Renin-angiotensin mediation of adrenal catecholamine secretion induced by haemorrhage

Effect of sympathoadrenal blockade on the hyperglycemic action of angiotensin II

The present experiments were designed to investigate the influence of the sympathoadrenal system on the hyperglycemic action of angiotensin II in freely moving rats divided into four experimental groups: (1) sham-operated animals submitted to intravenous administration of angiotensin II (1.9 nmol) which caused a rapid increase in plasma glucose reaching the highest values at 5 min after the injection (26.5% of the initial values; P < 0.01); (2) Sham-operated animals submitted to blockade of sympathetic noradrenergic pathways by treatment with guanethidine (10 mg/100 g body weight), which greatly decreased the baseline value of plasma glucose (85 +/- 5.5 mg% vs 136 +/- 5.1 mg% P < 0.01), and increased the hyperglycemic response to angiotensin II by 96% (P < 0.01); (3) Animals submitted to adrenodemedullation that did not alter the hyperglycemic response to angiotensin II; (4) Adrenodemedullated animals submitted to sympathetic blockade by guanethidine treatment which caused a 40.5% decrease in the hyperglycemic response to angiotensin II (P < 0.05). These data indicate that angiotensin II has a direct hyperglycemic effect in addition to its action on sympathetic nervous system activation and adrenomedullary secretion.

Effect of [1-Sar,8-Thr]-angiotensin II on the hyperglycemic response to hemorrhage in adrenodemedullated and guanethidine-treated rats

The present experiments were designed to further investigate the action of an angiotensin II antagonist on the hyperglycemic response to hemorrhage (1.2 ml/100 g b.wt./2 min). The animals were divided into 3 experimental groups; (1) sham-operated animals submitted to intravenous administration of [1-Sar,8-Thr]-angiotensin II (sarthran), an antagonist of angiotensin II (750 ng/100 g b.wt. as a bolus plus an infusion of 25 ng/100 g b.wt./min over 30 min), which greatly attenuated (51.8% lower than controls; P < 0.01) the hyperglycemic response to hemorrhage; (2) animals submitted to adrenodemedullation which decreased the hyperglycemic response to hemorrhage by 64% (P < 0.01). However, sarthran infusion into adrenodemedullated rats caused a 38.5% further decrease in hyperglycemic response to hemorrhage (P < 0.01); and (3) intact animals submitted to blockade of sympathetic noradrenergic pathways by treatment with guanethidine (10 mg/100 g b.wt.), which greatly decreased the baseline value of plasma glucose (64.1 +/- 3.5 mg% vs. 125.3 +/- 4.5 mg%, P < 0.01), and reduced the hyperglycemic response to hemorrhage by 34% (P < 0.01). Sarthran infusion into guanethidine-treated rats caused a further 34% decrease in hyperglycemic response to hemorrhage (P < 0.01). These data indicate that angiotensin II has a direct hyperglycemic effect in addition to its action on sympathetic nervous system activation and adrenomedullary secretion.

Long-term treatment with angiotensin I-converting enzyme inhibitors attenuates the loss of cardiac beta-adrenoceptor responses in rats with chronic heart failure

BACKGROUND

Cardiac contractile force in response to beta-adrenoceptor agonists and beta-adrenergic receptor density are decreased in failing human hearts. The effects of angiotensin I-converting enzyme (ACE) inhibitor on cardiac responsiveness to beta-adrenergic stimulation in failing hearts are not established. The present study was undertaken to determine whether ACE inhibitor may improve cardiac beta-adrenergic responsiveness in animals with chronic heart failure (CHF).

METHODS AND RESULTS

CHF was induced by left coronary artery ligation in rats. Cardiac output and stroke volume indices decreased 12 weeks after the operation. In sham-operated rats, dobutamine and isoprenaline increased cardiac output and stroke volume indices. In contrast, cardiac output and stroke volume responses to dobutamine and isoprenaline were severely blunted in the CHF rat. Cardiac beta 1-adrenergic receptor density was decreased while its dissociation constant (Kd) was not altered in the viable tissue of the left ventricle of the CHF rat, which is consistent with beta-adrenergic receptor downregulation. Cardiac norepinephrine content decreased in the CHF rats. Rats were treated orally with ACE inhibitors, 3 mg/kg trandolapril or 10 mg/kg enalapril once daily, or 5 mg/kg captopril twice daily from the 2nd to the 12th weeks after the operation. Treatment with ACE inhibitors attenuated the reduction in cardiac output and stroke volume indices and improved the inotropic response to dobutamine and isoprenaline and reversed partially the cardiac norepinephrine content in the CHF rat. ACE inhibitor treatment also attenuated the reduction in beta 1-adrenergic receptor density in the viable tissue of the left ventricle of the CHF rat.

CONCLUSIONS

The results suggest that ACE inhibitor treatment attenuates the blunting of cardiac responses to beta-adrenergic agonists in the CHF rat and that one of the mechanisms underlying this effect is prevention of cardiac beta 1-adrenergic receptor downregulation.

Hyperglycemic action of angiotensin II in freely moving rats

Angiotensin II has been implicated in the regulation of liver glycogen phosphorylase. Although it has been suggested that angiotensin II can raise blood glucose levels during hemorrhage, experimental data have not been presented. In the present study, the effect of angiotensin II on blood glucose levels was studied in freely moving rats, divided in three experimental groups: 1) intravenous administration of angiotensin II (0.48, 1.9, or 4.8 nmol) caused a dose-dependence response; 2) intracerebroventricular administration of angiotensin II (1.9 or 4.8 nmol) did not cause any significant change in glycemia compared with saline-treated controls; 3) intravenous administration of [Sar1,Thr8]angiotensin II, an antagonist of angiotensin II (750 ng/100 g b. wt. as a bolus plus a continuous injection of 25 ng/100 g b. wt./min over 30 min), greatly attenuated (39.2% lower than controls; p < 0.01) the hyperglycemic response to hemorrhage (1.2 ml/100 g b.wt.). These data indicate an in vivo involvement of angiotensin II in blood glucose regulation.

Interactions between ANG II, sympathetic nervous system, and baroreceptor reflexes in regulation of blood pressure

The renin-angiotensin system plays an important role in the regulation of arterial blood pressure and in the development of some forms of clinical and experimental hypertension. It is an important blood pressure control system in its own right but also interacts extensively with other blood pressure control systems, including the sympathetic nervous system and the baroreceptor reflexes. Angiotensin (ANG) II exerts several actions on the sympathetic nervous system. These include a central action to increase sympathetic outflow, stimulatory effects on sympathetic ganglia and the adrenal medulla, and actions at sympathetic nerve endings that serve to facilitate sympathetic neurotransmission. ANG II also interacts with baroreceptor reflexes. For example, it acts centrally to modulate the baroreflex control of heart rate, and this accounts for its ability to increase blood pressure without causing a reflex bradycardia. The physiological significance of these actions of ANG II is not fully understood. Most evidence indicates that the actions of ANG to enhance sympathetic activity do not contribute significantly to the pressor response to exogenous ANG II. On the other hand, there is considerable evidence that the actions of endogenous ANG II on the sympathetic nervous system enhance the cardiovascular responses elicited by activation of the sympathetic nervous system.

Hypovolaemia can potentiate hypoglycaemic stress-induced adrenaline release in the anaesthetized rat

The interaction between hypoglycaemic stress and hypovolaemic stress in promoting adrenaline secretion from the adrenal medulla has been studied in anaesthetised and conscious rats. In anaesthetized rats, insulin (1 IU/kg, i.v.) markedly increased plasma adrenaline concentrations whereas blood sampling alone did not. The hypoglycaemic response to insulin was greatly reduced if donor blood was used instead of saline to replace the withdrawn blood. The hypoglycaemic response was abolished by captopril or saralasin. The results suggest that adrenaline secretion in response to hypoglycaemic stress in anaesthetized rats is potentiated by hypovolaemic activation of the renin-angiotensin system. In contrast, in fully conscious rats, the response to hypoglycaemia was not abolished by captopril, indicating that in the absence of barbiturate anaesthesia, the hypoglycaemic release of catecholamines is not potentiated by the renin-angiotensin system.

Importance of endogenous angiotensin II in the cardiovascular responses to sympathetic stimulation in conscious rabbits

Pharmacological evidence indicates that angiotensin (Ang II) converting enzyme inhibitors attenuate cardiovascular responses to sympathetic stimulation. To investigate the physiological significance of this attenuation, the pressor and heart rate responses to bilateral carotid occlusion (BCO) were studied before and after administration of captopril and again during Ang II replacement in conscious, aortic nerve-sectioned rabbits with chronically implanted carotid occluders. In the control period, BCO produced increases (p less than 0.05) in mean arterial pressure (MAP) and heart rate (HR) of 37.3 +/- 3.0 mm Hg and 21.7 +/- 5.4 beats/min from baseline values of 79.1 +/- 2.5 mm Hg and 255.4 +/- 16.7 beats/min. Captopril (5 mg/kg i.v.) markedly reduced (p less than 0.05) both the pressor (10.2 +/- 2.6 mm Hg) and HR (5.0 +/- 4.0 beats/min) responses to BCO, in parallel with a decrease in plasma Ang II of 75%. Infusion of a subpressor dose of Ang II (5-25 ng/kg/min i.v.) increased plasma Ang II to precaptopril levels and fully restored (p less than 0.05) the pressor (33.0 +/- 5.7 mm Hg) and HR (19.8 +/- 7.7 beats/min) responses to BCO. In two additional series of experiments, the mechanism of the effects of captopril and Ang II were investigated. In the first series, cardiac baroreflex curves (pulse interval versus MAP) were generated by increasing or decreasing blood pressure with phenylephrine or nitroprusside (5-20 micrograms/kg/min i.v.). The slope of the linear region of the curve (2.9 msec/mm Hg) was not changed significantly by captopril treatment (3.1 msec/mm Hg) or Ang II replacement (3.2 msec/mm Hg), indicating that cardiac baroreflex sensitivity was not altered by blockade of the renin-angiotensin system. In the second series, the effect of captopril on the pressor response to exogenous norepinephrine (0.1-2.5 micrograms/kg/min i.v.) was tested. The response was reduced by less than 40%, indicating only a modest postsynaptic component to the action of captopril. These results provide physiological evidence for an important action of endogenous Ang II in facilitating the cardiovascular responses to sympathetic stimulation in conscious rabbits. This facilitation is not due to an action upon the baroreflex per se but results, at least in part, from a presynaptic action of Ang II.

Pressor effects of the alpha 2-adrenoceptor agonist B-HT 933 in anaesthetized and haemorrhagic rats: comparison with the haemodynamic effects of amidephrine

1. Blood pressure responses to single and multiple bolus doses of the alpha 2-adrenoceptor agonist B-HT 933 were analysed in intact anaesthetized rats which were either normotensive or hypotensive as a result of haemorrhage. Single bolus doses of B-HT 933 in normotensive rats induced a fall in blood pressure, whilst further doses induced dose-dependent pressor responses which were inhibited by the alpha 2-adrenoceptor antagonist yohimbine and unaffected by the alpha 1-adrenoceptor agonist prazosin. In the haemorrhagic, hypotensive animals, single bolus doses of B-HT 933 induced immediate dose-dependent pressor responses; the maximum pressor responses to the bolus of B-HT 933 and its ED50 values were the same in both the normotensive and hypotensive, haemorrhagic animals. 2. Cardiac output, its distribution and tissue blood flows were determined with tracer microspheres in intact anaesthetized normotensive and haemorrhagic, hypotensive rats during depressor (normotensive) and pressor (normotensive and hypotensive) responses to B-HT 933. Haemodynamics were also determined during pressor responses to the alpha 1-adrenoceptor agonist amidephrine. 3. In control normotensive rats, a single dose of B-HT 933 (1 mg kg-1) reduced blood pressure by reducing cardiac output (through a decrease in heart rate). It increased the fractional distribution of cardiac output to the spleen and stomach, reduced it to the heart and liver and reduced cardiac and hepatic blood flow. A further dose of B-HT 933 (1 mg kg-1 bolus followed by 100 micrograms min-1 infusion) increased blood pressure by increasing total peripheral resistance, which was accompanied by decreased proportions of cardiac output passing to the heart, liver and testes. There was also increased fractional distribution of cardiac output to the lungs, spleen, kidneys and stomach but blood flows through the liver and testes were reduced. Amidephrine (6 micrograms kg-1 bolus followed by 0.5 micrograms min-1 infusion) increased blood pressure by increasing cardiac output through an increased stroke volume. It increased cardiac output distribution to the kidneys and brain, increasing blood flow through the heart, lungs, brain, testes, epididimides, skin and large intestine. 4. Haemorrhage caused a fall in blood pressure which resulted from decreased total peripheral resistance and cardiac output (the latter due to decreases in both heart rate and stroke volume). It reduced the proportion of cardiac output distributed to the lungs, spleen, kidneys, testes and pancreas/mesentery and decreased blood flow through these organs as well as through the heart, liver, brain, epididimides, skin and the gastrointestinal tract.4

Effects of angiotensin II on cultured, bovine adrenal medullary cells

Primary cultures of bovine adrenal medullary cells have been used to study the effects of angiotensin II on catecholamine secretion and inositol phosphate accumulation. Angiotensin II induced a weak secretion of both adrenaline and noradrenaline, with a threshold of 10-100 pM and a shallow concentration-dependence up to 10 microM. The response was fully dependent on extracellular Ca++, was partially inhibited by 100 nM nifedipine, was completely blocked by [Sar1, Ala8]-angiotensin II (IC50 5-10 nM) and was unaffected by 0.1 mM hexamethonium. Angiotensin II also increased inositol phosphate accumulation over the range 1 pM-10 microM. Inositol trisphosphate levels increased in a biphasic manner after 15 sec and 1 min exposure to 10 nM angiotensin II, but were not significantly increased at 30 sec or 5, 15 or 30 min stimulation. Inositol bisphosphate was significantly increased after 1 min. Inositol monophosphate levels only increased after 1 min stimulation, but continued to rise during 30 min stimulation. Removal of extracellular Ca++ or addition of EGTA reduced basal inositol phosphate accumulation but not the ability of angiotensin II to stimulate inositol phosphate accumulation relative to basal. Nifedipine (100 nM) had no effect on basal or angiotensin II-induced inositol phosphate accumulation. The inositol phosphate response to angiotensin II was abolished by 1 microM [Sar1, Ala8]-angiotensin II. The results suggest that secretion of adrenal medullary catecholamines can be evoked by angiotensin II, at concentrations that are compatible with a role for circulating angiotensin II or for angiotensin II generated locally within the adrenal medulla. They do not support the suggestion that the secretory actions of angiotensin II on chromaffin cells are mediated by mobilization of intracellular Ca++ stores.

Localization of angiotensin II binding sites in the bovine adrenal medulla using a labelled specific antagonist

Angiotensin II binding sites have been localized in sections of bovine adrenal glands and on living cultured bovine adrenal medullary cells using [125I]-[Sar1,Ile8]-angiotensin II and autoradiographic techniques. Binding sites were observed over both adrenaline and noradrenaline chromaffin cells. However, they were present in higher density over adrenaline cells, as determined by the distribution of phenylethanolamine N-methyltransferase mRNA by in situ hybridization histochemistry and of glyoxylic acid-induced fluorescence of noradrenaline. Binding sites were also observed in low density over nerve tracts within the bovine adrenal gland. Living cultured bovine adrenal medullary cells possessed angiotensin II binding sites. Not all cells were labelled. At least 73% of identified dispersed chromaffin cells in these cultures were labelled. Some chromaffin cells were not labelled with the ligand, and at least some non-chromaffin cells in the cultures did possess angiotensin II binding sites. The results provide direct anatomical support for the known ability of angiotensin II to elicit catecholamine secretion from perfused adrenal glands and from cultured adrenal chromaffin cells. They also suggest that some of the effects of angiotensin II on calcium fluxes and second messenger levels measured in cultured adrenal medullary cell preparations may be due to angiotensin II acting on non-chromaffin cells present in these cultures.

Lack of effect of opioid compounds on angiotensin II responses of bovine adrenal medullary cells

Angiotensin II (10 nM) increased basal adrenaline and noradrenaline secretion from cultured bovine adrenal chromaffin cells by 2.5- to 3-fold and 4- to 6-fold, respectively, and stimulated basal accumulation of inositol phosphates more than 2-fold. Etorphine and diprenorphine in the range 10(-9) to 10(-5) M had no effect on the catecholamine secretion induced by angiotensin II, and, at 10(-8) and 10(-5) M, had no effect on angiotensin II-induced inositol phosphate accumulation. The functions of adrenal medullary opioid receptors remain to be determined.

Effects of hemorrhage and naloxone on adrenal release of methionine-enkephalin and catecholamines in halothane anesthetized dogs

Concurrent levels of methionine-enkephalin and catecholamines in adrenal vein, femoral vein and femoral artery were measured under baseline conditions and during graded hemorrhage in halothane anesthetized dogs and compared to a non-bled control group. Naloxone was administered in both groups at the end of the experiment. Normotensive hypovolemia with a remaining blood volume of 76% led to a moderate decrease in mean arterial blood pressure from baseline and a 15- to 20-fold increase in norepinephrine, epinephrine and dopamine, and a 5-fold increase in enkephalin in the adrenal vein. Subsequent induction of hypotensive hypovolemia with a remaining blood volume of 51% resulted in a profound drop in blood pressure and evoked a further increase in the level of catecholamines (40- to 50-fold from baseline) and enkephalin (8-fold from baseline) in the adrenal vein. In the control group only a 3- to 4-fold increase from baseline in adrenal vein hormone levels was observed over time. Naloxone administration at the end of the experiment, led to a 2- to 6-fold further increase in hormones at the 3 collection sites in both groups of dogs. Joint calculation of the partial correlation coefficients for the influence of preceding blood volume and blood pressure, and concurrent blood volume and blood pressure on hormone secretion in the adrenal vein revealed that these variables explained the variation in hormone levels between 56 and 92% during normotensive hypovolemia and 62-83% during hypotensive hypovolemia. In one dog with bilateral adrenalectomy, hemorrhage was poorly tolerated, and naloxone administration did not lead to increased systemic plasma levels of catecholamines and enkephalin or improved hemodynamics. In the hemorrhage group, molar ratios of norepinephrine/epinephrine in the adrenal vein showed a significant increasing trend during the experiment. Findings in these experiments support the idea of differential monoaminergic and enkephalinergic regulation in adrenal medullary cells.

Effect of bilateral nephrectomy on the recovery of blood pressure after acute hemorrhage in rats: role of renin-angiotensin system

The effect of bilateral nephrectomy, and administration of an inhibitor of angiotensin converting enzyme, on the recovery of arterial blood pressure after hemorrhage (loss of 1% of b.wt), was studied in male Sprague-Dawley rats. Neither manoeuver significantly affected the recovery of blood pressure within the first 10 min after hemorrhage. Thereafter, the recovery of the blood pressure was markedly suppressed. The study suggests that the initial recovery of blood pressure is unrelated to the kidneys, but the later one requires their presence and depends on the activity of the renin-angiotensin system.

Effects of the renin-angiotensin system on the reflex response of the adrenal medulla to hypotension in the dog

We have studied the influence of the renin-angiotensin system on the control of catecholamine release from innervated and denervated adrenal glands of anaesthetized dogs. Captopril reduced the resting release of catecholamines and inhibited release evoked either by lowering carotid sinus pressure or by stimulating the peripheral end of the cut splanchnic nerve. Both responses were restored by exogenous angiotensin II, and the reflex response could also be restored by corticotrophin. Cycloheximide, in the presence of captopril, further reduced the resting release of catecholamines and prevented the restoration of the reflex response by angiotension II. Plasma renin activity did not rise during baroreceptor tests lasting 10 min, but catecholamine release was evoked from the first minute. We conclude that the response of the adrenal medulla to sympathetic activity requires a minimum circulating concentration of angiotensin II. It is severely impaired by inhibition of the renin-angiotensin system but function can be restored either by exogenous angiotensin II or by corticotrophin.

High-affinity angiotensin receptors in rat adrenal medulla

Angiotensin II receptors have been quantitated in single rat adrenal medullas by incubation of tissue sections with 125I-[Sar1]-AII, autoradiography with exposure to 3H-sensitive Ultrofilm, computerized densitometry and comparison with 125I-labelled standards. Rat adrenal medulla contains a single class of high affinity AII receptors with a Ka of 0.84 +/- 0.02 X 10(9) M-1 and a Bmax of 3259 +/- 502 fmol/mg protein, one of the highest densities in AII receptors found in rat tissues. These observations provide evidence for a local site of action of AII in the release of adrenal medullary catecholamines.

Renal modulation of electrically stimulated adrenal medullary secretion

In acutely anephric dogs, depressed reflex adrenal medullary secretion (AMS) may be related to low plasma angiotensin. Either local (adrenal medulla) or central nervous system mechanisms are responsible. Local influences of acute bilateral nephrectomy on AMS were evaluated after left splanchnic nerve section. Two groups of five healthy, fasted mongrel dogs (16-20 kg) were prepared (Na pentobarbital iv anesthesia, 98% O2/2% CO2, Harvard volume ventilator) at celiotomy with blood pressure and sampling catheters per femoral arteriotomies and left adrenal-femoral venous T-shaped Silastic shunt. Group 2 dogs had acute bilateral nephrectomy. The acutely divided distal left splanchnic nerve was arranged for electrical stimulation (Grass, bipolar nerve stimulator, 1 0 V, 1-msec delay, 10 Hz, 10-msec square wave). Heparin anticoagulation was maintained and arterial pH monitored. Simultaneous adrenal vein, aortic blood sampling, and adrenal blood flow (F) determinations followed 10-min periods of alternating electrical stimulation and nerve rest. AMS for epinephrine (E) and norepinephrine (NE) was calculated [E and NE plasma concentration (single isotope radioenzymatic technique) differences of adrenal vein minus aorta multiplied by F]. Results were grouped, analyzed for variance, and compared (Wilcoxon unpaired rank sum, Student's test, Fischer's tables, ANOVA). Low aortic E, NE concentrations confirmed absent systemic adrenergic stimulation. The AMS ratio of NE:E was low in the anephric group. At the first stimulation interval NE:E was 0.28 +/- 0.14 (1 SD) in renal intact dogs vs 0.11 +/- 0.04 in anephric dogs, P less than 0.05. At rest NE:E was 0.33 +/- 0.12 in group 1 vs 0.17 +/- 0.02 in group 2 dogs, P less than 0.02. Plasma NE was also low in the anephric group (289 mg/liter +/- 126 (1 SD) vs 612 +/- 189, P = 0.033, resting).(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Effect of chronic sodium depletion on cerebrospinal fluid and plasma catecholamines

To test the role of central neurogenic factors in sodium-depleted states, cerebrospinal fluid (CSF) norepinephrine, epinephrine, and dopamine were measured in mongrel dogs first on a normal sodium intake (65 mEq sodium/day) and then on a 21-day regime of low sodium diet (4 mEq/day combined with diuretics). Plasma catecholamines were measured in the same group of dogs. Three weeks of sodium depletion supplemented with diuretics caused a 24-fold increase in plasma renin activity, hemoconcentration, and elevated serum protein concentration. Both plasma and CSF sodium decreased significantly. After sodium depletion, plasma norepinephrine rose 76% but epinephrine and dopamine did not change. The same pattern was observed whether samples were obtained in conscious or anesthetized animals. In CSF, norepinephrine rose 44% during sodium depletion, while epinephrine and dopamine remained unchanged. The CSF norepinephrine was related inversely to the CSF sodium concentration and directly to plasma renin activity. These observations support the view that the combined procedure of restricted dietary sodium intake and diuretic therapy causes alterations in CSF norepinephrine in a direction compatible with possible overactivity of central noradrenergic neurons.

Distinguishing traits in the Sabra hypertension-prone (SBH) and hypertension-resistant (SBN) rats

Distinct differences in central and peripheral noradrenaline (NA) were observed in the hypertension prone (SBH) and resistant (SBN) strain, derived from the Hebrew University SABRA rats. In the medulla oblongata NA concentration was 90% higher and tyrosine hydroxylase activity 88% lower in SBN when compared to SBH, suggesting marked strain differences in NA turnover. In this area, NA-induced cAMP generation was higher in SBH than in SBN, while the hypothalamus, the reverse situation was present. The relevance of hypertension of the reciprocal cAMP changes is still uncertain. The concentration of NA in heart tissue was significantly higher in SBN than in SBH. Doca-salt treatment caused hypertension and depletion of atrial NA in SBH, but had no effect on either blood pressure or atrial NA in SBN rats. The results suggest that resistance to hypertension in SBN rats is associated with decreased NA turnover in medulla oblongata and reduced activity of cardiac neuronal sympathetic endings.

The role of the renin-angiotensin system in mediation of adrenal catecholamine secretion in the cat induced by intrarenal beta-adrenergic stimulation

Isoproterenol infusion (0.1 microgram/kg per min) into the renal artery of the cat induced an increase in plasma renin concentration (PRC) from 14.3 +/- 5.7 (mean +/- SE) ng angiotensin I/ml per hr to 56.8 +/- 7.7 after 70 minutes (P < 0.05) and an increase in catecholamine secretion rate from 38.7 +/- 6.0 ng/kg per 10 min to 180.0 +/- 40.0 after 70 minutes (P < 0.001). Intravenous infusion of the same dose of isoproterenol had no significant effect on adrenomedullary catecholamine secretion rate. Isoproterenol induced preferential norepinephrine release: the ratio of norepinephrine to epinephrine secretion changed from 11.5:23.7 during the control period to 130.0:40.1 70 minutes after the start of isoproterenol administration. Intrarenal infusion of propranolol (3.0 mg/kg per min) inhibited renal renin release and adrenal catecholamine secretion in response to intrarenal isoproterenol. Intravenous infusion (0.4 microgram/kg per min) of an angiotensin II antagonist [Sar1, Ileu8]angiotensin II abolished the catecholamine response to intrarenal isoproterenol infusion. It is suggested that intrarenal isoproterenol infusion stimulates renal renin release and angiotensin production which, in turn, stimulates a preferential secretion of adrenomedullary norepinephrine.

Reversal by naloxone of hemorrhagic shock in anephric cats

Hemorrhage, 15 ml/kg induced a rapid fall of blood pressure in intact and anephric cats, but only intact cats demonstrated significant blood pressure recovery following bleeding. Naloxone, 0.1 mg/kg x min, i.v., had a mild promoting effect on blood pressure recovery in the intact cats whereas, in the hemorrhaged anephric cats naloxone re-established an almost complete recovery of blood pressure. These results suggest that endogenous opioid substances play a significant depressor role in hemorrhagic shock, especially in anephric animals.

Renin-angiotensin mediation of adrenal catecholamine secretion induced by hypoglycaemia in the cat

1 The mechanism involved in catecholamine (CA) release from the cat adrenal gland in response to insulin hypoglycaemia was studied. In intact cats, hypoglycaemia induced an 11 fold increase in adrenomedullary CA secretion. 2 Acute bilateral nephrectomy nearly abolished the increased CA release from the adrenal gland during hypoglycaemia. 3 Infusion of Sar1-Ileu8-Angiotensin II (AII), a competitive AII antagonist, suppressed the adrenomedullary response to the insulin-induced hypoglycaemia. After termination of the antagonist infusion CA secretion from the adrenal medulla increased rapidly, reaching the same level as in insulin-treated cats. 4 Infusion of rabbit anti-angiotensin I antibodies suppressed CA release from the adrenal gland of hypoglycaemic cats. This effect was more prolonged than that of Sar1-Ileu8-AII. 5 These results indicate that CA release from the adrenal medulla of the cat in response to insulin-induced hypoglycaemia, is mediated through the renal reninangiotensin system. Since hypoglycaemia causes sympathetic stimulation through a central mechanism, angiotensin may act through the central nervous system.

The mechanism of alpha-adrenergic inhibition of catecholamine release

1 The effect of alpha-adrenoceptor agonists on membrane adenosine triphosphatase (ATPase) activity was studied in membranes from the bovine adrenal medulla and the rat submaxillary gland. 2 alpha-Adrenoceptor agonists (10(-7) to 10(-5) M) enhanced significantly Na,K-ATPase activity but not Mg-ATPase activity in adrenal medulla. This effect was not observed in membranes from phaeochromyocytoma. Phenylephrine (10(-5) M), naphazoline (10(-5) M) and clonidine (10(-5) M) caused a significant increase of the activity of Na,K-ATPase (but not of Mg-ATPase) in the submaxillary gland. The enhancement became more prominent after ligature of the submaxillary duct but disappeared completely after superior cervical ganglionectomy. Thus, the effect of the alpha-adrenoceptor agonists was due to an action on adrenergic nerve terminals in the submaxillary gland. 3 Phenylephrine and naphazoline did not affect 45Ca uptake but enhanced the rate of 45Ca efflux from adrenal medullary slices in vitro. 4 Phenylephrine enhanced the rate of 45Ca efflux from slices of submaxillary gland (with previous ligation of the duct); this was blocked by phentolamine and sympathetic denervation. Therefore phenylephrine was acting on the adrenergic nerve terminals. 5 It is suggested that the inhibition by alpha-adrenoceptor agonists of the exocytotic release of catecholamines from adrenergic nerve terminals and from chromaffin cells may be due to activation of the sodium pump, which results in enhancement of calcium efflux, causing a reduction of free intracellular Ca2+.

Selective reduction of adrenal medulla response to angiotensin induced by suppression of renin-angiotensin

Angiotensin II (AII) induced an increase of adrenaline (A) release from rat adrenal glands in vitro. The response of the adrenal glands was completely abolished 22--24 h after bilateral nephrectomy. Adrenal glands from DOCA-salt-treated rats did not respond to AII as well, whereas adrenal glands from rats treated with furosemide and a low salt diet retained this response. High potassium in the medium increased significantly the release of A from in vitro incubated adrenal glands of nephrectomized rats from 0.43 +/- 0.04 to 0.63 +/- 0.06 microgram/gland and salbutamol (in calcium-free medium) increased the release from 0.34 +/- 0.03 to 0.43 +/- 0.03 microgram/gland. These results indicate that the adrenal medulla develops subsensitivity to AII in vitro, following a reduction in levels of this agonist in vivo.

Endogenous prostaglandins modulate adrenal catecholamine secretion

Adrenal catecholamine secretion induced by haemorrhage in the cat was increased by administration of indomethacin to intact or to bilaterally nephrectomized animals. Infusion of PGE2 (but not of PGF2 alpha) suppressed the increased catecholamine secretion caused by indomethacin.

Experimental hypertension and catecholamine distribution in the rat brain

Hypertension was induced in rats (Hebrew University strain) by three different procedures: (1) deoxycorticosterone acetate (DOCA)--salt treatment; (2) unilateral renal artery clip or (3) chronic salt-loading. Noradrenaline (NA) and dopamine (DA) distribution in different brain areas was assayed following induction of hypertension. NA content increased significantly in various areas: the increase of NA in the pons-medulla was common to all procedures inducing hypertension. NA content increased also in the mesencephalon, the hypothalamus and the rest of the forebrain (DOCA--salt hypertension), in the mesencephalon, the hypothalamus and the cortex (in renal clip hypertension). No significant changes in DA content were observed in any region of the brain following induction of hypertension by the three different methods. In two substrains, selected from the Hebrew University strain, for their respective sensitivity (H) or immunity (N) to hypertension induced by DOCA--salt treatment, there were no significant increases in NA or DA in any part of the brain following DOCA--salt treatment. Comparison of NA concentrations in these strains showed that NA was significantly higher in the pons-medulla of the untreated N strain rats than in the medulla of untreated H strain or in untreated rats of the original strain (Hebrew University). A model is presented suggesting that central NA-containing neurons plays a major role in controlling hypertension.

Mechanism of PGE inhibition of catecholamine release from adrenal medulla

Catecholamine (CA) secretion from the adrenal medulla was induced in vitro by acetylcholine (10(-4)M) (ACh), by incubation in potassium-free medium, by addition of ouabain (10(-3)M), by theophylline (10(-2)M) or by salbutamol (10(-6) and 6 x 10(-6) M). Theophylline and salbutamol, but not ACh, released CA in a calcium-free medium supplemented with 2mM EGTA. PGE2 significantly inhibited both CA secretion evoked by ACh and that evoked by salbutamol, i.e. both secretion dependent on, and independent of, extracellular calcium, PGE2 counteracted the increase of cAMP levels caused by ACh or salbutamol in adrenal medullary slices. PGE2 also diminished the salbutamol-induced activation of adenylate cyclase in an adrenal medullary membrane preparation, PGE2 reduced the rate of 45Ca efflux from slices of adrenal medulla preloaded with 45CaCl2. It is suggested that PGE2 inhibits CA secretion through the following sequence: inhibition of adenylate cyclase, a fall of cellular cAMP resulting in reduced release of calcium from intracellular binding sites and reduced free cytoplasmic calcium.

Modification by SQ 14225 of blood pressure and adrenal catecholamine response to hemorrhage

SQ 14225 infusion to cats exposed to hemorrhage suppressed adrenomedullary catecholamine release. The immediate compensatory blood pressure response was unaffected, while the later compensatory response was of a higher magnitude in intact cats that in SQ 14225-treated animals. Adrenal gland blood flow was better preserved in SQ 14225-treated hemorrhaged cats than in cats exposed to hemorrhage only. These data emphasize the role of angiotensin II in adrenal catecholamine and blood pressure responses to hemorrhage.

Effects of verapamil, dantrolene and lanthanum on catecholamine release from rat adrenal medulla

1. The release of catecholamines (CA) from rat adrenal incubated in vivo in Locke solution was studied. 2. Acetylcholine-induced release of CA and CA release by 56 mM KCl were inhibited by verapamil and lanthanum chloride which block calcium permeability. 3. CA secretion induced by salbutamol or by theophylline was unaffected by either verapamil or lanthanum chloride. 4. Dantrolene-sodium inhibited the CA secretion induced by theophylline but only partially reduced potassium-induced release of CA. 5. Verapamil enhanced the secretion of CA induced by salbutamol (in a calcium-free medium). 6. Tyramine-induced secretion of CA was unaffected by lanthanum chloride, verapamil or dantrolene-sodium. 7. It is suggested that cyclic adenosine 3',5'-monophosphate-mediated CA secretion (induced by theophylline or salbutamol) depends on release of calcium from intracellular stores, and that CA secretion induced by tyramine is independent of intra- or extracellular calcium.

The effect of saline loading on blood pressure and catecholamine secretion in the rat and the cat

The effects of acute and chronic saline loading on blood pressure and catechalomine (CA) response in the rat and cat were studied. In the rat, both acute and chronic saline loading stimulated urinary CA excretion without affecting blood pressure; adrenal tyrosine hydroxylase (TH) activity was increased. Pentolinium bitartrate abolished the increase in CA excretion. Chronic saline loading reduced the plasma renin concentration (PRC) and the heart noradrenaline concentration and elevated the plasma cholesterol. Acute infusion of isotonic or hypertonic saline in the cat stimulated adrenomedullary CA secretion. This response was not significantly affected after bilateral cervical vagotomy, but was totally abolished following bilateral nephrectomy. Blood pressure was increased only during the isotonic saline infusion but not during hypertonic saline administration. It is concluded that stimulation of adrenal CA secretion by saline loading is mediated through a renal factor.

Angiotensin II mediation of adrenal catecholamine secretion induced by intrarenal isoprenaline infusion

Isoprenaline infusion into the renal artery of the cat induced an increase in the catecholamine (CA) secretion rate from the adrenal gland. CA release by isoprenaline was preferentially noradrenaline. Intravenous infusion of an angiotensin II (A II) antagonist abolished adrenal CA response to intrarenal isoprenaline injection. It is suggested that intrarenal isoprenaline infusion stimulates renal renin release and A II production which, in turn, stimulates adrenal CA secretion.

Reversal of prostaglandin E2 effect on adrenal catecholamine release after hypophysectomy

Prostaglandin E2 induced increased catecholamine release in vitro from adrenals of hypophysectomized rats, while in adrenals of intact rats catecholamine release was suppressed by prostaglandin E2.