Effect of acute sympathectomy on responses to angiotensin and norepinephrine

The structural reconformation of peptides in enhancing functional and therapeutic properties: Insights into their solid state crystallizations

Therapeutic peptides derived proteins with alpha-reconformation states like antibody shape have shown potential effects in combating terrible diseases linked with earlier signs of angiogensis, mutagenesis and transgenesis. Alpha reconformation in material design refers to the folding of the peptide chains and their transitions under reversible chemical bonds of disulfide chemical bridges and further non-covalence lesions. Thus, the rational design of signal peptides into alpha-helix is intended in increasing the defending effects of peptides into cores like adjuvant antibiotic and/or vaccines. Thereby, the signal peptides are able in displaying multiple eradicating regions by changing crystal-depositions and deviation angles. These types of molecular structures could have multiple advantages in tracing disease syndromes and impurities by increasing the host defense against the fates of pathogens and viruses, eventually leading to the loss in signaling by increasing peptide susceptibility levels to folding and unfolding and therefore, formation of transgenic peptide models. Alpha reconformation peptides is aimed in triggering as well as other regulatory functions such as remodulating metabolic chain disorders of lipolysis and glucolysis by increasing the insulin and leptin resistance for best lipid storages and lipoprotein density distributions.

Immunohistochemical Localization of AT1a, AT1b, and AT2 Angiotensin II Receptor Subtypes in the Rat Adrenal, Pituitary, and Brain with a Perspective Commentary

Angiotensin II increases blood pressure and stimulates thirst and sodium appetite in the brain. It also stimulates secretion of aldosterone from the adrenal zona glomerulosa and epinephrine from the adrenal medulla. The rat has 3 subtypes of angiotensin II receptors: AT1a, AT1b, and AT2. mRNAs for all three subtypes occur in the adrenal and brain. To immunohistochemically differentiate these receptor subtypes, rabbits were immunized with C-terminal fragments of these subtypes to generate receptor subtype-specific antibodies. Immunofluorescence revealed AT1a and AT2 receptors in adrenal zona glomerulosa and medulla. AT1b immunofluorescence was present in the zona glomerulosa, but not the medulla. Ultrastructural immunogold labeling for the AT1a receptor in glomerulosa and medullary cells localized it to plasma membrane, endocytic vesicles, multivesicular bodies, and the nucleus. AT1b and AT2, but not AT1a, immunofluorescence was observed in the anterior pituitary. Stellate cells were AT1b positive while ovoid cells were AT2 positive. In the brain, neurons were AT1a, AT1b, and AT2 positive, but glia was only AT1b positive. Highest levels of AT1a, AT1b, and AT2 receptor immunofluorescence were in the subfornical organ, median eminence, area postrema, paraventricular nucleus, and solitary tract nucleus. These studies complement those employing different techniques to characterize Ang II receptors.

Renin: at the heart of the mast cell

Cardiac mast cells proliferate in cardiovascular diseases. In myocardial ischemia, mast cell mediators contribute to coronary vasoconstriction, arrhythmias, leukocyte recruitment, and tissue injury and repair. Arrhythmic dysfunction, coronary vasoconstriction, and contractile failure are also characteristic of cardiac anaphylaxis. In coronary atherosclerosis, mast cell mediators facilitate cholesterol accumulation and plaque destabilization. In cardiac failure, mast cell chymase causes myocyte apoptosis and fibroblast proliferation, leading to ventricular dysfunction. Chymase and tryptase also contribute to fibrosis in cardiomyopathies and myocarditis. In addition, mast cell tumor necrosis factor-alpha promotes myocardial remodeling. Cardiac remodeling and hypertrophy in end-stage hypertension are also induced by mast cell mediators and proteases. We recently discovered that cardiac mast cells contain and release renin, which initiates local angiotensin formation. Angiotensin causes coronary vasoconstriction, arrhythmias, fibrosis, apoptosis, and endothelin release, all demonstrated mechanisms of mast-cell-associated cardiac disease. The effects of angiotensin are further amplified by the release of norepinephrine from cardiac sympathetic nerves. Our discovery of renin in cardiac mast cells and its release in pathophysiological conditions uncovers an important new pathway in the development of mast-cell-associated heart diseases. Several steps in this novel pathway may constitute future therapeutic targets.

Cardiac mast cell-derived renin promotes local angiotensin formation, norepinephrine release, and arrhythmias in ischemia/reperfusion

Having identified renin in cardiac mast cells, we assessed whether its release leads to cardiac dysfunction. In Langendorff-perfused guinea pig hearts, mast cell degranulation with compound 48/80 released Ang I-forming activity. This activity was blocked by the selective renin inhibitor BILA2157, indicating that renin was responsible for Ang I formation. Local generation of cardiac Ang II from mast cell-derived renin also elicited norepinephrine release from isolated sympathetic nerve terminals. This action was mediated by Ang II-type 1 (AT1) receptors. In 2 models of ischemia/reperfusion using Langendorff-perfused guinea pig and mouse hearts, a significant coronary spillover of renin and norepinephrine was observed. In both models, this was accompanied by ventricular fibrillation. Mast cell stabilization with cromolyn or lodoxamide markedly reduced active renin overflow and attenuated both norepinephrine release and arrhythmias. Similar cardioprotection was observed in guinea pig hearts treated with BILA2157 or the AT1 receptor antagonist EXP3174. Renin overflow and arrhythmias in ischemia/reperfusion were much less prominent in hearts of mast cell-deficient mice than in control hearts. Thus, mast cell-derived renin is pivotal for activating a cardiac renin-angiotensin system leading to excessive norepinephrine release in ischemia/reperfusion. Mast cell-derived renin may be a useful therapeutic target for hyperadrenergic dysfunctions, such as arrhythmias, sudden cardiac death, myocardial ischemia, and congestive heart failure.

Pressor and intra-renal effects of angiotensins I and II, and noradrenaline, in anaesthetized and conscious sheep

The pressor and intra-renal actions and effects of octa - and deca-peptides angiotensins II and I and of the catecholamine noradrenaline, in anaesthetized and conscious sheep, are considered. The halothane anaesthetic substantially lowers pressor sensitivity to both peptides but does not influence their ability to liberate K(+) ions into the circulating plasma. In comparison with angiotensin II, both angiotensin I and noradrenaline -- with direct presentation to the kidney -- are ineffective in decreasing intra-renal blood flow. However, with left ventricular injection, both pressor compounds immediately increase the blood pressure, as does angiotensin II. Combined doses of the decapeptide and catecholamine are thus highly effective in raising the blood pressure while having a minimal effect on blood flow through the kidney. This overall situation could provide a basis for treating clinical shock, especially regarding septicaemia and septic shock. The lowered hind-limb blood flow, with administration of the pressor compounds into the femoral artery, contrasts strongly with the raised flow resulting from intravenous injection. Experimental procedures to establish, or otherwise, relevant hypothetical situations are detailed.

Pressure-independent cardiac effects of angiotensin II in pigs

BACKGROUND

Angiotensin II (Ang II) is a potent vasoconstrictor with an important role in the development of cardiovascular disease. Earlier results have shown a positive acute inotropic effect of Ang II in anaesthetized pigs together with significant vasoconstriction. This investigation was designed to study cardiac effects of Ang II, when blood pressure was maintained constant by experimental means.

METHODS

Ang II (200 microg h(-1)) was infused in anaesthetized pigs (n = 10) at two different arterial blood pressures, the first determined by the effects of Ang II alone, and the second maintained at baseline blood pressure with nitroprusside. Cardiac systolic and diastolic function was evaluated by analysis of left ventricular pressure-volume relationships.

RESULTS

Heart rate, end-systolic elastance (Ees) and pre-load adjusted maximal power (PWRmax EDV(-2)) increased at both blood pressure levels, although less when blood pressure was kept constant with nitroprusside. The time constant for isovolumetric relaxation (tau(1/2)) was prolonged with Ang II alone and shortened with Ang II infused together with nitroprusside.

CONCLUSION

Ang II infusion in the pig has inotropic and chronotropic properties independent of arterial blood pressure levels, although the effects seem to be blunted by pharmacological actions of the nitric oxide donor nitroprusside.

Ischemia promotes renin activation and angiotensin formation in sympathetic nerve terminals isolated from the human heart: contribution to carrier-mediated norepinephrine release

We recently reported that in the ischemic human heart, locally formed angiotensin II activates angiotensin II type 1 (AT(1)) receptors on sympathetic nerve terminals, promoting reversal of the norepinephrine transporter in an outward direction (i.e., carrier-mediated norepinephrine release). The purpose of this study was to assess whether cardiac sympathetic nerve endings contribute to local angiotensin II formation, in addition to being a target of angiotensin II. To this end, we isolated sympathetic nerve endings (cardiac synaptosomes) from surgical specimens of human right atrium and incubated them in ischemic conditions (95% N(2,) sodium dithionite, and no glucose for 70 min). These synaptosomes released large amounts of endogenous norepinephrine via a carrier-mediated mechanism, as evidenced by the inhibitory effect of desipramine on this process. Norepinephrine release was further enhanced by preincubation of synaptosomes with angiotensinogen and was prevented by two renin inhibitors, pepstatin-A and BILA 2157BS, as well as by the angiotensin-converting enzyme inhibitor enalaprilat and the AT(1) receptor antagonist EXP 3174 [2-N-butyl-4-chloro-1-[2'-(1H-tetrazol-5-yl)biphenyl-4-yl] methyl]imidazole-5-carboxylic acid]. Western blot analysis revealed the presence of renin in cardiac sympathetic nerve terminals; renin abundance increased ~3-fold during ischemia. Thus, renin is rapidly activated during ischemia in cardiac sympathetic nerve terminals, and this process eventually culminates in angiotensin II formation, stimulation of AT(1) receptors, and carrier-mediated norepinephrine release. Our findings uncover a novel autocrine/paracrine mechanism whereby angiotensin II, formed at adrenergic nerve endings in myocardial ischemia, elicits carrier-mediated norepinephrine release by activating adjacent AT(1) receptors.

The cardiac effects of intracoronary angiotensin II infusion

Angiotensin II (Ang II) is a potent vasoconstrictor, which recently has been shown to also have significant inotropic effects. Previous results regarding the mechanisms of the acute inotropic effects of Ang II are not conclusive. We designed this study to investigate the local cardiac effects of intracoronary Ang II infusion in doses not affecting systemic circulation. Ang II (2.5-40 microg/h) was infused in the left coronary artery of Yorkshire pigs (n = 9) reaching calculated intracoronary Ang II concentrations of 842 +/- 310, 3342 +/- 1238, and 12448 +/- 4393 pg/mL, respectively. Cardiac systolic and diastolic function was evaluated by analysis of the left ventricular pressure-volume relationship. Coronary flow was measured by using a coronary sinus catheter and the retrograde thermodilution technique. No significant changes were seen in the systolic and diastolic function variables of heart rate, end-systolic elastance, preload recruitable stroke work, the time constant for isovolumetric relaxation, or in coronary vascular resistance and flow. The positive inotropic and chronotropic effects of Ang II seen in previous studies seem thus to be mediated via extracardiac actions of Ang II. Coronary vascular tone is not affected by local Ang II infusion in anesthetized pigs.

IMPLICATIONS

The positive inotropic and chronotropic effects of angiotension II (Ang II) seen in previous studies seem to be mediated via extracardiac actions of Ang II. Coronary vascular tone is not affected by local Ang II infusion in anesthetized pigs.

Splanchnic vasoconstriction by angiotensin II is arterial pressure dependent

BACKGROUND

Our hypothesis was that splanchnic vasoconstriction by exogenous angiotensin II (Ang II) is significantly potentiated by local mechanisms increasing vasomotor tone and that splanchnic tissue oxygenation during administration of Ang II is perfusion pressure dependent. The aim was to study local splanchnic circulatory effects and tissue oxygenation during intravenous infusion of Ang II at different levels of regional arterial driving pressure in a whole-body large animal model.

METHODS

Ang II was infused in incremental doses (0-200 microg x h-1) in anaesthetised instrumented pigs (n=8). Mean superior mesenteric arterial pressure (PSMA) was adjusted by a local variable perivascular occluder. Perivascular ultrasound and laser-Doppler flowmetry were used for measurements of mesenteric venous blood flow and superficial intestinal blood flow, respectively. Intestinal oxygenation was evaluated by oxygen tissue tension (PtiO2) and lactate fluxes.

RESULTS

Ang II produced prominent and dose-dependent increases in mesenteric vascular resistance (RSMA) when the intestine was exposed to systemic arterial pressure, but Ang II increased RSMA only minimally when PSMA was artificially kept constant at a lower level (50 mmHg) by the occluder. Although Ang II decreased PtiO2 at a PSMA of 50 mmHg, splanchnic lactate production was not observed.

CONCLUSION

We demonstrate that splanchnic vasoconstriction by exogenous Ang II is dependent on arterial driving pressure, suggesting significant potentiation through autoregulatory increases in vasomotor tone. Intestinal hypoxaemia does not seem to occur during short-term infusion of Ang II in doses that significantly increases systemic arterial pressure.

Acute effects of angiotensin II on myocardial performance

BACKGROUND

Specific angiotensin II (Ang II) receptors exist in many organs including peripheral blood vessels, cardiac myocytes and the central nervous system. This suggests multiple sites of actions for Ang II throughout the cardiovascular system. Cardiac effects of Ang II are not completely understood, though its prominent vasoconstrictor actions are well described. This study was designed to assess left ventricular function during administration of Ang II using relatively load-independent methods in a whole-animal model.

METHODS

Ang II was infused in incremental doses (0-200 microg x h(-1)) in anaesthetised instrumented pigs (n=10). Cardiac systolic and diastolic function were evaluated by analysis of the left ventricular pressure-volume relationship.

RESULTS

Heart rate (HR), mean arterial pressure (MAP) and systemic vascular resistance (SVR) increased dose-dependently with Ang II, while cardiac output (CO) remained unchanged. Systolic function indices, end-systolic elastance (Ees) and preload recruitable stroke work (PRSW), demonstrated dose-dependent increases. The diastolic function parameter tau (tau) did not change with increasing Ang II dose.

CONCLUSION

Ang II infusion caused increases in contractility indices in anaesthetised pigs in the doses used in this study. The mechanisms for these systolic function effects may be a direct myocardial effect or modulated through changes in autonomic nervous system activity.

At receptor inhibition affects the noradrenaline sensitivity in isolated portal vein of normotensive rat

Short term treatments of normotensive Wistar Kyoto rats with angiotensin II (ANGII) or in combination with the AT1 receptor antagonist, losartan or PD123319, the AT2 receptor antagonist on systemic arterial blood pressure (MABP) and their influence on noradrenaline sensitivity in isolated mesenteric portal vein were evaluated. ANGII increased MABP as well as the contractile response to noradrenaline in vessels from ANGII-treated animals. MABP and the maximal effect of the concentration response curve for noradrenaline were prevented by losartan. However, PD123319 did not influence the blood pressure, but completely removed the vessels sensitivity to noradrenaline. ANGII combined with the AT1 and/or AT2 receptors blockade completely prevented the pressure response to ANGII, but the concentration response curve for noradrenaline did not differ from the vehicle-treated control curve. In conclusion both AT1- and AT2 receptor activation seems to be important in controlling noradrenaline sensitivity of rat portal vein smooth muscle.

Angiotensin II mesenteric and renal vasoregulation: dissimilar modulatory effects with nitroprusside

BACKGROUND

The role of systemic arterial pressure for the vascular effects of angiotensin II (Ang II) and the interactions between Ang II and perfusion pressure-dependent local vascular control mechanisms are not well understood. This study addresses these aspects of exogenous Ang II in the mesenteric and renal regional circulations.

METHODS

Ang II was infused in incremental doses (0-200 microg/h) in anesthetized instrumented pigs (n=10). Renal and portal blood flows were measured by perivascular ultrasound. In the second part of the study, sodium nitroprusside (SNP) was infused at doses titrated to keep mean arterial pressure constant, in spite of concurrent Ang II administration.

RESULTS

Powerful dose-dependent vasoconstrictions by Ang II were found in renal and mesenteric vascular beds (at highest Ang II doses vascular resistances increased by 109% and 88% respectively). Ang II-induced vasoconstriction was fully inhibited in the mesenteric, but not in the renal circulation, during conditions of constant mean arterial pressures achieved by SNP infusion.

CONCLUSIONS

Mesenteric, but not renal, vasoconstriction by Ang II was inhibited by pharmacological maintenance of perfusion pressure. This could reflect differences between these vascular beds as regards the importance of co-acting myogenic pressure-dependent vasoconstriction. Alternatively, as the drug chosen for pressure control, sodium nitroprusside, serves as a nitric oxide donor, the relative balance between nitric oxide-mediated vasodilation and Ang II-induced vasoconstriction could have regional differences.

Short treatments of normotensive and hypertensive rats by angiotensin II and nitric oxide inhibitor induce an increase of noradrenaline sensitivity in isolated vena portae preparations

The aim of the study was to examine the influence of a short-term treatment of conscious Wistar Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) by angiotensin II (ANG II) and by ANG II in combination with either l -NAME, HOE 140 or minoxidil on the mean arterial blood pressure (MABP) and the noradrenaline sensitivity in isolated portal vein preparations. MABP was significantly increased by ANG II treatment and ANG II plus l -NAME. However, it was slightly affected by ANG II in association with HOE 140, and significantly lowered by ANG II plus minoxidil. In control animals noradrenaline increased the frequency and the tone of contractile force. While ANG II enhanced the contractile response to noradrenaline, neither in combination with l -NAME, HOE 140 nor minoxidil prevented such an increase in the response to noradrenaline. In the presence of ergotamine, the contractile response to noradrenaline was completely blocked not only in control animals, but also in animals treated with ANG II alone or in combination with minoxidil. However, ergotamine (3 microm) failed to block completely the contractile response to noradrenaline in vessels from animals treated by ANG II in combination with l -NAME or HOE 140. These data suggest that ANG II causes an increase of noradrenaline sensitivity in the isolated portal vein of rat. l -NAME and HOE 140 unmask a contractile response to noradrenaline in the presence of ergotamine which seems to be mediated not only by alpha-adrenoceptors, but may be compensated by an endothelial relaxation.

Sympathomimetic effects of Parquetina nigrescens (Periplocaceae) extract in isolated portal vein smooth muscle

The purpose of this study was to examine the mechanisms underlying the pharmacological effects of the extract of Parquetina nigrescens (Expar) on vascular smooth muscle contractility. To evaluate the Expar effect, the contractile activity of portal veins isolated from Wistar Kyoto rats was isometrically recorded. Isolated portal vein preparations developed rhythmic and spontaneous contractile activity. Expar increased the contractile response of the portal vein preparations in a dose-dependent manner. The maximal effect of the dose-response curve for Expar was prevented by the alpha1-adrenergic blocking agent prazosin at 10 nM and 30 nM concentration dependently. The contractile responses of the muscle to Expar were partly blocked after chemical sympathectomy of the preparations with 6-hydroxydopamine, and those obtained in the same conditions with tyramine were completely abolished, whereas responses to noradrenaline were unaffected by the 6-hydroxydopamine pretreatment. It is concluded that Expar contains principles, which can be characterized as direct and indirect sympathomimetic.

Regulatory role of brain angiotensins in the control of physiological and behavioral responses

Considerable evidence now indicates that a separate and distinct renin-angiotensin system (RAS) is present within the brain. The necessary precursors and enzymes required for the formation and degradation of the biologically active forms of angiotensins have been identified in brain tissues as have angiotensin binding sites. Although this brain RAS appears to be regulated independently from the peripheral RAS, circulating angiotensins do exert a portion of their actions via stimulation of brain angiotensin receptors located in circumventricular organs. These circumventricular organs are located in the proximity of brain ventricles, are richly vascularized and possess a reduced blood-brain barrier thus permitting accessibility by peptides. In this way the brain RAS interacts with other neurotransmitter and neuromodulator systems and contributes to the regulation of blood pressure, body fluid homeostasis, cyclicity of reproductive hormones and sexual behavior, and perhaps plays a role in other functions such as memory acquisition and recall, sensory acuity including pain perception and exploratory behavior. An overactive brain RAS has been identified as one of the factors contributing to the pathogenesis and maintenance of hypertension in the spontaneously hypertensive rat (SHR) model of human essential hypertension. Oral treatment with angiotensin-converting enzyme inhibitors, which interfere with the formation of angiotensin II, prevents the development of hypertension in young SHR by acting, at least in part, upon the brain RAS. Delivery of converting enzyme inhibitors or specific angiotensin receptor antagonists into the brain significantly reduces blood pressure in adult SHR. Thus, if the SHR is an appropriate model of human essential hypertension (there is controversy concerning its usefulness), the potential contribution of the brain RAS to this dysfunction must be considered during the development of future antihypertensive compounds.

Angiotensin II facilitates tricyclic antidepressant-induced changes in QRS-duration in the rat

Experimental evidence indicates that angiotensin II can modulate sodium channel and gap junction function. This raises the possibility that variations in angiotensin II could alter the effect of drugs that act on these mechanisms. In this study, the influence of changing salt status and angiotensin II activity has been investigated by evaluating the QRS prolonging effects of the sodium channel blocking drug, desmethylimipramine. In control rats with a normal salt intake, intravenous infusion of desmethylimipramine (0.8 mg/kg/min) over 60 min increased QRS duration over time to 150% of control by 60 min; mean arterial blood pressure and pulse rate were decreased. In salt-deplete rats, the response to desmethylimipramine was similar to controls for 30 min. Thereafter, QRS duration increased more rapidly than controls. In rats on a high salt diet, the same desmethylimipramine infusion produced no change in QRS duration for 30 min, despite equivalent reductions in mean arterial blood pressure. Thereafter, QRS duration increased, reaching values similar to control by 60 min. In rats on a normal salt diet pretreated with captopril, there was a similar blunting of the QRS response to desmethylimipramine to that observed in salt-loaded rats. The QRS response to desmethylimipramine and salt-loaded rats on normal salt diets receiving captopril returned to the control pattern after a subpressor infusion of angiotensin II (3 ng/min), while a higher rate of angiotensin II (10 ng/min) further enhanced the QRS prolonging effect of desmethylimipramine. These data demonstrate that endogenous angiotensin II contributes to the regulation of the cardiac electro-physiological response to DMI.

Angiotensin-induced hypertension in the rat. Sympathetic nerve activity and prostaglandins

To elucidate mechanisms of angiotensin II (Ang II)-related hypertension, we infused angiotensin (76 ng/min s.c.) into rats with minipumps for 10-14 days. Control rats received sham pumps. We measured blood pressure by tail-cuff, and the excretion of aldosterone and prostaglandins (PG) (PGE2, prostacyclin derivative 6kPGF1 alpha, and thromboxane [Tx] derivative TxB2). Angiotensin II increased blood pressure by 20 mm Hg by day 2 and by 90 mm Hg by day 10. Aldosterone excretion increased from 10 to 70 ng/day in Ang II rats by day 7. Urine PGE2 did not increase in angiotensin rats; however, both 6kPGF1 alpha and TxB2 excretion increased with angiotensin. Control rats had no changes in any of these parameters. A sympathetic component was tested in a separate group of angiotensin rats that received phenoxybenzamine (300 micrograms/kg/day) during angiotensin infusion; their increase in blood pressure of 40 mm Hg at 10 days was less than in those rats with angiotensin alone but more than in control rats. Phenoxybenzamine did not influence the angiotensin-induced increases in excretion of 6kPGF1 alpha or TxB2. Additional groups of conscious angiotensin and control rats were equipped with splanchnic nerve electrodes on day 14 for recording of sympathetic nerve activity. Angiotensin rats had greater basal sympathetic nerve activity than the control rats. Incremental methoxamine injections demonstrated altered baroreceptor reflex function in rats receiving angiotensin. We conclude that increased blood pressure with chronic angiotensin infusion is accompanied by increased production of aldosterone and increased sympathetic tone. The latter may be modulated by PG.

Angiotensin II-noradrenergic interactions in renovascular hypertensive rats

This study tested the hypothesis that interactions of endogenous angiotensin II (AII) with the noradrenergic neuroeffector junction are important in renin-dependent hypertension. In the in situ blood-perfused rat mesentery, in normal rats exogenous AII potentiated mesenteric vascular responses to periarterial (sympathetic) nerve stimulation (PNS) more than vascular responses to exogenous norepinephrine (NE). In 2-kidney-1-clip (2K-1C) rats with renovascular hypertension mesenteric vascular responses to PNS and NE were greater than in sham-operated rats, and renovascular hypertension mimicked the effects of exogenous AII with respect to enhancing responses to PNS more than responses to NE. In 2K-1C rats, but not in sham-operated rats, 1-Sar-8-Ile-AII markedly suppressed vascular responses to PNS, without influencing responses to NE. Finally, 1-Sar-8-Ile-AII attenuated sympathetic nerve stimulation-induced neuronal spillover of NE in 2K-1C rats, but not in sham-operated rats. These data indicate that renovascular hypertension enhances noradrenergic neurotransmission, and that this enhancement is mediated in part by AII-induced facilitation of NE release.

Neurogenic actions of angiotensin II

It has become increasingly evident that blood-borne angiotensin II has major effects upon brain cardiovascular centers. With the discovery of an angiotensin I-forming enzyme or isoenzymes in the central nervous system of mammals, alternative concepts have emerged regarding the role of this peptide in the regulation of central adrenoreceptor activity, pituitary function, and hydromineral metabolism. These concepts are reviewed, and a framework for future research is suggested by the author.

Neural contribution to renal hypertension following acute renal artery stenosis in conscious rats

To assess the hemodynamic changes during acute renal artery stenosis (RSt) and their dependence on alterations in the renin-angiotensin and sympathetic nervous systems, we studied conscious rats chronically instrumented with miniaturized pulsed-Doppler flow probes. Probes were implanted on the superior mesenteric and both renal arteries, and on the lower abdominal aorta for measurement of mesenteric (MR), renal (RR), and hindquarters (HQR) vascular resistance. Unilateral RSt, with a pneumatic cuff occluder that reduced flow by approximately 50%, increased mean arterial pressure (MAP) by 32%, reduced heart rate, and increased MR, nonstenotic (contralateral) RR and HQR. The hypertension was renin-dependent since plasma renin activity increased 6-fold and the angiotensin II (AII) antagonist, saralasin, significantly reduced MAP and regional resistances. The acute hypertension was also associated with increased neurogenic vasoconstrictor tone since hexamethonium markedly reduced MAP, MR, HQR and non-stenotic RR. Hexamethonium similarly decreased MAP during hypertension induced by AII infusion, whereas hypertension produced by the "pure" peripheral vasoconstrictor, phenylephrine, was unaffected by ganglionic blockade. In animals with peripheral sympathectomy produced by 6-hydroxydopamine, acute RSt produced hemodynamic changes similar in magnitude to intact animals; however, PRA increased 3-fold more than in intact rats. We conclude that hypertension induced by acute RSt in conscious rats is not only renin-dependent, but is also associated with inappropriately high neurogenic vasoconstrictor tone, presumably activated by indirect neural actions of AII.

Effects of angiotensin II on the cardiac responses to sympathetic nerve stimulation in dogs

In anesthetized dogs with the left cardiac sympathetic nerves and both vagal nerves intact, angiotensin II (AII) induced a substantial, dose-dependent increase in arterial blood pressure and small increments in cardiac cycle length and ventricular contractile force. In dogs in which the cardiac sympathetic and vagal nerves had been interrupted, AII produced similar increases in blood pressure and larger increases in contractile force, but it decreased the cardiac cycle length. In both groups of dogs, AII augmented substantially the positive inotropic responses to sympathetic nerve stimulation, but it enhanced the positive chronotropic responses only slightly. However, AII did not appreciably prolong the cardiac responses to sympathetic nerve stimulation, nor did it alter significantly the cardiac responses to norepinephrine infusions. Hence, at the dosage levels used, AII probably did not inhibit the neuronal uptake of norepinephrine appreciably nor did it enhance the responsiveness of the cardiac effector sites to norepinephrine. Therefore, the potentiation of the cardiac responses to sympathetic nerve stimulation by AII in these experiments was probably achieved principally by facilitating norepinephrine release from the adrenergic nerve terminals in the heart.

Rat (Ile5) but not bovine (Val5) angiotensin raises plasma norepinephrine in rats

A part of the vasoconstrictor activity of angiotensin II (AII) may result from its ability to enhance norepinephrine (NE) release from sympathetic noradrenergic nerve terminals. To investigate this proposed pressor mechanism of AII, the effects of intravenous (i.v.) infusion of AII on blood pressure and plasma catecholamines in pithed rats were determined. Two naturally occurring angiotensins, valine5 AII (bovine) and isoleucine5 AII (rat), were administered in equal (72 ng/min) doses. Valine5 AII caused an 80% increase in mean arterial pressure (MAP) from 54 +/- 4 to 97 +/- 19 mm Hg. Isoleucine5 AII caused an 82% increase in MAP from 49 +/- 5 to 89 +/- 18 mm Hg. Neither angiotensin caused a change in heart rate, suggesting that pithing completely destroyed the central baroreceptor reflex mechanism. Plasma catecholamines were differentially affected by the peptides:isoleucine5 AII significantly increased plasma NE concentration by 82% compared to saline-infused rats (p less than 0.01). Valine5 AII did not significantly affect plasma NE concentration. Plasma dopamine and epinephrine concentrations were not significantly altered by infusion of either analog. Despite the significant increases in plasma NE concentrations with isoleucine5 in AII-infusion rats, there was no correlation between plateau MAP or the percent increase in MAP and plasma NE concentrations of individual animals within this group. The ability of angiotensin to elevate MAP, increase NE release from sympathetic nerve terminals, as well as potential differences in the actions of angiotensins in different species, and angiotensin receptor heterogeneity, are discussed.

Humoral and neurogenic factors in two-kidney renovascular hypertension

A "bolus" dose (110 microgram) of the angiotesin II (A II)-blocker 1-Sar-8-Ala-A II (Saralasin, S) followed by its slow rate infusion (5 microgram/min/rat) for thirty min, was injected before and after the complete ganglionic blockade by pentolinium (P) in unanaesthetized unilaterally clipped renal hypertensive rats (the opposite kidney remained untouched). Pentolinium was also injected like a "bolus" dose (3 mg) followed by slow infusion (0.1 mg/min/rat) for thirty min. The observations were made until the fifth week after clipping the left renal artery. A consistent maximal hypotensive response was observed after the "bolus test" with both drugs. When S was the first drug injected, an inverse correlation was found between the percent decrease in arterial pressure (BP) by S and the percent decrease in BP by P (r = --0.83, P < 0.01, n = 8). Thus whenever a greater hypotensive effect was obtained by S, a smaller neural pressor component remained to be blocked by P. On the other hand, when P was the first drug injected a lesser A II pressor component remained to be blocked by S in the hypertensive rats. The results suggest that a considerable A II pressor effect in two-kidney renovascular hypertension is mediated via neurogenic mechanisms from the first week. A direct pressor vasoconstriction was found to be significant in cases with very high plasma-renin activity.

Bartter's syndrome. New insights into pathogenesis and treatment

Discussed here is a patient with normotension, hypokalemic alkalosis, hyperreninemia, hyperaldosteronism, juxtaglomerular cell hyperplasia and insensitivity to the pressor effects of angiotensin (Bartter's syndrome). The hyperreninemia and hyperaldosteronism were both suppressible with volume expansion. Hypokalemia was correctible both short-term with potassium chloride infusions and long-term with spironolactone. Nevertheless, the abnormal pressor response to infused angiotensin could not be corrected by these maeuvers, suggesting that this defect is likely to be of primary pathophysiologic significance. We found that potassium loading markedly stimulated aldosterone excretion. This may explain the inadequacy of potassium supplementation alone to correct the hypokalemia and the observed "escape" from the potassium conserving effects of spironolactone seen in patients with Bartter's syndrome. The administration of propranolol in large doses only partially suppressed the marked hyperreniemia of our patient and failed to prevent a subsequent rise in the renin level which was associated with spironolactone therapy. In contrast, suppression of the renin level to normal was demonstrated by sodium loading. It is suggested that patients with Bartter's syndrome be treated simultaneously with large doses of spironolactone and a high sodium intake.

The effect of propranolol on vascular responses to sympathetic nerve stimulation

1. In an attempt to clarify the role of the sympathetic neurone in the antihypertensive action of propranolol, the effect of this drug on responses to lumbar sympathetic nerve stimulation has been studied in the perfused hind-limb of the dog. 2. No consistent reduction of maximal or submaximal responses to nerve stimulation was produced by propranolol (10 to 100 mug/kg). In contrast, potentiation of nerve-evoked response, as well as those to injected noradrenaline, usually occurred. Dexpropranolol (50 mug/kg) had no effect. 3. When neuronal uptake of noradrenaline was inhibited by desmethylimipramine or cacaine, no reduction in responses to sympathetic nerve stimulation was observed with propranolol. 4. No evidence was found, using alpha-adrenoceptor blocking drugs, that released transmitter stimulates beta-adrenoceptors in the blood vessels of the hind-limb. 5. No evidence has been found for the existence of an adrenergic neurone-blocking action of propranolol that might contribute to the antihypertensive activity in man.

Mechanism of enhancement by angiotensin II of sympathetic adrenergic transmission in the guinea pig

An electrophysiological analysis was made of the mechanism underlying the potentiation by angiotensin II of responses to adrenergic nervous stimulation of the isolated vas deferens and the uterine artery of the guinea pig. Concentrations of angiotensin II up to 5 x 10 -7 g/ml did not affect resting muscle cell membrane potential, cause spontaneous transmitter release, or increase transmitter release induced by single nerve stimuli. The time courses of excitatory junction potentials were not increased, indicating that the neuronal mechanism for transmitter inactivation was not inhibited. Angiotensin II did, however, increase the degree of facilitation of successive excitatory junction potentials during repetitive low-frequency nerve stimulation, leading to greater depolarization of the muscle cells than under control conditions. That this enhancement of facilitation was the primary mechanism underlying potentiation of transmission by angiotensin II was supported by the finding that the potentiation of mechanical responses to nerve stimulation decreased markedly as the frequency of stimulation was raised over the range where facilitation is replaced by summation as the major factor in depolarizing the muscle cells to firing threshold (2-6 pulses/sec). This mechanism of action of angiotensin II means that its potentiating effect on transmission is relatively specific for the low frequencies of nervous activity which are thought to be important in tonic cardiovascular control.