Effect of renal nerve stimulation, renal blood flow and adrenergic blockade on plasma renin activity in the cat

The denervation or activation of renal sympathetic nerve and renal blood flow

The denervation or activation of the sympathetic nerve in the kidney can affect renal hemodynamics. The sympathetic nervous system regulates the physiological functions of the kidneys. Stimulation of sympathetic efferent nerves affects various parameters related to renal hemodynamics, including sodium excretion, renin secretion, and renal blood flow (RBF). Hence, renal sympathetic fibers may also play an essential role in regulating systemic vascular resistance and controlling blood pressure. In the absence of renal nerves, the hemodynamics response to stimuli is negligible or absent. The effect of renal sympathetic denervation on RBF is dependent on several factors such as interspecies differences, the basic level of nerve activity in the vessels or local density of adrenergic receptor in the vascular bed. The role of renal denervation has been investigated therapeutically in hypertension and related disorders. Hence, the dynamic impact of renal nerves on RBF enables using RBF dynamic criteria as a marker for renal denervation therapy.

Renal medullary oxygenation decreases with lower body negative pressure in healthy young adults

One in three Americans suffer from kidney diseases such as chronic kidney disease, and one of the etiologies is suggested to be long-term renal hypoxia. Interestingly, sympathetic nervous system activation evokes a renal vasoconstrictor effect that may limit oxygen delivery to the kidney. In this report, we sought to determine if sympathetic activation evoked by lower body negative pressure (LBNP) would decrease cortical and medullary oxygenation in humans. LBNP was activated in a graded fashion (LBNP; -10, -20, and -30 mmHg), as renal oxygenation was measured (T2*, blood oxygen level dependent, BOLD MRI; n = 8). At a separate time, renal blood flow velocity (RBV) to the kidney was measured (n = 13) as LBNP was instituted. LBNP significantly reduced RBV (P = 0.041) at -30 mmHg of LBNP (Δ-8.17 ± 3.75 cm/s). Moreover, both renal medullary and cortical T2* were reduced with the graded LBNP application (main effect for the level of LBNP P = 0.0008). During recovery, RBV rapidly returned to baseline, whereas medullary T2* remained depressed into the first minute of recovery. In conclusion, sympathetic activation reduces renal blood flow and leads to a significant decrease in oxygenation in the renal cortex and medulla.NEW & NOTEWORTHY In healthy young adults, increased sympathetic activation induced by lower body negative pressure, led to a decrease in renal cortical and medullary oxygenation measured by T2*, a noninvasive magnetic resonance derived index of deoxyhemoglobin levels. In this study, we observed a significant decrease in renal cortical and medullary oxygenation with LBNP as well as an increase in renal vasoconstriction. We speculate that sympathetic renal vasoconstriction led to a significant reduction in tissue oxygenation by limiting oxygen delivery to the renal medulla.

A Chymase Inhibitory RNA Aptamer Improves Cardiac Function and Survival after Myocardial Infarction

We have reported that mast cell chymase, an angiotensin II-generating enzyme, is important in cardiovascular tissues. Recently, we developed a new chymase-specific inhibitory RNA aptamer, HA28, and we evaluated the effects of HA28 on cardiac function and the mortality rate after myocardial infarction. Echocardiographic parameters, such as the left ventricular ejection fraction, fractional shortening, and the ratio of early to late ventricular filling velocities, were significantly improved by treatment with HA28 after myocardial infarction. The mortality rate was significantly reduced in the HA28-treated group. Cardiac chymase activity and chymase gene expression were significantly higher in the vehicle-treated myocardial infarction group, and these were markedly suppressed in the HA28-treated myocardial infarction group. The present study provides the first evidence that a single-stranded RNA aptamer that is a chymase-specific inhibitor is very effective in the treatment of acute heart failure caused by myocardial infarction. Chymase may be a new therapeutic target in post-myocardial infarction pathophysiology.

Eppur Si Muove: The dynamic nature of physiological control of renal blood flow by the renal sympathetic nerves

Tubuloglomerular feedback and the myogenic response are widely appreciated as important regulators of renal blood flow, but the role of the sympathetic nervous system in physiological renal blood flow control remains controversial. Where classic studies using static measures of renal blood flow failed, dynamic approaches have succeeded in demonstrating sympathetic control of renal blood flow under normal physiological conditions. This review focuses on transfer function analysis of renal pressure-flow, which leverages the physical relationship between blood pressure and flow to assess the underlying vascular control mechanisms. Studies using this approach indicate that the renal nerves are important in the rapid regulation of the renal vasculature. Animals with intact renal innervation show a sympathetic signature in the frequency range associated with sympathetic vasomotion that is eliminated by renal denervation. In conscious rabbits, this sympathetic signature exerts vasoconstrictive, baroreflex control of renal vascular conductance, matching well with the rhythmic, baroreflex-influenced control of renal sympathetic nerve activity and complementing findings from other studies employing dynamic approaches to study renal sympathetic vascular control. In this light, classic studies reporting that nerve stimulation and renal denervation do not affect static measures of renal blood flow provide evidence for the strength of renal autoregulation rather than evidence against physiological renal sympathetic control of renal blood flow. Thus, alongside tubuloglomerular feedback and the myogenic response, renal sympathetic outflow should be considered an important physiological regulator of renal blood flow. Clinically, renal sympathetic vasomotion may be important for solving the problems facing the field of therapeutic renal denervation.

Pathophysiology of Hyponatremia in Children

Hyponatremia is a common electrolyte disorder in children. It is generally defined as plasma sodium of less than 135 mmol/l. Sodium homeostasis is essential for maintaining intravascular volume and is tightly linked to water balance. Plasma water volume is regulated mainly by the secretion of an antidiuretic hormone (ADH) and by the thirst mechanism. ADH is synthesized in the hypothalamus and stored in the posterior hypophysis. It binds to V2 receptors in the distal nephron and induces translocation of aquaporin water channels in the plasma membrane to retain water. There are two main types of receptors involved in the control of the body water balance-osmoreceptors and baroreceptors. Osmoreceptors reside in hypothalamus and respond to changes of extracellular fluid (ECF) osmolality. Baroreceptors are mechanoreceptors that sense blood pressure in the vessel wall. Response reflexes from baroreceptors influence sympathetic outflow, vessel tonus, and cardiac output. An increase of 1% of plasma osmolality may cause an increase in ADH levels, while the threshold of volume receptors for ADH secretion is higher. However, significant hypotension is a more potent stimulus for ADH secretion than increased osmolality. The main cause of pediatric hyponatremia is an abundance of free water. This may occur in hypovolemic children with low ECF volume, normovolemic patients with inappropriately increased ADH secretion, and also in hypervolemic individuals with decreased effective circulating volume and appropriately increased ADH levels. Proper understanding of the pathophysiology of hyponatremic states is essential for establishing the correct diagnosis and appropriate therapy.

Renal nerve stimulation leads to the activation of the Na+/H+ exchanger isoform 3 via angiotensin II type I receptor

Renal nerve stimulation at a low frequency (below 2 Hz) causes water and sodium reabsorption via α1-adrenoreceptor tubular activation, a process independent of changes in systemic blood pressure, renal blood flow, or glomerular filtration rate. However, the underlying mechanism of the reabsorption of sodium is not fully understood. Since the sympathetic nervous system and intrarenal ANG II appear to act synergistically to mediate the process of sodium reabsorption, we hypothesized that low-frequency acute electrical stimulation of the renal nerve (ESRN) activates NHE3-mediated sodium reabsorption via ANG II AT1 receptor activation in Wistar rats. We found that ESRN significantly increased urinary angiotensinogen excretion and renal cortical ANG II content, but not the circulating angiotensinogen levels, and also decreased urinary flow and pH and sodium excretion via mechanisms independent of alterations in creatinine clearance. Urinary cAMP excretion was reduced, as was renal cortical PKA activity. ESRN significantly increased NHE3 activity and abundance in the apical microvillar domain of the proximal tubule, decreased the ratio of phosphorylated NHE3 at serine 552/total NHE3, but did not alter total cortical NHE3 abundance. All responses mediated by ESRN were completely abolished by a losartan-mediated AT1 receptor blockade. Taken together, our results demonstrate that higher NHE3-mediated proximal tubular sodium reabsorption induced by ESRN occurs via intrarenal renin angiotensin system activation and triggering of the AT1 receptor/inhibitory G-protein signaling pathway, which leads to inhibition of cAMP formation and reduction of PKA activity.

Autonomic regulation of kidney function

The kidneys play a central role in cardiovascular homeostasis by ensuring a balance between the fluid taken in and that lost and excreted during everyday activities. This ensures stability of extracellular fluid volume and maintenance of normal levels of blood pressure. Renal fluid handling is controlled via neural and humoral influences, with the former determining a rapid dynamic response to changing intake of sodium whereas the latter cause a slower longer-term modulation of sodium and water handling. Activity in the renal sympathetic nerves arises from an integration of information from the high and low pressure cardiovascular baroreceptors, the somatosensory and visceral systems as well as the higher cortical centers. Each sensory system provides varying input to the autonomic centers of the hypothalamic and medullary areas of the brain at a level appropriate to the activity being performed. In pathophysiological states, such as hypertension, heart failure and chronic renal disease, there may be an inappropriate sympathoexcitation causing sodium retention which exacerbates the disease process. The contribution of the renal sympathetic nerves to these cardiovascular diseases is beginning to be appreciated with the demonstration that renal denervation of resistant hypertensive patients results in a long-term normalization of blood pressure.

The complex interaction between overweight, hypertension, and sympathetic overactivity

There is ample evidence in the epidemiological and clinical literature that hypertension and overweight are closely and causally interrelated. Sympathetic nervous system (SNS) overactivity has been well documented in both hypertension and overweight, but it is not clear whether this is a coincidental finding or whether the association reflects a mechanistic role of SNS in these two interrelated clinical conditions. Whereas in this review we focus on the evidence for a primary role of SNS in the development of hypertension and overweight, it is clear that the process can be initiated from other starting points such as primary overeating or sleep apnea. After overweight evolves, hormones secreted by fat cells further accelerate SNS overactivity, weight gain, and blood pressure increase. The main thesis of this article is that regardless of where the process started, the same clinical picture of hypertension, overweight, and SNS overactivity will emerge. There is good evidence that in genetically prone individuals, prolonged SNS stimulation elicits a down regulation of beta-adrenergic receptors. This in turn decreases the ability to dissipate calories and diminishes the beta-adrenoceptor-mediated vasodilatation. We hypothesize that beta-adrenoceptor downregulation is the linchpin in the association of SNS with overweight and hypertension.

LONG-TERM RECORDING OF SYMPATHETIC NERVE ACTIVITY: THE NEW FRONTIER IN UNDERSTANDING THE DEVELOPMENT OF HYPERTENSION?

1. With increasing evidence that the sympathetic nervous system plays a critical role in the development of hypertension, focus is turning to how these signals translate to a chronic increase in arterial pressure. 2. The kidney's role in the control of salt and water homeostasis makes it an obvious target for such investigations. However, to date, many studies have been restricted to experiments lasting only a few hours or, at most, a few days, whereas others may use indirect methods of assessing sympathetic activity rather than direct recordings. 3. We review current approaches used to determine the effects of renal sympathetic nerve activity (SNA) on arterial pressure and suggest possible avenues of future investigation. We propose that although afferent inputs, such as from chemoreceptors and baroreceptors, are important for the short-term control of blood pressure via regulation of SNA to multiple organs, it is highly likely that alternative signals are important for setting the long-term level of renal SNA. 4. Emerging evidence indicates circulating angiotensin II is hormone that may act on the central nervous system to regulate renal SNA, renal function and, thus, blood pressure. 5. We propose that an integral part of future studies seeking an understanding of the genesis of hypertension should include chronic direct recordings of renal SNA.

Role of angiotensin II in the neural control of renal function

The aim of the present study was to distinguish between the direct effects of the renal nerves on renal function and indirect effects via neurally mediated increased systemic angiotensin II. We applied low-level electrical stimulation (1 Hz) to the left renal nerves in pentobarbitone-anesthetized rabbits for 180 minutes and measured renal blood flow, sodium excretion, and urine flow rate from both the stimulated and the nonstimulated contralateral kidney in the presence and the absence of ACE inhibition (enalaprilat). Stimulation resulted in an angiotensin II-mediated rise in arterial pressure and decreases in renal blood flow, urine flow rate, and sodium excretion on the stimulated side. On the nonstimulated denervated side, we found no change in renal blood flow, but found a decrease in urine flow rate. With ACE inhibition, renal stimulation no longer caused an increase in arterial pressure, the antidiuretic responses of the stimulated kidney were attenuated, and, importantly, the decrease in urine flow rate on the nonstimulated kidney was completely abolished. We therefore propose that although a direct effect of the renal nerves on sodium excretion is clearly present, the antidiuresis and antinatriuresis observed during renal activation is further supported by a neurally mediated increase in systemic angiotensin II.

MLR stimulation and exercise pressor reflex activate different renal sympathetic fibers in decerebrate cats

Although mesencephalic locomotor region (MLR) stimulation and the exercise pressor reflex have been shown to increase whole nerve renal sympathetic activity, it is not known whether these mechanisms converge onto the same population of renal sympathetic postganglionic efferents. In decerebrate cats, we examined the responses of single renal sympathetic postganglionic efferents to stimulation of the MLR and the exercise pressor reflex (i.e., static contraction of the triceps surae muscles). We found that, in most instances (24 of 28 fibers), either MLR stimulation or the muscle reflex, but not both, increased the discharge of renal postganglionic sympathetic efferents. In addition, we found that renal sympathetic efferents that responded to static contraction while the muscles were freely perfused responded more vigorously to static contraction during circulatory arrest. Moreover, stretch of the calcaneal (Achilles) tendon stimulated the same renal sympathetic efferents as did static contraction. These findings suggest that MLR stimulation and the exercise pressor reflex do not converge onto the same renal sympathetic postganglionic efferents.

Sympathetic modulation of blood pressure variability

Although sympathetic nervous activity (SNA) displays oscillations synchronous with the heart beat and respiration, and between 0.1-0.4 Hz, it is apparent that each of these frequencies does not have the same effect on the vasculature. Frequencies above 1 Hz do not produce oscillations in the vasculature but instead contribute to the mean level of vasoconstriction. Slower oscillations in SNA result in a cycle of vasoconstriction and vasodilation within the vasculature, the amplitude of which, generally decreases with increasing frequency. Some studies indicate that, within the same species, differences exist in the frequency responses between vascular beds, such as the skin and gut. This differential responsiveness is also found between the medullary and cortical vasculature regions of the rabbit kidney. Low-pass filter properties have been described in the iliac circulation of rats, and evidence has been provided that noradrenaline reuptake mechanisms are not the frequency limiting step of the vasculature response. Recent studies on isolated rat vascular smooth muscle cells suggest that sympathetic modulation of vascular tone is limited by the alpha-adrenoceptor signal transduction into the cells and not by an intrinsic inability of the cells to contract and relax at higher rates.

Long-term control of renal blood flow: what is the role of the renal nerves?

We have developed a system for long-term continuous monitoring of cardiovascular parameters in rabbits living in their home cage to assess what role renal sympathetic nerve activity (RSNA) has in regulating renal blood flow (RBF) in daily life. Blood pressure, heart rate, locomotor activity, RSNA, and RBF were recorded continuously for 4 wk. Beginning 4-5 days after surgery a circadian rhythm, dependent on feeding time, was observed. When averaged over all days RBF to the innervated and denervated kidneys was not significantly different. However, control of RBF around these mean levels was dependent on the presence of the renal sympathetic nerves. In particular we observed episodic elevations in heart rate and other parameters associated with activity. In the denervated kidney, during these episodic elevations, the increase in renal resistance was closely related to the increase in arterial pressure. In the innervated kidney the renal resistance response was significantly more variable, indicating an interaction of the sympathetic nervous system. These results indicate that whereas overall levels of RSNA do not set the mean level of RBF the renal vasculature is sensitive to episodic increases in sympathetic nerve activity.

Neural regulation of renal blood flow: a re-examination

1. The importance of renal sympathetic nerve activity (RSNA) in the regulation of renal function is well established. However, it is less clear how the renal vasculature responds to the different mean levels and patterns of RSNA. While many studies have indicated that small to moderate changes in RSNA preferentially regulate renin secretion or sodium excretion and only large changes in RSNA regulate renal blood flow (RBF), other experimental evidence suggests that small changes in RSNA can influence RBF 2. When RSNA has been directly measured in conjunction with RBF, it appears that a range of afferent stimuli can induce reflex changes in RBF. However, many studies in a variety of species have measured RBF only during stimuli designed to reflexly increase or decrease sympathetic activity, but have not recorded RSNA. While this approach can be informative, it is not definitive because the ability of the vasculature to respond to RSNA may, in part, reflect the resting level of RSNA and, therefore, the vasoconstrictive state of the vasculature under the control conditions. 3. Further understanding of the control of RBF by RSNA has come from studies that have analysed the underlying rhythms in sympathetic nerve activity and their effect on the cardiovascular system. These studies show that the frequency-response characteristic of the renal vasculature is such that higher frequency oscillations in RSNA (above 0.6 Hz) contribute to setting the mean level of RBF. In comparison, lower frequency oscillations in RSNA can induce cyclic vasoconstriction and dilation in the renal vasculature, thus inducing oscillations in RBF. 4. In summary, the present review discusses the neural control of RBF, summarizing evidence in support of the hypothesis that RBF is under the influence of RSNA across the full range of RSNA.

Neural regulation of kidney function by the somatosensory system in normotensive and hypertensive rats

This investigation examined the renal sympathetic nerve and renal excretory responses to somatosensory stimulation in normotensive and stroke-prone spontaneously hypertensive rats (SHRSP). Somatosensory activation was achieved by either subcutaneous capsaicin administration or exposure of the airways tract to irritant fumes from acetic acid in chloralose-urethan-anesthetized animals. In Wistar rats, blood pressure increased between 10 and 20% (P < 0.001-0.01), renal perfusion pressure was maintained unchanged, renal hemodynamics were unaltered, and urine flow and sodium excretion were decreased by 25 to 50% (P < 0.001-0.05). In the SHRSP, the somatosensory-induced increases in blood pressure were slightly larger (approximately 15-20% P < 0.05) than those of the Wistar rats, whereas the excretory responses were one-half those of the normotensive animals (P < 0.05). The somatosensory challenges reflexly increased integrated renal sympathetic nerve activity in both normotensive and hypertensive rats. The power spectral analysis demonstrated that the increases in percentage power at heart rate frequency and total power were two to three times more (P < 0.05) in the Wistar rats compared with the SHRSP. The reduced ability of the SHRSP to modulate the energy in the renal sympathetic nerve signal at heart rate frequency might explain in part the attenuated functional responses to the somatosensory challenges.

Effect of renal nerves on expression of renin and angiotensinogen genes in rat kidneys

In this study, we try to determine the influence of renal nerve activity on renal function, plasma renin activity (PRA), and the corresponding expression of renin and angiotensinogen genes in the kidney. In pentobarbitone-anesthetized rats, the left renal nerves were stimulated (15 V, 0.2 ms) at frequencies to reduce left renal blood flow by 15, 30, and 45%. There were corresponding reductions in glomerular filtration rate from 12 to 52% and absolute and fractional sodium excretions from 20 to 75%. PRA levels in control rats were 10.8 +/- 1.5 and were increased to 65.9 +/- 9.1, 144.2 +/- 19.7, and 277.2 +/- 22.0 ng angiotensin I.h-1.ml-1 after 1 h at each of the three levels of nerve stimulation. Renal renin mRNA was similar in innervated and denervated kidneys and was not affected by the lowest level of nerve stimulation; however, neurally induced decreases in blood flow to 30 and 45% increased renin mRNA levels by 3.0- and 3.4-fold (both P < 0.05), respectively. Angiotensinogen mRNA levels were higher (P < 0.05) in kidneys subjected to the lowest level of nerve stimulation, but when renal blood flow was reduced by 30 and 45%, expression of this gene was unchanged. Stimulation of the renal nerves in the presence of the beta 1-adrenoceptor antagonist atenolol only doubled PRA at the highest rates of stimulation. Neither renal renin nor angiotensinogen mRNA were changed during neurally mediated reductions in renal blood flow of 15 or 30% after administration of atenolol.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Effects of renal denervation and reinnervation on ganglionic gene expression of neurotransmitter proteins and c-fos in rat

To investigate the molecular basis for reno-renal interactions, Northern blot analyses of sympathetic ganglia were used to study the changes in levels of mRNA encoding tyrosine hydroxylase (TH), neuropeptide Y (NPY), and c-fos at 4, 14, 21, and 56 days after denervation of the left kidney, and of c-fos mRNA at 1 and 4 h after denervation. Ganglia included in the study were right and left paravertebral chain ganglia (PVG, T11 to L2), celiac-mesenteric plexus (CMP), and right and left superior cervical ganglia (SCG). Levels of TH mRNA in the left PVG and CMP were decreased at 4 and 14 days compared to controls. Levels were elevated at 21 days and similar to control levels at 56 days. In the right PVG, TH mRNA levels were elevated at 4 and 14 days, diminished from this elevated level at 21 days, and similar to control levels at 56 days. No differences were found in TH mRNA levels of left or right SCG compared to controls. In long-term experiments (days), no differences in NPY or c-fos mRNA levels were found in any of the ganglia from experimental rats compared to controls. Levels of c-fos mRNA in the left PVG and CMP were decreased at 1 hour compared to control levels. By 4 h, differences in mRNA levels were no longer apparent. In the right PVG, c-fos mRNA levels were elevated at 1 hour and no longer different from control levels at 4 h. No differences were found in c-fos mRNA levels of left or right SCG compared to controls.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the renal nerves in renin synthesis

1. Renin synthesis and secretion were studied in Balb/c mice with a denervated left kidney. 2. Denervation inhibited renin secretion. 3. Denervation reduced the renal renin content. 4. Denervation reduced renal renin mRNA. 5. Renal denervation inhibits renin secretion by blocking the synthetic system prior to mRNA formation.

Effects of centrally administered angiotensin on sympathetic nerve activity and blood flow to the kidney in conscious rats

The effects of intracerebroventricular (i.c.v.) administration of different doses (10 pg-100 ng) of angiotensin II (AII) on renal sympathetic nerve activity (RSNA), mean arterial blood pressure (MAP), heart rate (HR), renal blood flow and femoral blood flow have been examined in conscious rats. Administration of AII (10 ng) through a chronically implanted cannula induced an increase in MAP (20-22 mmHg), a decrease in HR (24 bpm), a decrease in RSNA by 57%, a decrease of femoral blood flow by 21% but no change in renal blood flow. The effects on MAP, HR and RSNA are greatly attenuated by the prior i.c.v. injection of an AII-antagonist saralasin. In anesthetized rats, renal denervation significantly attenuated an increase in urinary sodium excretion induced by i.c.v. injection of AII. Since activation of the renal nerve is known to induce sodium reabsorption from the renal tubule and renin release, the relevance of the present finding is discussed in relation to the effect of AII on sodium excretion.

Resetting of pressure-dependent renin release by intrarenal alpha 1-adrenoceptors in conscious dogs

We investigated the influence of a stimulation of intrarenal alpha 1-adrenoceptors on the relationship between renin release and renal artery pressure in 8 conscious, chronically instrumented dogs receiving a normal salt diet. Renin stimulus-response curves were determined by a stepwise reduction of renal artery pressure down to 70 mm Hg (1) under control conditions, (2) during a bilateral common carotid occlusion combined with an intrarenal prazosin infusion, and (3) during an intrarenal methoxamine infusion. Both drug infusions did not alter resting renal blood flow. (1) The control renin stimulus-response curve revealed a flat portion (plateau-level) around and above the resting blood pressure and a very steep portion (slope) below a well-defined threshold pressure 10-15 mm Hg below the resting blood pressure. (2) An intrarenal alpha 1-adrenoceptor blockade by prazosin prevented the resetting of the threshold pressure which is regularly observed during bilateral common carotid occlusion. (3) An intrarenal infusion of the alpha 1-adrenoceptor agonist methoxamine increased the threshold pressure. We suggest that the neural control of renin release within the autoregulatory range of renal blood flow involves two independent mechanisms: the direct release of renin from juxtaglomerular granular cells by beta 1-adrenoceptors, and the modulation of the threshold pressure of pressure-dependent renin release by intrarenal alpha 1-adrenoceptors. The small changes in renal nerve activity necessary to reset the threshold pressure and the close relationship between the threshold pressure and resting blood pressure imply an important function of intrarenal alpha 1-adrenoceptors in the regulation of renin release.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of enalapril on changes in cardiac output and organ vascular resistances induced by alpha 1- and alpha 2-adrenoceptor agonists in pithed normotensive rats

1. Cardiac output, its distribution and regional vascular resistances were determined with tracer microspheres in pithed rats in the presence of the angiotensin converting enzyme inhibitor enalapril. The effects of enalapril on the cardiovascular responses elicited by either the alpha 1-adrenoceptor agonist phenylephrine or the alpha 2-adrenoceptor agonist xylazine were determined. 2. Enalapril decreased diastolic and mean blood pressure by decreasing cardiac index and total peripheral resistance. It induced vasodilatation in the kidney, epididimides, epididimidal fat and pancreas/mesentery. Vasoconstriction in the lungs, testes and liver was evident following enalapril administration as well as a decrease in the proportion of cardiac output passing to them, whilst the pancreas and mesentery received a greater proportion of the cardiac output. All the above effects of enalapril were reversed by infusion of angiotensin II at a rate of 75 ng kg-1 min-1. 3. Xylazine increased blood pressure by increasing both cardiac output and total peripheral resistance. Enalapril did not affect the increase in cardiac output caused by xylazine but decreased the effect of the alpha 2-agonist on blood pressure by preventing the increase in total peripheral resistance. Inhibition by enalapril of xylazine-induced vasoconstriction in the kidneys, testes, fat and gastrointestinal tract contributed to the decrease in total peripheral resistance. Enalapril also inhibited xylazine-induced changes in cardiac output distribution to the liver, lungs and heart. All the above effects of enalapril were reversed by infusion of angiotensin II. 4. Enalapril decreased the sustained phase of the pressor response to an infusion of phenylephrine whilst having no effect on the initial peak pressor response to a bolus injection of phenylephrine. Phenylephrine increased both cardiac output and total peripheral resistance and enalapril abolished its effect on total peripheral resistance whilst having no effect on the increase in cardiac output. Enalapril inhibited phenylephrine-induced vasoconstriction in the testes, fat, muscle, spleen and gastrointestinal tract. Enalapril also inhibited phenylephrine-induced changes in cardiac output distribution to the lungs and liver. The infusion of angiotensin II did not fully reverse the inhibitory effect of enalapril either on the phenylephrine-induced increases in diastolic blood pressure or on the vasoconstriction in the fat, spleen and gastrointestinal tract, but did reverse all other effects of enalapril.

The role of angiotensin II in the antidiuresis and antinatriuresis induced by stimulation of the sympathetic nerves to the rat kidney

1 Stimulation of the renal sympathetic nerves in pentobarbitone anaesthetized rats at low frequencies, which did not statistically change renal blood flow and glomerular filtration rate, significantly reduced urine flow by 35%, absolute sodium excretion by 44% and fractional sodium excretion by 40%. 2 In rats fed a low sodium diet for 2 to 3 weeks, similar renal nerve stimulation caused no consistent changes in renal haemodynamics but decreased urine flow by 38%, absolute sodium excretion by 44% and fractional sodium excretion by 38%, which were identical responses to those obtained in sodium replete animals. In contrast, stimulation of the renal nerves in sodium depleted rats given a constant infusion of captopril at 500 micrograms/kg/h had no statistically significant effect on either water or sodium excretion. 3 Renal nerve stimulation in rats given saline to drink and DOCA for 2 to 3 weeks did not significantly change either renal haemodynamics or the output of water or sodium. However, in other animals maintained on a high salt intake but given a constant infusion of angiotensin II (20 ng/kg/min), renal nerve stimulation caused minimal changes in renal haemodynamics but significantly reduced urine flow by 41%, absolute sodium excretion by 54% and fractional sodium excretion by 49%. 4 These results show that the neurally-mediated tubular responses require the presence of a minimal circulating level of angiotensin II since, when its production is blocked, either acutely or chronically, the renal nerve-induced antinatriuresis and antidiuresis is inhibited but can be restored by the infusion of angiotensin II. These findings provide direct evidence that angiotensin II has an important potentiating action at the renal nerve junctions, most probably at the epithelial cells of the renal tubule.

Active and inactive renin in the cat

Pentobarbitone-anesthetized cats underwent peritoneal dialysis and had blood samples removed and kidneys deep frozen at sacrifice. Inactive renin is easily measurable in cat plasma and peritoneal dialysate fluid. Only small amounts are found after acid activation at pH 4.0, but large amounts after trypsin 2 mg/ml at 4 degrees C for 10 minutes. Mean active renin in pentobarbitone-anesthetized cats was 1.8 +/- 0.4 pmoles AI/ml/hr, while inactive renin was 2.3 +/- 0.5 pmoles AI/ml/hr. The increased angiotensin I producing activity after trypsin in peritoneal dialysate was most active at pH 7.0 (plasma and kidney active renin 7.25 and 7.85), and had an apparent molecular weight of 39-40,000. (Plasma active renin had an apparent MW of 33,500 and kidney active renin 36,000. Plasma inactive renin had an apparent MW of 35,500) Cat plasma after cibacron-blue affinity chromatography showed mainly active renin in the breakthrough buffer (30% of total renin eluted), and renin which is almost entirely inactive in the bound peak (70% of total renin eluted). Active renin from plasma and kidney, and activated inactive renin from concentrated peritoneal fluid, showed exactly similar inhibition by the renin inhibitor H77 (IC50 0.3 microM). Cat plasma angiotensinogen had an apparent MW of 53,000.

Preclinical studies on angiotensin converting enzyme inhibitors

The identification of the renin-angiotensin-aldosterone system in the control of blood pressure, and the preclinical development of the angiotensin converting enzyme inhibitors for therapeutic use are reviewed. The properties of these compounds are discussed with respect to their in vitro enzyme inhibitory potency; prevention of the pharmacological effects of angiotensin I; potentiation of those of bradykinin; tissue enzyme inhibition; mechanism of effect on blood pressure both alone and in combination with other antihypertensive agents; and effect on cardiac parameters.

Dissociation of the responses of the renin-angiotensin system and sympathetic nervous system to a vasodilator stimulus in congestive heart failure

The ability of neurohumoral reflex control mechanisms to respond to a vasodilator mediated alteration in hemodynamic status was studied. A sodium nitroprusside infusion was administered to 5 normal subjects and 47 patients with severe congestive heart failure resulting in significant decreases in mean arterial pressure and in systemic vascular resistance. As expected in normals the vasodilator stimulus caused a reflex activation in both the renin-angiotensin system and sympathetic nervous system as measured by increased plasma renin activity and plasma norepinephrine, respectively. In the patients with heart failure, plasma renin activity rose similarly in response to nitroprusside (+63% in heart failure, 100% in normals, P = NS) while plasma norepinephrine remained essentially unchanged (+11% in heart failure, 98% in normals, P less than 0.01). These data demonstrate that the neurohumoral dysfunction seen in patients with heart failure is not uniform. In patients with severe congestive heart failure the renin-angiotensin system apparently is activated by mechanisms other than sympathetic nervous stimulation. This intact reflex humoral response may still function in opposition to the beneficial hemodynamic effects produced by direct vasodilators such as nitroprusside.

Potentiation of hemorrhage-evoked catecholamine release by prior blood loss in cats

The effect of prior blood loss on the plasma catecholamine response to acute hemorrhage (H) was assessed in alpha-chloralose-urethane anesthetized cats. Animals sustained an initial H period of 0 (samples only), 10, or 20% H total blood volume. Ninety minutes after reinfusion of the shed blood, all animals sustained a rapid 20% H. The catecholamine and arterial pressure responses to this second 20% H were assessed every 2 min for 20-min duration. Plasma norepinephrine increased modestly in the 0/20% group (+0.63 +/- 0.13 ng/ml) and 10/20% group (+0.66 +/- 0.07 ng/ml), whereas the 20/20% group showed a much larger (P less than 0.01) mean increase of 3.58 +/- 1.16 ng/ml. Plasma epinephrine did not increase after 0/20% (+0.05 +/- 0.02 ng/ml), increased slightly after 10/20% H (+0.10 +/- 0.05 ng/ml), and demonstrated a large significant increase after 20/20% H (+0.48 +/- 0.18 ng/ml). The differential effect of 20% H on catecholamine release, depending on the magnitude of prior blood loss, was not the result of altered mean arterial or pulse pressure responses to H. Correlation analyses revealed that the mean increases in epinephrine and norepinephrine during the post-H sampling period were well correlated in each animal (r = 0.864, P less than 0.001). The data indicate that the priming effect of prior blood loss on H evoked catecholamine release: requires an initial blood loss of 20% of total blood volume; occurs rapidly, as it is seen by 90 min after the initial H period; and affects epinephrine and norepinephrine similarly. We conclude that the immediate past secretory history of the sympathoadrenal system affects its responsiveness to subsequent blood loss.

The influence of diltiazem and nifedipine on the haemodynamic and tubular responses of the rat kidney to renal nerve stimulation

An investigation was undertaken in the pentobarbitone anaesthetized rat to determine the influence of calcium entry blockade on the haemodynamic and tubular responses of the kidney to renal sympathetic nerve stimulation. Electrical activation of the nerves, at rates causing a 12% reduction in renal blood flow, did not change glomerular filtration rate but significantly reduced urine flow (32%) and absolute (34%) and fractional sodium excretion (33%). Intravenous administration of diltiazem (10 and 20 micrograms/kg/min) and nifedipine (1.0 and 2.0 micrograms/kg/min) caused significant reductions of systemic blood pressure. Stimulation of the renal nerves, to reduce renal blood flow between 15% and 18% in the presence of both low and high doses of diltiazem, caused significant falls in glomerular filtration rate of 9% and 23%, respectively. During the low dose of nifedipine glomerular filtration rate did not change but in animals receiving the higher dose it fell by 17%. The magnitude of the neurally induced changes in urine flow, absolute and fractional sodium excretions were not different at either dose level of diltiazem or nifedipine from that observed in the absence of drugs. Stimulation of the renal nerves at low rates, which did not change renal blood flow, had no effect on glomerular filtration rate but significantly reduced urine flow (38%) and absolute (39%) and fractional sodium excretion (35%). At these low rates of nerve stimulation glomerular filtration rate remained unchanged during the infusion of either dose level of diltiazem. However, during administration of both the low and high doses of nifedipine there were significant reductions of glomerular filtration rate of 20% and 17%, respectively. The magnitude of the neurally induced changes in urine flow, absolute and fractional sodium excretions in the presence of both low and high doses of diltiazem and nifedipine were the same as those observed in the absence of drugs. The results of this study provide no evidence to indicate that the nerve mediated increases in tubular sodium reabsorption, a response involving alpha-adrenoreceptors, is dependent on the movement of calcium into the epithelial cells. The data did not indicate that blockade of calcium entry into cells impaired the ability of the kidney to regulate glomerular filtration rate which appeared to be due to a lack of renal efferent arteriolar vasoconstriction.

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.

Cardiovascular responses to intrathecal administration of arginine vasopressin in rats

Arginine vasopressin (AVP) has been localized in numerous extrahypothalamic brain regions and in the spinal cord. The results of intracerebroventricular AVP injections and microinjection of AVP into the brain stem suggest that this peptide, acting centrally at higher levels, may influence cardiovascular function. No function for the AVP occurring at spinal levels has been reported. In this study we report that AVP, in picomole quantities, increased arterial blood pressure and integrated heart rate in a dose-dependent manner following intrathecal application to the thoracic region in the rat. This response was not blocked by intravenous administration of the AVP antagonist d(CH2)5-D-Tyr-VAVP. These results suggest that AVP, acting within the spinal cord, may alter neural outflow regulating blood pressure and heart rate.

Further studies on renal nerve stimulation induced release of noradrenaline and dopamine from the canine kidney in situ

The renal venous outflow of dopamine and noradrenaline were studied in the canine kidney in situ in connection with renal nerve stimulation (RNS). RNS (0.5-4 Hz) caused frequency-dependent increases in the outflow of both catecholamines, which could be detected already at 0.5 Hz. The ratio dopamine/noradrenaline in renal venous plasma (approximately 0.15) was not influenced by varying the RNS parameters but was significantly enhanced (to about 0.25) by pretreatment with guanethidine according to a procedure previously used to demonstrate renal dopaminergic vasodilation. The unstimulated kidney removed conjugated dopamine (which represents 98-99% of the total dopamine in plasma). During RNS the conjugated dopamine outflow to renal venous blood increased, but measurements of conjugated dopamine were less reliable than measurements of free dopamine to assess dopamine release from the kidney. When studying the renal nerve contributions to the renal venous outflow of dopamine and noradrenaline more accurate estimates may be obtained by correcting for the removal of catecholamines delivered to the kidney in arterial plasma. Such corrections were performed with endogenous adrenaline or radiolabelled noradrenaline. The two methods of correction yielded similar results and showed that RNS reduced catecholamine extraction in the kidney. The high ratio of dopamine/noradrenaline in kidney tissue (with a preferential distribution of dopamine to the cortex) and the dopamine outflow to renal venous plasma during RNS support the existence of specific dopaminergic nerves in the dog kidney.

An in vivo study of the alpha-adrenoreceptor subtypes on the renal vasculature of the anaesthetized rabbit

An attempt has been made to classify the subtypes of alpha-adrenoreceptors mediating renal vasoconstriction in vivo using renal nerve stimulation and alpha-adrenoreceptor agonists and antagonists of varying selectivity in anesthetised rabbits. In the first series of experiments prazosin more potently inhibited the phenylephrine than the nerve induced renal vasoconstriction while phentolamine inhibited the response to phenylephrine and nerve stimulation nearly equally. In the second series of experiments low to moderate doses of yohimbine potently inhibited the renal vasoconstriction of clonidine, was less potent on that induced by noradrenaline and did not significantly affect that of phenylephrine. Prazosin was a potent antagonist of phenylephrine induced vasoconstriction but was less potent on nerve stimulation and noradrenaline, and was without effect on clonidine. In the third series of experiments, prazosin partially inhibited the renal vasoconstriction produced by all frequencies of nerve stimulation and that produced by high doses of noradrenaline. The prazosin resistant component of nerve stimulation and exogenous noradrenaline was significantly reduced by the addition of yohimbine. These results have been taken to suggest the presence of both alpha 1- and alpha 2-adrenoreceptors in the renal vasculature of the rabbit which mediate the renal vasoconstriction of exogenous and endogenous noradrenaline. They further show that in the rabbit the alpha 2-adrenoreceptors mediate a much greater proportion of the total renal vasoconstriction compared with the dog, cat or rat in which the alpha 1-adrenoreceptor population appears to be the predominant receptor.

The effect of graded renal nerve stimulation on renal function in the anaesthetized rabbit

The renal nerves of the rabbit were stimulated to cause less than 5, 9 or 22% reduction in renal blood flow. Glomerular filtration rate was reduced by 0, 3 and 11% respectively when the renal nerves were stimulated at these increasing rates. At low and moderate rates of renal nerve stimulation absolute and fractional sodium excretions were reduced by between 17 and 22%, but at the highest rates they were reduced by 55 and 53% respectively. At increasing rates of renal nerve stimulation plasma renin activity was increased by 40, 70 and 180%.

Effects of adrenoceptor agonists and antagonists on smooth muscle cells and neuromuscular transmission in the guinea-pig renal artery and vein

In the guinea-pig renal artery and vein, the membrane potential was -66.8 mV and -46.8 mV, the length constant 0.54 mm and 0.43 mm, and the time constant 240 ms and 98 ms, respectively. The maximum slope of the depolarization produced by a 10 fold increase [K]o was 46 mV in the renal artery and 39 mV in the renal vein. Noradrenaline (NA over 5 X 10(-7)M in the artery and over 10(-7)M in the vein) depolarized the membrane and slightly reduced the membrane resistance, assessed from relative changes in the amplitude of electrotonic potential. The action of NA was suppressed by prazosin in the artery but by yohimbine in the vein, i.e. the alpha 1-adrenoceptor is present in the extrajunctional muscle membrane in the renal artery while the alpha 2-adrenoceptor is present in the renal vein. Dopamine and isoprenaline did not modify the membrane properties. In the renal artery, repetitive perivascular nerve stimulation (0.1 ms, 50 Hz, 5 shocks) evoked excitatory junction potential (e.j.p.). Applications of guanethidine (10(-6) M) or tetrodotoxin (3 X 10(-7) M) abolished the generation of the e.j.p.. Low concentrations of phentolamine (5 X 10(-7) M), prazosin (10(-7) M) and yohimbine (5 X 10(-7) M) enhanced the e.j.p. amplitude, while high concentrations of phentolamine (10(-5) M) and prazosin (greater than 10(-5) M) reduced the amplitude of e.j.p.s. NA, dopamine and clonidine consistently suppressed the amplitude of e.j.ps, at any given concentration over 10(-7) M. Spontaneous generated miniature e.j.ps (m.e.j.ps) were recorded on rare occasions. Phentolamine and yohimbine both at 5 x 10(-7) M and prazosin 10(-7) M increased the appearance of m.e.j.ps. 5 In the renal vein, repetitive nerve stimulation failed to generate the e.j.p. Sympathetic innervation to this tissue seems to be sparse. 6 Specificity of innervation and adrenoceptors present on smooth muscle cells in both the renal artery and vein are discussed, and the presynaptic regulation ofNA release is compared with findings in other vascular tissues.

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.

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.

Effect of converting enzyme inhibition on the renal haemodynamic responses to noradrenaline infusion in the rat

1 The renal haemodynamic responses to a close arterial infusion of noradrenaline (29.7-177.9 nmol kg-1 h-1) were measured in rats anaesthetized with pentobarbitone. Systemic blood pressure was unaffected by noradrenaline infusion at this dose level. Renal blood flow was significantly reduced by 16% while glomerular filtration rate remained unchanged. These responses resulted in a rise in filtration fraction of some 10%. 2 In a separate group of animals, noradrenaline infusion in this manner and at similar dose rate increased plasma renin activity approximately 3 fold. 3 Continuous infusion of the angiotensin converting enzyme inhibitor, teprotide (3.36 mumol kg-1 h-1), had no measurable effect on systemic blood pressure, renal blood flow, glomerular filtration rate or filtration fraction. 4 Infusion of noradrenaline into these animals receiving teprotide caused a significant fall in renal blood flow of 16%. There was a fall in glomerular filtration rate of some 17% which was significantly different from the response observed in the animals not receiving teprotide. There was a consequent small but insignificant fall in filtration fraction. 5 These data show that the regulation of glomerular filtration rate in response to the vasoconstrictor drug, noradrenaline, is partly mediated via the renin-angiotensin system. They provide evidence for a role of intrarenal angiotensin II in regulating glomerular filtration by causing efferent arteriolar vasoconstriction.

Renal blood-flow changes during renal nerve stimulation in rats treated with alpha-adrenergic and dopaminergic blockers

1. Blood flow through the inner cortex and outer medulla of the rat kidney was measured by the hydrogen wash-out technique. 2. Renal nerve stimulation caused vasoconstriction in both cortex and medulla. This constriction was abolished or reduced by phenoxybenzamine (9 mumole/kg I.V.), phentolamine (100 n-mole/kg) or prazosin (1.5 mumole/kg). 3. After prazosin (6 mumole/kg I.V.), renal nerve stimulation caused small but significant renal cortical vasodilatation. This vasodilatation was reversed by sulpiride (0.7 mumole kg-1 min-1), but was unaffected by propranolol (10 mumole/kg) or atropine (4.3 mumole/kg). 4. These results indicate the existence of dopaminergic vasodilator nerves to the renal cortex of the rat.

Mechanism of renin release during renal nerve stimulation in dogs

During renal nerve stimulation, a predominant vasoconstrictory effect on small arteries would lower blood pressure in the afferent arterioles and induce arteriolar dilation and renin release by the autoregulation mechanism. This hypothesis was examined in anaesthetized dogs by stimulating renal nerves at 4 Hz which permitted continuous reduction of renal blood flow (RBF) by 30-40%; renin release increased almost equally at control and low blood pressure, and in the non-filtering kidney during ureteral occlusion. Examinations of the relationship between RBF and arterial perfusion pressure during mechanical constriction of the renal artery showed that the lowest autoregulating pressure was 25-35 mmHg higher during nerve stimulation than in control experiments, consistent with the hypothesis of arteriolar dilation. Phenoxybenzamine, an inhibitor of alpha-adrenoceptors, abolished vasoconstriction and the effect of nerve stimulation on renin release at control blood pressure; renin release rose from 0.9 +/- 0.4 to 17 +/- 5 microgram/min before, and from 1.7 +/- 0.5 to 4.6 +/- 1.4 microgram/min after phenoxybenzamine infusion. At pressures below the range of autoregulation, phenoxybenzamine did not alter renin release response to nerve stimulation. Propranolol, a Beta-adrenergic inhibitor, attenuated the effect of nerve stimulation on renin release both at control and low blood pressure. We conclude that during renal nerve stimulation (1) renin release is caused by beta-adrenergic stimulation provided the afferent arterioles are dilated and (2) that alpha-adrenergic stimulation dilated the afferent arterioles as a consequence of a predominant vasoconstrictory effect on small arteries. Hence, by inhibiting the beta-adrenergic effect by propranolol, renin release does not increase during renal nerve stimulation. Phenoxybenzamine prevents renin release at control blood pressure because afferent arterioles are not dilated during nerve stimulation. In contrast, phenoxybenzamine does not reduce renin release during nerve stimulation at low blood pressure because afferent arterioles are dilated by the autoregulating mechanism.

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.