAVP = 0.72 (pOsmc - 282.0); and control, thirst = 4.0 (pOsmc - 279.4) versus CRF, thirst = 3.5 (pOsmc - 281.8). The apparent sensitivity (slope) of AVP release was greater in severe CRF than in normal controls (P = 0.04). There was no significant difference between the groups in thirst sensitivity, threshold for thirst onset and threshold for AVP release. Osmoregulated thirst was normal in severe CRF, but increasing osmolality leads to higher concentrations of AVP than would be expected.

Vasoactive intestinal peptide and renin secretion

VIP has now been shown to produce an increase in renin release in a number of species, including humans. Our work suggests that VIP is capable of producing this effect by a direct action on the renin-secreting juxtaglomerular cells of the kidney. We have found no evidence to support the possibility that VIP produces this effect as a neurotransmitter in the kidney. In this regard, it should be noted that VIP has been identified as a cotransmitter primarily in cholinergic neurons. The kidney is thought to lack cholinergic innervation, and acetylcholine has no effect on renin secretion. We have explored two conditions where renin secretion is known to increase and found that circulating levels of VIP did not increase along with the increase in PRA. Thus, at least in hemorrhage and dietary sodium restriction, VIP does not appear to affect renin secretion through a humoral mechanism. There could be other untested situations where a humoral effect of VIP might come into play since we have shown that the whole animal is capable of increasing plasma VIP to levels that affect renin release. Studies employing recently developed VIP antagonists have the potential to determine in which physiological or pathological situations VIP contributes to the control of renin secretion. For example, in endotoxic shock, plasma levels of both VIP and PRA are significantly elevated. Could the increase in PRA be partly dependent on an action of circulating VIP?

Changes in glomerular hemodynamic response to angiotensin II after subacute renal denervation in rats

We examined the changes in glomerular hemodynamics produced by angiotensin II (AII) in both normal Munich-Wistar rats and rats which were unilaterally renal denervated (measured kidney) 4-6 d prior to the measurement periods. Measurements of glomerular dynamics were performed in a control period after plasma volume expansion and during infusion of 11 ng X 100 g body wt-1 X min-1 of AII. The glomerular hydrostatic pressure gradient increased from 38 +/- 1 to 49 +/- 1 mmHg in denervated rats compared with a lesser response in controls (from 39 +/- 1 to 45 +/- 1 mmHg, P less than 0.05). Single nephron plasma flow decreased from 213 +/- 17 to 87 +/- 4 nl X min-1 X g kidney wt (KW)-1 in denervated kidneys versus a more modest decrease in control kidneys (from 161 +/- 9 to 102 +/- 5 nl X min X gKW-1). These changes were due to a greater increase in both afferent and efferent arteriolar resistance after AII infusion in denervated compared with control kidneys. Glomerular AII receptor maximum binding was 1,196 +/- 267 fmol/mg protein in denervated kidneys compared with 612 +/- 89 fmol/mg protein (P less than 0.01) in controls with no change in receptor affinity. We conclude the subacute unilateral renal denervation results in renal vasodilation, denervation magnifies the vasoconstrictive effect of AII infusion on glomerular hemodynamics, and the observed increased response to AII after denervation is associated with increases in glomerular AII receptors.

Effects of dobutamine on renal function in normal man

The effects of graded doses of dobutamine on renal function were studied in eight male volunteers. The infusion rates were 2.5, 5 and 10 micrograms/kg/min. Systolic blood pressure increased by 19% (P less than 0.01), 31% (P less than 0.01), and 44% (P less than 0.01), respectively, while diastolic blood pressure decreased by 17% (P less than 0.02), 17% (P less than 0.02) and 25% (P less than 0.01), respectively. Heart rate increased at the highest dosage by 32% (P less than 0.01). Glomerular filtration rate (GFR) diminished at all three infusion speeds by 10% (P less than 0.02), 9% (P less than 0.05) and 14% (P less than 0.02), respectively, while renal blood flow (RBF) was unchanged. Urine flow rate (UF) decreased by 36% (P less than 0.05) and fractional free water clearance (CH2O/CIn) diminished by 37% (P less than 0.05) at the rate of 10 micrograms/kg/min. Fractional potassium excretion (CK/CIn) decreased by 34% (P less than 0.01) and 44% (P less than 0.01) at the two highest rates. Fractional sodium excretion (CNa/CIn) and fractional chloride excretion (CCl/CIn) were unchanged. Catecholamine levels were unaltered. Plasma renin activity (PRA) rose significantly (P less than 0.05) at the highest infusion rate of dobutamine. It is concluded that dobutamine influences GFR, the clinical significance of which, however, is difficult to evaluate.

Immunoelectron microscopic localization of renin in the juxtaglomerular cells of the amphibian Bufo bufo

The ultrastructural localization of renin in the juxtaglomerular apparatus of the kidney of the toad Bufo bufo has been examined using an immunogold staining method for electron microscopic immunocytochemistry and an antiserum to renin isolated from the submandibular gland of the mouse. Renin immunoreactivity was confined to lamellated granules in the cytoplasm of epitheloid or juxtaglomerular cells in the glomerular afferent arterioles and also in the media cells of larger arteries. Mouse kidney tissue, examined for purposes of comparison, showed immunolabeling limited to the granules of the juxtaglomerular cells. The presence of renin or a renin-like substance in the juxtaglomerular granules of the toad kidney is discussed in relation to the lysosomal nature of these granules. A model is presented linking the lysosomal function of the juxtaglomerular granules and the release of renin mediated by beta-adrenergic receptors present on the surface of the juxtaglomerular cells.

Effect of intravenous epinephrine infusion on plasma renin activity in adrenalectomized dogs

Previous experiments have shown that circulating epinephrine stimulates renin secretin and increases plasma renin activity (PRA) when it is infused intravenously, but not when it is infused directly into the renal artery at similar infusion rates. The present experiments were designed to test the hypothesis that the adrenal glands mediate the PRA response to intravenous epinephrine infusion. Accordingly, anesthetized dogs were prepared with either an acute bilateral adrenalectomy or a sham-adrenalectomy procedure. Epinephrine was then infused intravenously into each animal for 45 minutes at a rate of 25 ng X kg-1 X min-1. Time control experiments in which epinephrine was not infused were also conducted. In sham-adrenalectomized dogs, PRA (in nanograms per ml h-1) rose from 4.1 +/- 1.4 in the control period to 13.0 +/- 3.0 during intravenous epinephrine infusion (means +/- SE; p less than 0.01). In adrenalectomized dogs, PRA rose from 2.1 +/- 0.4 during the control period to 5.5 +/- 0.9 during intravenous epinephrine infusion (p less than 0.01). Neither the absolute increments in PRA nor the percent increases in PRA were significantly different between the two groups receiving epinephrine. PRA remained unchanged in time control experiments. These data demonstrate that the adrenal glands need not be present in order for intravenous epinephrine infusion to elicit an increase in PRA. The data do not support the hypothesis, therefore, that epinephrine-induced increases in PRA are initiated by receptors located within the adrenal glands.

Autoradiographic localization of beta-adrenoceptor subtypes in guinea-pig kidney

The distribution of beta-adrenoceptor subtypes in slide-mounted sections of guinea-pig kidney has been examined by the technique of in vitro labelling combined with autoradiography. Binding of (-)-[125I]-cyanopindolol (Cyp) to kidney sections equilibrated and dissociated slowly, was saturable and stereoselective with respect to the isomers of propranolol and pindolol. These characteristics were appropriate for binding to beta-adrenoceptors. Delineation of beta-adrenoceptor subtypes was achieved by use of betaxolol (beta 1-adrenoceptors) and ICI 118,551 (beta 2-adrenoceptors) and computer assisted curve fitting techniques. Both beta 1- and beta 2-adrenoceptors were present in the proportions 1:2. 3H-Ultrofilm images of (-)-[125I]-Cyp binding to guinea-pig kidney sections showed localized patches of binding in the cortex and concentrated binding in the outer stripe of the medulla. Cortical receptors were of the beta 1 subtype and those associated with the outer stripe of the medulla were of the beta 2-adrenoceptor subtype. beta 1-Adrenoceptors were concentrated over glomeruli and beta 2-adrenoceptors over the straight portion of the proximal tubule.

Influence of renal perfusion pressure on alpha- and beta-adrenergic stimulation of renin release

Experiments were performed in pentobarbital-anesthetized dogs to 1) determine if neural stimulation of renin release can be mediated by renal alpha-adrenoceptors at renal nerve stimulation (RNS) frequencies that have little or no effect on total renal blood flow (less than or equal to 1.2 Hz) and 2) ascertain whether alpha-adrenergic control of renin release is affected by renal perfusion pressure (RPP). The renal nerves were electrically stimulated both in the absence of RPP control and with RPP controlled near 85 mmHg. Decreased RPP lowered the threshold for neurogenic stimulation of renin release from less than or equal to 1.2 to 0.3 Hz. beta-Adrenoceptor blockade with propranolol blunted the renin secretion rate (RSR) response to graded RNS (0.3-5.0 Hz), but the extent of inhibition during low-frequency RNS was dependent on RPP. Propranolol prevented increased RSR at 0.6-1.2 Hz RNS when RPP was 111-120 mmHg but not when RPP was 85 mmHg. Combined alpha- and beta-blockade with prazosin and propranolol totally prevented increased RSR during 0.6-1.2 Hz RNS at reduced RPP. In summary, both alpha- and beta-adrenoceptors mediate neural stimulation of renin release at RNS frequencies that do not decrease total renal blood flow when RPP is 85 mmHg.

Central neuroendocrine regulation of renin secretion rate: a hypothesis

A literature survey is presented from which is proposed a kidney-anterior hypothalamic endocrine feedback loop may be involved in regulation of renin secretion.

Threshold pressure for the pressure-dependent renin release in the autoregulating kidney of conscious dogs

The effect of varying renal artery pressure between 160 and 40 mm Hg on renal blood flow and renin release was studied in seven conscious foxhounds under beta-adrenergic blockade receiving a normal sodium diet (4.1 mmol/kg/day). Pressure was either increased by bilateral common carotid occlusion or reduced in steps and maintained constant by a control-system using an inflatable renal artery cuff. Carotid occlusion itself had no influence on renal blood flow and renin release when renal artery pressure was kept constant and the beta-receptors in the kidney were blocked. Between 160 mm Hg and resting pressure there was no change in renal blood flow; between resting blood pressure and the lower limit of autoregulation (average 63.9 mm Hg) renal blood flow increased slightly (average 7%) indicating a high efficiency of renal blood flow autoregulation. The relationship between renal artery pressure and renin release could be approximated by two linear sections: a low sensitivity to a pressure change (average slope: -0.69 +/- 0.26 ng AI/min/mm Hg) was found above a threshold pressure (average: 89.8 +/- 3.3 mm Hg) and a high sensitivity to a pressure change (average slope: -64.4 +/- 20.8 ng AI/min/mm Hg) was observed between threshold pressure and 60 mm Hg. There was no further increase of renin release between 60 and 40 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Time dependence of mechanisms in the renin response to renal nerve stimulation

The renin release responses to 1 and 5 min of renal nerve stimulation were determined. The left kidneys of alpha-chlorolose-anesthetized cats were pump perfused with blood while stimulating the decentralized renal nerves at different frequencies (0.5 ms, 10 V). With renal blood flow (RBF) held constant, 1 min of renal nerve stimulation increased renin secretion rate (RSR) at 1.0 (128%), 4.0 (168%) and 12.0 (160%) Hz. After 5 min of stimulation the responses were not different. Propranolol pretreatment prevented the increase in RSR at 1.0 Hz, and resulted in decreased RSR at 4.0 and 12.0 Hz. This response pattern occurred after 1 and 5 min of renal nerve stimulation. When renal perfusion pressure (RPP) was held constant, RSR at 1 min into the stimulation period was similar to that found in the constant RBF group. However, after 5 min the 4.0 Hz and 12.0 Hz responses were significantly greater than the 1 min responses (242% and 508%). Propranolol pretreatment resulted in renin responses after 1 min of stimulation which were similar to the beta-blocked constant RBF group. After 5 min of stimulation at 4.0 and 12.0 Hz RSR was greater than control levels. The results illustrate that renal nerve evoked renin release is time-dependent under constant RPP conditions. The data indicate the presence of 3 mechanisms in these responses. A beta-adrenergic receptor operates at all frequencies to increase renin release. When renal vasoconstriction occurs additional mechanisms are involved. One is inhibitory, independent of renal hemodynamic conditions and rapidly activated. The second is excitatory, occurs only under constant RPP conditions, and is activated slowly.

Evidence from binding studies for beta 1-adrenoceptors associated with glomeruli isolated from rat kidney

The beta adrenoceptor antagonist radioligand [3H] dihydroalprenolol (DHA) has been used to characterise beta adrenoceptors in membranes prepared from rat renal glomeruli. Association of the ligand was rapid and had reached equilibrium within 10 mins at 37 degrees C. Dissociation occurred in two distinct phases, a rapidly dissociating phase (low affinity site) and a slowly dissociating phase (high affinity site). The KD value for the high affinity site calculated from the kinetic experiments was 0.8 nM. Saturation analysis of binding gave comparable values for KD (1.77 nM) and demonstrated that membranes from glomeruli had four times the density of binding sites measured in renal cortex. In all saturation studies Hill coefficients were not significantly different from unity. Binding was stereoselective with respect to the (-) isomers of isoprenaline and propranolol and the potency of the selective displacing agents betaxolol (beta 1 adrenoceptors) and ICI 118,551 (beta 2 adrenoceptors) indicated that the receptors are of the beta 1 subtype.

Renal venous outflow and urinary excretion of norepinephrine, epinephrine, and dopamine during graded renal nerve stimulation

The effect of renal nerve stimulation (RNS) on renal venous outflow and urinary excretion of endogenous norepinephrine, epinephrine, and dopamine was examined in anesthetized dogs. In the unstimulated denervated kidney, there was a negative venoarterial concentration difference for all catecholamines. Low-level RNS (LLRNS) caused small changes in renal hemodynamics and renal venous outflow of dopamine and increased norepinephrine outflow by 3.22 +/- 0.95 pmol X min-1 X g-1 (n = 5, P less than 0.05). High-level RNS (HLRNS) reduced renal blood flow by 50% and increased renal venous outflow of norepinephrine and dopamine by 9.58 +/- 0.67 and 0.46 +/- 0.05 pmol X min-1 X g-1, respectively (n = 27, P less than 0.01 for both). Renal uptake of epinephrine was unchanged by HLRNS. The urinary excretion of norepinephrine but not dopamine was increased to a similar degree following RNS at both levels. HLRNS caused a similar increase of the urinary norepinephrine excretion from the contralateral denervated and unstimulated kidney. This could be explained by the increase in arterial norepinephrine (from 0.74 +/- 0.08 to 1.20 +/- 0.14 nM, P less than 0.01) caused by HLRNS as shown by experiments with intravenous infusions of norepinephrine. The alpha-adrenoceptor antagonist phenoxybenzamine counteracted the hemodynamic response to HLRNS and enhanced the renal venous outflow and urinary excretion of norepinephrine and dopamine. Our results indicate that renal nerves release dopamine as well as norepinephrine and that urinary catecholamine excretion is a poor indicator of intrarenal catecholamine release.

Captopril: an update review of its pharmacological properties and therapeutic efficacy in congestive heart failure

Captopril is the first angiotensin-converting enzyme inhibitor for oral administration. In combination with continued digitalis and diuretic therapy it has been demonstrated to be effective in the management of severe heart failure refractory to optimal digitalis, diuretic and, in many patients, vasodilator treatment. Most studies to date have been open trials of several weeks or months duration, but a number of patients have received continued treatment, with sustained benefit, for up to 1 year or more. A placebo-controlled trial in a limited number of patients with less severe heart failure has confirmed the results of open trials. Captopril administration improves cardiac performance as a result of a reduction in systemic vascular resistance (afterload) and the various determinants of left ventricular filling pressure (preload). Improvements in exercise tolerance and functional classification, with associated reduction of clinical symptomatology, occur with simultaneous decreases in myocardial oxygen consumption. At present, captopril is worthy of a trial in patients refractory to more traditional medical management. Whether it should be considered a 'first-line' agent after failure of optimal digitalis and diuretic therapy, and before instituting other vasodilator therapy, is less clear. In patients with severe or resistant heart failure, a response to captopril is usually accompanied by a general improvement in the quality of life. The effect of captopril treatment on 1- and 2-year survival rates in patients with severe heart failure appears similar to that reported for other vasodilators. Most patients tolerate captopril treatment well, but hypotension, reduced renal function, skin rash, dysgeusia, and neutropenia have been reported.

Potential role of increased sympathetic activity in impaired sodium and water excretion in cirrhosis

In a study of 26 patients with cirrhosis, plasma norepinephrine concentrations were significantly higher in 19 patients who abnormally excreted an acute water load than in seven who excreted the load normally (8324 +/- 116 vs. 306 +/- 33 pg per milliliter; P less than 0.001). There was also a significant positive correlation between plasma levels of norepinephrine and arginine vasopressin after the water load, as well as a negative correlation between plasma norepinephrine and the percentage of the load excreted. A positive correlation between plasma norepinephrine and plasma renin activity, as well as between norepinephrine and aldosterone, was observed. In addition, there was a negative correlation between plasma norepinephrine and urinary sodium excretion. These findings indicate that increased sympathetic activity, as assessed by plasma levels of norepinephrine, correlates closely with sodium and water retention in cirrhotic patients and thus may be of pathogenetic importance.

Effects of chronic prazosin treatment on the renin-angiotensin-aldosterone system in man

The effects of chronic prazosin treatment (3 mg/day for three weeks) on plasma renin activity (PRA) and plasma aldosterone (PA) levels were evaluated in 12 hypertensive patients, under conditions of metabolic balance. After three weeks of drug administration no significant change occurred in PRA as well as PA levels, with respect to pretreatment values, both in basal conditions and following 2 hours of ambulation. No change was observed in heart rate, while a fall in both systolic (P less than 0.02) and diastolic (P less than 0.05) blood pressure occurred in supine as well as in deambulation-stimulating condition. A mild increase in body weight (P less than 0.05) and a decrease in serum sodium (P less than 0.05) was induced by prazosin treatment. These findings are in keeping with the pharmacologic properties of prazosin, which is a selective blocker of postsynaptic alpha adrenoreceptors and therefore lowers vascular resistance without reflex sympathetic overactivity. The moderate volume expansion after prazosin does not appear to be aldosterone mediated.

Conditions for stimulation of renin release by cyclic AMP in anaesthetized dogs

Cyclic AMP (cAMP) is the intracellular mediator of beta-adrenergic stimulation in most tissues. Stimulation of beta-adrenoceptors increases renin release much more at low than at control arterial perfusion pressure. If beta-adrenergic stimulation is mediated by cAMP, this nucleotide should also potentiate renin release at low perfusion pressure. In anaesthetized, propranolol treated dogs, the dibutyryl derivative of cAMP (DB-cAMP), which penetrates cell membranes more readily than cAMP, increased renin release significantly during renal arterial constriction at a perfusion pressure below the range of autoregulation, but no significant effect was observed at control blood pressure. A dose-response relationship could be demonstrated in propranolol treated dogs by administering DB-cAMP at 10, 100 and 1000 micrograms/min at low but not at control blood pressure. Since sodium excretion increased, stimulation of a macula densa mechanism is unlikely, whereas arteriolar dilation, caused by autoregulation at low blood pressure, may condition the juxtaglomerular apparatus for renin release. Infusion of cAMP had no effect on renin release either at control or low blood pressure, whereas 5'AMP exerted a marked inhibitory effect at low blood pressure. We conclude that infusion of DB-cAMP rather than cAMP stimulates renin release at low but not at control blood pressure and that this effect is not mediated by beta-adrenergic receptors; cAMP may be an intracellular mediator of renin release.

Neurohumoral control mechanisms in congestive heart failure

Neurohumoral vasoconstrictor systems may play an important role in the hemodynamic derangement and natural history of congestive heart failure (CHF) by raising impedance to left ventricular ejection and shifting blood centrally to augment cardiac filling. Activation of the sympathetic nervous system, and renin-angiotensin system, and the antidiuretic hormone-vasopressin system can be demonstrated in clinical CHF by increased plasma levels of norepinephrine, renin activity, and arginine vasopressin. Because the magnitude of increase in each of these hormones varies widely from patient to patient, profiling of the neurohumoral response might provide new insight into the mechanisms of regulation of the circulation in CHF and into specific management with drugs to inhibit or reverse the vasoconstrictor process. Preliminary encouraging experience with converting-enzyme inhibitors to block formation of angiotensin II and alpha-receptor blockers to inhibit norepinephrine-induced vasoconstriction raise the possibility that selective therapy may eventually have a place in long-term management of CHF. Controlled trials in a larger patient population are now required.

Renin release after furosemide and ethacrynic acid in man. Evidence for neural reflex control mechanisms

The mechanisms of renin release after furosemide (F) and ethacrynic acid (EA) in man were examined. We evaluated whether acute volume shifts within the low pressure system after F induce renin release via neural pathways. Immersion in a water-bath or beta-blockade reduced the increase of plasma renin concentration after F but not after EA. It is concluded that acute renin release after F but not after EA in man is partially due to neurally mediated reflexes originating from volume receptors in the cardiopulmonary area.

The role of prostaglandins in the alpha- and beta-adrenoceptor mediated renin release response to graded renal nerve stimulation

The role of prostaglandins in the renin release response to renal nerve stimulation (RNS) at different intensities was examined in the anaesthetized dog. The animals were divided into two groups receiving either low or high level RNS, defined by the frequencies of stimulation producing reduction in renal blood flow by 5% or less and 50%. Indomethacin or diclofenac sodium (5 mg/kg i.v.), prostaglandin synthesis inhibitors, did not affect the renin release response to low level RNS but decreased the renin release response to high level RNS by 31 +/- 8% (P less than 0.01). Addition of metoprolol, (0.5 mg/kg i.v.) beta-1-adrenoceptor antagonist, to indomethacin or diclofenac sodium resulted in a greater reduction (68 +/- 6% P less than 0.01) of the renin release response to high level RNS compared to that produced by either drug alone. Metoprolol, alone, reduced the renin release response to high level RNS by 37 +/- 14% (P less than 0.05). Phenoxybenzamine (0.6 microgram . kg-1 . min-1), alpha-adrenoceptor antagonist, into the renal artery practically abolished the renal vasoconstrictor response to high level RNS and reduced the renin release response by 50 +/- 7% (P less than 0.01). Addition of metoprolol to phenoxybenzamine practically abolished the renal vasoconstrictor response and the renin release response to high level RNS; 94 +/- 4% (P less than 0.01). Addition of phenoxybenzamine to indomethacin or diclofenac sodium practically abolished the renal vasoconstrictor response to high level RNS but did not produce any greater reduction of the renin release response than that produced by either drug alone. These findings suggest that low level RNS results in renin release which is not dependent on prostaglandins. High level RNS results in renin release which is partly mediated by beta-1-adrenoceptors and partly related to alpha-adrenoceptors mediated renal vasoconstriction. Prostaglandins are not involved in the renin release deriving from alpha-adrenoceptor mediated renal vasoconstriction.

The influence of circulating catecholamines and prostaglandins on canine renal hemodynamics during hemorrhage

The relationship between circulating catecholamines and prostaglandins and the independent contribution of circulating catecholamines to renal vasoconstriction during hemorrhage is unknown. The renal hemodynamic effects of a 30% decrease in blood pressure by hemorrhage were therefore studied in three groups of anesthetized dogs which had undergone prior bilateral renal denervation. A constant unilateral infusion of the catecholamine antagonist phenoxybenzamine (POB, 0.2 micrograms/kg per min) into the renal artery during hemorrhage was also performed. In the control (C) dogs (n = 6), hemorrhage was not associated with significant changes in glomerular filtration rate (GFR) or renal blood flow (RBF) in either POB-infused and denervated or noninfused, denervated kidneys. In the second group of dogs (n = 8), pretreated with the prostaglandin inhibitor indomethacin (IN, 10 mg/kg, iv), POB-infused and denervated kidneys had a significantly higher GFR (30 vs. 23 ml/min, P less than 0.05) and RBF (180 vs. 130 ml/min, P less than 0.0-5) than contralateral denervated kidneys during the hemorrhage period. Similar results were observed in the third group of dogs (n = 6) pretreated with the chemically dissimilar prostaglandin inhibitor meclofenemate (M). Circulating plasma catecholamines increased to a similar degree in C (116 to 530 pg/ml, P less than 0.005), IN (116 to 488 pg/ml, P less than 0.005), and M (75 to 315 pg/ml, P less than 0.01) groups; the major part of this increase was due to an increase in plasma norepinephrine (NE). These results indicate that, in this model of hemorrhage, plasma NE exerts a moderate but significant renal vasoconstrictor effect which is unmasked by prostaglandin inhibition.

The role of beta-1-adrenoceptors in the renin release response to graded renal sympathetic nerve stimulation

The contribution of beta-adrenoceptor activation to renin release was examined in anaesthetized dogs using renal nerve stimulation (RNS) at different discharge rates in the presence of i.v. beta-adrenoceptor blockade. The animals were divided into 2 groups, which received either low or high level of RNS, defined by the frequency of stimulation producing decrease in renal blood flow of 5 and 50%, respectively. Low level RNS increased renin release tenfold. The renin release response was almost abolished by 0.5 mg/kg of metoprolol or dl-propranolol but unaffected by 0.5 mg/kg of d-propranolol. The increase in renin release to high level RNS was equally reduced by 33% by 0.5 mg/kg and 2.0 mg/kg of metoprolol. dl-propranolol, 0.5 mg/kg, reduced the renin release response to about the same extent, 44%, while 2.0 mg/kg reduced it somewhat more, 59%. This was probably due to its membrane stabilizing properties as d-propranolol, 2.0 mg/kg and lidocaine 2.0 mg/kg + 0.1 mg x kg-1 x min-1, also reduced the renin release response. These data suggest that the renin release response to low level RNS is almost completely mediated by beta-adrenoceptors which are of the beta-1 subtype. High level RNS results in a renin release, which is only partly mediated by beta-1-adrenoceptors. The remainder is apparently related to other mechanisms activated by high level RNS and is probably a consequence of the associated renal vasoconstriction.

Mechanisms of renin secretion during hemorrhage in the dog

The importance of renal perfusion pressure (RPP), the sympathetic beta adrenergic nervous system and renal prostaglandins (PG) on renin release during a uniform 15-17% reduction in blood pressure by hemorrhage (HH) was studied systematically in anesthetized dogs. All groups of animals had similar decrements in systemic and renal hemodynamics with HH. In control dogs (n = 7), both plasma renin activity (PRA, 4.1-9.0 ng angiotensin I/ml per h, P < 0.05) and renin secretory rate (RSR, 26-228 ng/ml per h.min, P < 0.005) increased significantly with HH. This increase in renin release during HH was not abolished by any single maneuver alone including beta adrenergic blockade with d,l-propranolol (n = 6), renal PG inhibition with indomethacin (n = 7), or control of RPP (n = 6). However, when beta adrenergic blockade was combined with control of RPP (n = 7) during HH, neither PRA (1.9-2.7 ng/ml per h, NS) nor RSR (16-53 ng/ml per h.min, NS) increased significantly. Similarly, a combination of beta adrenergic blockade and PG inhibition (n = 6) also abolished the increase in PRA (1.5-1.4 ng/ml per h, NS) and RSR (14-55 ng/ml per h.min, NS) during HH despite significant decreases in sodium excretion. Finally, a combination of PG inhibition and RPP control was associated with significant increases in PRA and RSR during HH. These results support a multifactorial mechanism in renin release during HH and implicate both the beta adrenergic receptors, renal baroreceptors, and possibly the macula densa as constituting the primary pathways of renin release during HH of this magnitude. Because either constant RPP or PG inhibition blunted renin release during HH in the setting of beta adrenergic blockade, the present results strongly suggest that the renal baroreceptor, and probably the macula densa mechanism are PG mediated.

Mechanisms of portal hypertension-induced alterations in renal hemodynamics, renal water excretion, and renin secretion

Clinical states with portal venous hypertension are frequently associated with impairment in renal hemodynamics and water excretion, as well as increased renin secretion. In the present investigation, portal venous pressure (PVP) was increased in anesthetized dogs undergoing a water diuresis. Renal arterial pressure was maintained constant in all studies. As PVP was increased from 6 to 20 mm Hg, decreases in cardiac output (2.5-2.0 liter/min, P less than 0.05) and mean arterial pressure (140-131 mm Hg, P less than 0.05) were observed. Increases in PVP were also associated with decreases in glomerular filtration rate (GFR, 40-31 ml/min, P less than 0.001), renal blood flow (RBF, 276-193 ml/min, P less than 0.001), and increases in renin secretion (232-939 U/min, P less than 0.025) in innervated kidneys. No significant change in either GFR or RBF and a decrease in renin secretion occurred with increases in PVP in denervated kidneys. To dissociate the changes in cardiac output and mean arterial pressure induced by increase PVP from the observed decreases in GFR and RBF, studies were performed on animals undergoing constriction of the thoracic inferior vena cava. In these studies, similar decreases in cardiac output and mean arterial pressure were not associated with significant changes in GFR or RBF. Increases in PVP also were associated with an antidiuresis as urine osmolality increased from 101 to 446 mosmol/kg H2O (P less than 0.001). This antidiuresis was significantly blunted but not abolished by acute hypophysectomy. In hypophysectomized animals, changes in free water clearance and urine flow were linearly correlated as PVP was increased. These studies indicate that increases in PVP result in decreases in GFR and RBF and increases in renin secretion mediated by increased renal adrenergic tone. Increased PVP is also associated with antidiuresis; this antidiuresis is mediated both by vasopressin release and by diminished tubular fluid delivery to the distal nephron.