Evidence for a beta-adrenoceptor-mediated regulation of human natural killer cells

Pretreatment of lymphocytes (16 hr, 37 degrees C) with adrenaline at final concentrations of 10(-7) to 10(-9) M, followed by removal of the drug, increased natural killer (NK) cell activity vs K562 leukemic cells in a 4-hr 51Cr-release assay. The most efficient concentration of adrenaline was 10(-8) M; mean increase of NK activity over base-line activity for all donors examined was 30%. However, the individual response to adrenaline pretreatment was variable; in some donors, the effect was equal to maximal interferon (IFN) stimulation. Effects of adrenaline pretreatment were consistently reduced to base-line activity by co-incubation with the nonselective beta-adrenoceptor antagonist propranolol at 100-fold higher concentrations. The enhancing effect of adrenaline (10(-8) M) pretreatment was also observed after 1-hr pretreatment; this effect was prevented by simultaneous incubation with propranolol but was not affected by dex-propranolol. Direct addition of adrenaline to lymphocyte/target cell mixtures was inhibitory at 10(-6) M adrenaline concentration. The inhibitory effect of adrenaline in this assay was again completely prevented by propranolol and unaffected by dex-propranolol. The observed stimulatory effect of adrenaline pretreatment could not be ascribed to IFN production. Data presented indicate a dual effect of adrenaline on NK cell activity and suggest both a positive and a negative beta-adrenoceptor-mediated regulation of human NK cells.     

A role for alternative pathway catecholamines in the regulation of steroidogenesis in cow luteal cells

Incubation of bovine luteal cells with the alternative pathway catecholamines octopamine, synephrine and deoxyadrenaline at concentrations of 10(-6) to 10(-3) M enhanced the production of progesterone (P less than 0.05). Tryamine did not alter basal progesterone production (P greater than 0.05). Addition of noradrenaline and adrenaline at concentrations of 10(-4) to 10(-7) M significantly elevated the production of progesterone (P less than 0.05). The steroidogenic response to noradrenaline and adrenaline was greater than that for octopamine, synephrine and deoxyadrenaline (P less than 0.05). Response to both primary (10(-6) M) and alternative (10(-4) M) pathway catecholamines was inhibited by propranolol (10(-5) M, P less than 0.05) but not phentolamine (10(-5) M, P greater than 0.05). These results demonstrate that octopamine, synephrine and deoxyadrenaline can affect steroidogenesis by bovine luteal cells, and their action is mediated by beta-adrenergic receptors.     

Radioprotective action of catecholamines on a Chinese hamster fibroblast culture

The radioprotective ability of adrenaline, noradrenaline and isoproterenol in various concentrations has been studied in experiments on cultured Chinese hamster fibroblasts in vitro. Radioprotective effect of isoproterenol is pronounced at 10(-8)M concentration; adrenaline and especially noradrenaline are effective at higher concentrations. The antagonist of beta-adrenergic receptors propranolol blocks the radioprotective effect of catecholamines on cells. The beta-adrenergic mechanism of catecholamines radioprotective action on the mammals cells is under discussion.     

The inotropic action of adrenergic agonists on the heart ventricles of the chick embryo]

The effects of adrenergic drugs on the twitch tension of the electrically driven (1.2-1.5 Hz) ventricular preparations from 2-20-day old chick embryos and hatched chicks were studied. Agonists evoked positive inotropic responses of 3-day embryonic ventricles and of ventricles from older animals. 2-day embryonic ventricles were unresponsive. 5-day embryonic ventricles were most sensitive to agonists (EC50 value of adrenaline = 4.5 x 10(-9) M), while ventricles from 14-20-day old embryos had a minimal sensitivity (1-2 x 10(-9) M), while ventricles from 14-20-day old embryos had a minimal sensitivity (1-2 x 10(-7) M). The order of agonists activity (isoproterenol greater than noradrenaline greater than adrenaline much greater than phenylephrine) and the high potency of propranolol as antagonist of adrenaline indicate that responses are mediated with beta-adrenoceptors. The role of GTP-binding protein for the regulation of adrenoreactivity in embryonic chick heart during ontogenesis is discussed.     

Stimulation of progesterone secretion by cultured human granulosa cells with melatonin and catecholamines

Granulosa cells, aspirated from the follicles of patients undergoing treatment for in-vitro fertilization, were cultured in serum-supplemented medium. Adrenaline and noradrenaline stimulated a dose-related increase in progesterone secretion with a maximum stimulation at 10(-5) M, a response that was prevented by the beta-antagonist, propranolol. Adrenaline and hCG showed similar characteristics in their stimulation of progesterone secretion but there was no further increase in progesterone when the 2 compounds were added together. Melatonin stimulated progesterone secretion and, like adrenaline, this stimulation was prevented by propranolol. The ability of both adrenaline and melatonin to increase progesterone secretion was dependent on the degree of follicular development, as determined by peripheral oestradiol concentrations, on the day of laparoscopy. These results suggest that adrenaline and melatonin may have a physiological role in modulating luteal function and that melatonin may act by a beta-adrenergic-related mechanism.     

Requirement of Indoleamines and a V-type Proton ATPase for the Expression of the Circadian Glow Rhythm in Gonyaulax polyedra

In the dinoflagellate Gonyaulax polyedra , bioluminescence is known to be controlled by proton transfer from an acidic vacuole system to the scintillons. We demonstrate that bafilomycin A 1 , a specific blocker of V-type proton ATPases, inhibits at low concentrations (down to 2 × 10 -8 M) bioluminescence and, in particular, the circadian glow maximum. For many hours bafilomycin A 1 does not interfere with the capacity of the bioluminescent system. Therefore, we conclude on the participation of a V-type ATPase in proton accumulation in the acidic vacuoles. Inhibition of tryptophan hydroxylase by p-chlorophenylalanine, p-fluorophenylalanine, or 5-fluorotryptophan also suppresses the circadian glow maximum. After inhibition of the enzyme by p-chlorophenylalanine, the glow peak can be restored, without any additional unspecific effects on bioluminescence, by supplementation with 5-hydroxytryptophan. Therefore, the availability of indoleamines is required for the expression of the glow maximum. Since 5-methoxytryptamine is the only physiologically occurring indoleamine with substantial effects on bioluminescence at low concentrations (below 10 -7 M), and since this substance accumulates in the second half of the night to stimulatory concentrations, this indolic metabolite may represent the physiologically active substance involved in the expression of the glow maximum.     

Effect of ascorbic acid on the binding of 3H-GABA and 3H-glutamic acid to synaptosomes of the rat cerebral cortex

The effects of different concentrations of L-ascorbic acid (Asc) on Na+-dependent binding of 3H-GABA and 3H-DL-glutamic acid to rat brain cortical synaptosomes were studied. Asc, at a concentration nearly equal to brain extracellular one (3 X 10(-4) M), had no effect on specific and nonspecific 5H-GABA binding. At higher concentrations (10(-3) M) Asc strongly inhibited, and at lower concentrations (10(-6) M) considerably stimulated 3H-GABA binding. At a concentration of 10(-5)-10(-3) M Asc tended to decrease 3H-DL-glutamic acid binding.     

Potentiation of the responses to transmural nerve stimulation by alpha-agonists: a possible role in vivo

This study was undertaken to assess the effects of exogenous alpha-agonists on the effector response to transmural nerve stimulation in canine saphenous vein rings. The response to a fixed train (5 s duration) of transmural nerve stimulation (8 Hz, 0.3 ms, 9 V) applied every 5 min was determined in the control state and in the presence of subthreshold (for contraction) concentrations of noradrenaline, adrenaline, clonidine, and methoxamine. The maximum potentiations achieved by the three drugs were 246.2 +/- 36.9, 220.5 +/- 38.8, 384.3 +/- 78.7, and 353.3 +/- 68.0%, respectively. The potentiation observed was significantly inhibited by indomethacin (10(-6) mol/L) and propranolol (5 X 10(-6) mol/L). Both indomethacin and propranolol potentiated the response to transmural nerve stimulation. The potentiation of the responses to transmural nerve stimulation by alpha-agonists suggests that, presynaptic alpha 2-inhibition by circulating catecholamines is likely to be of limited biological significance in modulating the effector responses in the canine saphenous vein.     

Effects of carbon monoxide and heme oxygenase inhibitors in cerebral vessels of rats and mice

Carbon monoxide (CO) has been postulated to be a signaling molecule in many tissues, including the vasculature. We examined vasomotor responses of adult rat and mouse cerebral arteries to both exogenously applied and endogenously produced CO. The diameter of isolated, pressurized, and perfused rat middle cerebral arteries (MCAs) was not altered by authentic CO (10(-6) to 10(-4) M). Mouse MCAs, however, dilated by 21 +/- 10% at 10(-4) M CO. Authentic nitric oxide (NO., 10(-10) to 10(-7) M) dilated both rat and mouse MCAs. At 10(-8) M NO., rat vessels dilated by 84 +/- 4%, and at 10(-7) M NO., mouse vessels dilated by 59 +/- 9%. Stimulation of endogenous CO production through heme oxygenase (HO) with the heme precursor delta-aminolevulinic acid (10(-10) to 10(-4) M) did not dilate the MCAs of either species. The metalloporphyrin HO inhibitor chromium mesoporphyrin IX (CrMP) caused profound constriction of the rat MCA (44 +/- 2% at 3 x 10(-5) M). Importantly, this constriction was unaltered by exogenous CO (10(-4) M) or CO plus 10(-5) M biliverdine (both HO products). In contrast, exogenous CO (10(-4) M) reversed CrMP-induced constriction in rat gracilis arterioles. Control mouse MCAs constricted by only 3 +/- 1% in response to 10(-5) M CrMP. Magnesium protoporphyrin IX (10(-5) M), a weak HO inhibitor used to control for nonspecific effects of metalloporphyrins, also constricted the rat MCA to a similar extent as CrMP. We conclude that, at physiological concentrations, CO is not a dilator of adult rodent cerebral arteries and that metalloporphyrin HO inhibitors have nonspecific constrictor effects in rat cerebral arteries.     

Relaxing effect of insulin in renal arteries from diabetic rats

Abnormal renal vasomotor tone exists in the early stages of diabetes mellitus. Insulin has been proposed to modulate renal function and to possess vasodilatory effects. The present study was initiated in order to evaluate the direct effect of insulin on isolated renal arteries. Twelve insulin-treated streptozotocine diabetic rats with diabetes for 50 days were compared with 15 weight-matched control rats. The contractile responses to 60 mM K+ and 10(-4) M noradrenaline, and the insulin- (0.8-6.4 I.U./ml) induced relaxation of vessels precontracted with noradrenaline, were similar in diabetic and control rats. There was a tendency towards greater relaxation in diabetic (71%) than in control rats (54%). Nw-nitro-L-arginine methyl ester (L-NAME) (10(-4) M) given before noradrenaline tended to attenuate the insulin-induced relaxation, while addition of L-arginine (10(-6) M) to L-NAME attenuated the relaxation in diabetic but increased it in control rats (P < 0.05). The effect of insulin was tested further in control rats and was not influenced by administration of a single dose (10(-6) M) of indomethacin or propranolol given instead of L-NAME. The effect of a single dose of methylene-blue, given before noradrenaline, was tested in control rats in varying doses between 2 x 10(-6) and 2 x 10(-4) M. In the highest concentration it made no difference whether insulin was given or not and there was a similar relaxing effect in diabetic and control arteries. In conclusion, the present study showed that insulin per se has a relaxing effect on renal arteries. There was a tendency to greater relaxation in diabetic than in control rats, an effect which was attenuated by in-vitro-pretreatment with L-NAME as well as with L-NAME and L-arginine in diabetic vessels, while relaxation was increased in control vessels. This may indicate that the effect of insulin may be mediated through nitric oxide in diabetic but not in control rats. The effects of insulin in control vessels were not modified in vitro by indomethacin, propranolol or methylene-blue.     

Innervation of holothurian body wall muscle: inhibitory effects and localization of 5-HT

We investigated innervation to body wall muscles as well as distribution of 5-HT (serotonin) and its effects on longitudinal muscles of body wall (LMBW) of the sea cucumber Apostichopus japonicus. With serial sections we found neural branches and fibers extending from hyponeural part of radial nerve towards LMBW and circular muscles of body wall. With the aqueous aldehyde (Faglu) method yellow fluorescence indicating indolamines was observed in LMBW and in the mesentery connecting LMBW to the body wall. With indirect immunohistochemistry 5-HT-like immunoreactivity was observed in LMBW and in mesentery. These results strongly suggested that both LMBW and mesentery contained 5-HT. The effects of monoamine neurotransmitters were studied in LMBW. Putative neurotransmitters tested were 5-HT, adrenaline, noradrenaline, dopamine, and DOPA at the concentration of 10(-6) M. The application of 5-HT caused no contraction or relaxation, but it inhibited the contraction induced by 10(-6)-10(-5) M acetylcholine (ACh). Catecholamines were ineffective by themselves and had no effects on the contraction induced by ACh. The present histological, histochemical, and pharmacological studies strongly suggested that holothurian LMBW was innervated by inhibitory serotonergic neurons of the hyponeural nervous system.     

Requirement of Indoleamines and a V-type Proton ATPase for the Expression of the Circadian Glow Rhythm in Gonyaulax polyedra

In the dinoflagellate Gonyaulax polyedra, bioluminescence is known to be controlled by proton transfer from an acidic vacuole system to the scintillons. We demonstrate that bafilomycin A 1, a specific blocker of V-type proton ATPases, inhibits at low concentrations (down to 2 × 10 –8 M) bioluminescence and, in particular, the circadian glow maximum. For many hours bafilomycin A 1 does not interfere with the capacity of the bioluminescent system. Therefore, we conclude on the participation of a V-type ATPase in proton accumulation in the acidic vacuoles. Inhibition of tryptophan hydroxylase by p-chlorophenylalanine, p-fluorophenylalanine, or 5-fluorotryptophan also suppresses the circadian glow maximum. After inhibition of the enzyme by p-chlorophenylalanine, the glow peak can be restored, without any additional unspecific effects on bioluminescence, by supplementation with 5-hydroxytryptophan. Therefore, the availability of indoleamines is required for the expression of the glow maximum. Since 5-methoxytryptamine is the only physiologically occurring indoleamine with substantial effects on bioluminescence at low concentrations (below 10 –7 M), and since this substance accumulates in the second half of the night to stimulatory concentrations, this indolic metabolite may represent the physiologically active substance involved in the expression of the glow maximum.     

Lipolytic response of fat cells to catecholamines in sedentary and exercise-trained women

It has been shown that adipose tissue lipolytic activity is increased in endurance-trained subjects. In women, adipose tissue is extensive and it was thought interesting to confirm that endurance training increases the capacity of female adipose tissue to mobilize lipids, and moreover to more fully understand the mechanisms involved. So, biopsies of fat were obtained from the periumbilical region of 13 trained female runners (T) and 17 sedentary women (S) and the in vitro response to catecholamines of the collagenase-isolated fat cells was studied. Glycerol release, chosen as adipocyte lipolysis indicator, was measured by bioluminescence for various epinephrine and norepinephrine concentrations. In both groups, these substances provoked an increase in lipolysis, but the response was significantly higher in T. In both groups, isoproterenol increased the lipolytic activity above basal concentrations at 10(-8) M and above. Lipolytic activity in T was significantly higher (P less than 0.01) than the S control at 10(-7) M and above. Epinephrine plus propranolol decreased lipolysis in both groups, but at 10(-5) M, lipolytic activity was significantly lower in S than in T (P less than 0.05). It is concluded that in female subjects, endurance training increases the sensitivity of subcutaneous abdominal adipose tissue to the lipolytic action of catecholamines; this effect seems to be related both to a decreased efficiency of the alpha 2-adrenergic pathway and to an increased efficiency of the beta-adrenergic pathway. This latter effect seems to take place at a step beyond the receptor-adenylate cyclase system in the lipolytic cascade.     

Molecular mechanisms of regulatory action of adrenergic receptor agonists on functional activity of adenylyl cyclase signaling system of the ciliates Dileptus anser and Tetrahymena pyriformis

Adenylyl cyclase signaling system (ACS) of the higher eukaryotes involves the following main components: receptor, heterotrimeric G protein, adenylyl cyclase (AC), and protein kinase A. At present, these components have been found in cells of different species of the lower eukaryotes. Hence, the signal transduction through ACS of unicellular eukaryotes may have some features in common with those of the higher eukaryotes. We showed earlier that agonists of adrenergic receptors (ARs) regulate AC activity of ciliates Dileptus anser and Tetrahymena pyriformis. The aim of this work was to study molecular mechanisms of AR ligand action on the functional activity of different components of ACS of the ciliates. It has been shown that beta-AR antagonist [3H]-dihydroalprenolol binds membranes of the ciliates with a comparatively lower affinity than those of the higher eukaryotes (Kd for D. anser was 13.4 nM, for T. pyriformis--27 nM). Beta-AR ligands--agonist (-)-isoproterenol and antagonists propranolol and atenolol in competition manner displace [3H]-dihydroalprenolol with IC50 that are 10-100 times higher than corresponding IC50 of beta-AR of the higher eukaryotes. In the presence of GTP, the right shift of competition curves of [3H]-dihydroalprenolol displacement by isoproterenol was obtained, being most considerable in the case of D. anser. Adrenaline and isoproterenol in a dose-dependent manner stimulated GTP-binding in cell cultures of D. anser and T. pyriformis. Suramin (10(-5) M), the inhibitor of heterotrimeric G proteins, completely blocked effects of these hormones. In D. anser culture, adrenaline and isoproterenol in a dose-dependent manner, stimulated AC activity, and its stimulating effects in the presence of beta-AR blockers vanished (propranolol) or decreased to a great extent (atenolol). At the same time the effects were unchanged in the presence of alpha2-AR antagonists yohimbine and idazoxan. These data show the involvement of G protein-coupled beta-AR in signal transduction induced by AR agonists in D. anser cells. In cell culture of T. pyriformis isoproterenol weakly stimulated AC activity, and its effect was completely blocked by beta-AR blockers. Adrenaline in T. pyriformis cells in a dose-dependent manner inhibited AC activity. Inhibiting effect of hormone was decreased in the presence of alpha2-AR blockers. On the basis of the obtained data we concluded that adrenaline in T. pyriformis cells inhibited AC activity through G protein-coupled receptor, being close to alpha2-AR of vertebrate animals.     

Alpha 2-adrenoceptor control of ion and water transport in the newt renal distal tubule.

To study the nature of adrenergic stimulation of ions and water reabsorption in the newt renal distal tubule, stationary microperfusion of the nephron and electron probe analysis were used. After application of norepinephrine (NE 10(-6) M) to the tubule surface, the fractional reabsorption of fluid increased from 15.0 +/- 3.1 to 41.30 +/- 10.4% (n = 7, p < 0.01), of Na+ from 69.30 +/- 6.6 to 79.10 +/- 7.5% (p < 0.05), Cl- from 63.30 +/- 7.6 to 72.40 +/- 7.9% (p < 0.05). Instead of secretion (control), there was reabsorption of K+. Fractional reabsorption of Ca2+ decreased from 51.00 +/- 6.0 to 43.00 +/- 7.0% (p < 0.05). The nonspecific alpha-adrenergic antagonist dibenamine 10(-6) M completely inhibited the effect of NE while, under the action of propranolol (2 x 10(-6) M) NE increased ion and water reabsorption significantly. When applied alone, or with NE, the specific alpha 2-adrenoblocker idazoxan, 2 x 10(-6) M, did not interfere with reabsorption in the distal tubule. At the same time, under the action of alpha 1-adrenoblocker prazosin 2 x 10(-6) M NE, increased the fractional reabsorption of fluid from 24.10 +/- 3.4 to 44.40 +/- 4.0% (n = 6, p < 0.001). These results serve as evidence that there exist specific alpha 2-adrenoceptors in the newt distal tubule the stimulation of which increases membrane permeability of the distal tubule to water, Na+, K+, Cl-, but not to Ca2+.     

Effect of biologically active agents on the feeding response of Dileptus anser]

A study was made of the effects of pharmacological agents--adrenaline, noradrenaline, serotonine, dibenamine, theophylline, and emidazole--on the phagocytar activity of Dileptus anser. These effects were estimated in terms of food response changes towards chemical food models (CFM) made in cysteine, lecithine or tween-40 solutions. The above pharmacological agents were also tested as phagocytosis inductors. Of these, only adrenalin appeared to be an effective inductor of the food response. The CFM, made in adrenaline, was stimulated by 10(-6) M theophylline, and inhibited with 10(-4)--10(-8) M imidazole. The addition of 10(-3)--10(-12) M adrenaline or 10(-8)--10(-10) M serotonine to the Dileptus-containing medium stimulated phagocytosis of CFM. 5.10(-6) M dibenamin decreased phagocytotic intensity of CFM. 10(-6) M theophylline stimulated, while 10(-4) M inhibited the food response. It is proposed that protozoans have receptors capable of accepting hormones. A possible role of the system of cyclic AMP in transporting hormonal and food stimules in protozoans is discussed.     

Centrally acting leptin induces a resuscitating effect in haemorrhagic shock in rats

Centrally acting leptin induces the activation of the sympathetic nervous system with a pressor effect in normotensive rats. The purpose of the study was to examine central leptin-evoked action in critical haemorrhagic hypotension. In anaesthetized male Wistar rats subjected for irreversible haemorrhagic shock with mean arterial pressure (MAP) 20-25 mmHg haemodynamic parameters and plasma concentrations of adrenaline and noradrenaline were measured. Leptin given intracerebroventricularly (20 μg) evoked long-lasting rises in MAP and heart rate (HR), with a subsequent increase in renal, mesenteric and hindquarters blood flows and a 100% survival at 2 h. MAP and peripheral blood flow changes were inhibited by a pre-treatment with α(1)- and α(2)-adrenoceptor antagonists prazosin (0.5 mg/kg) and yohimbine (1 mg/kg), while β-adrenoceptor antagonist propranolol (1 mg/kg) blocked leptin-induced HR changes, without influence on MAP, peripheral blood flows and survival. Twenty min after leptin treatment, there were higher plasma concentrations of noradrenaline, but not adrenaline, in comparison with the saline-treated control group. In conclusion, centrally acting leptin induces a long-lasting pressor effect with an improvement in the survival rate in haemorrhage-shocked rats. The effect may be associated with the activation of the sympathetic nervous system.     

The beta-adrenergic regulation of the Na, K-ATPase activity in the sarcolemma of the heart muscle

Using a sensitive potentiometric method the effect of isoproterenol upon the activity of Na, K-ATPase in cardiomyocytes has been studied. The activity of the enzyme in rat sarcolemma at isoproterenol-induced myocarditis decreases by 42%. A direct action of isoproterenol on the Na, K-ATPase activity in sarcolemma in vitro has been investigated. In the concentration range 10(-9)-10(-3) M (from receptor-binding up to cardiotoxic) a gradual decrease of the activity reaching the complete inhibition at 10(-3) M is revealed. Antagonist of beta-adrenoreceptors propranolol in concentrations required for displacing the agonist (10(-9) M) provides for the recovery of the Na, K-ATPase activity up to 76% of its normal activity. This action transforms into nonspecific inhibition at rising concentration of the antagonist. Possible mechanisms of the beta-adrenergic regulation effect in cardiomyocytes on Na, K-ATPase of the sarcolemma are discussed.     

Galanin potentiates electrical stimulation and exogenous norepinephrine-induced contractions in the rat vas deferens

In order to evaluate the mode of action of galanin (GAL) on the neuroeffector mechanism of peripheral sympathetic nerve fibers, the effects of this peptide were tested on the electrical stimulated and the unstimulated preparations of the isolated rat vas deferens in the presence of 10(-7) M atropine. The contractile responses, which were mediated predominantly by activation of postganglionic noradrenergic nerve fibers were dose-dependently potentiated by GAL in concentrations ranging from 1 to 50 nM. The facilitatory action induced by GAL in high concentrations (greater than 10 nM) usually returned to the control level at 2-3 min and were tachyphylactic. The potentiating action of GAL was not modified by pretreatment with 10(-7) M propranolol. Contractions produced by exogenous norepinephrine (NE) in the unstimulated preparations were not affected by pretreatment with low concentrations (less than 5 nM) of GAL. On the other hand, the contractions were dose-dependently potentiated 1 min after pretreatment with higher concentrations (greater than 10 nM) of GAL, which recovered 15 min after constant flow washout. Contractions developed by exogenous 5-hydroxytryptamine were not affected, or slightly inhibited, by GAL (1-50 nM). In some preparations without electrical stimulation, high concentrations of GAL caused a slight contraction, which was not blocked by pretreatment with 10(-6) M phentolamine and 10(-6) M tetrodotoxin. These results suggest that GAL receptors exist presynaptically in the rat vas deferens and that stimulation of the receptors by GAL potentiates the release of NE from the nerve terminals during postganglionic sympathetic nerve stimulation. Other mechanisms for GAL action, such as influence on neuronal uptake and catecholamine metabolism, cannot be ruled out.     

Concentration-dependent effects of angiotensin II on sinus rate in canine isolated right atrial preparations.

To investigate the concentration-response relationship of angiotensin II with respect to its chronotropic effects, the sinus rate was recorded from canine isolated right atrial preparations perfused through the sinus node artery. Nicotine (5 x 10(-5) M) injection induced an early, atropine-sensitive bradycardic response and a more delayed propranolol-sensitive tachycardic response, suggesting that the preparations contained both cholinergic and adrenergic neurons. The former response, but not the latter, was markedly reduced in preparations in which the right atrial ganglionated plexus was removed. Positive chronotropic responses were induced by angiotensin II over a wide range of concentrations (10(-12) - 5 x 10(-6) M), with a maximum increment of 29.9 +/- 9.6 beats/min. Responses to low concentrations (angiotensin II, 10(-11) M) were monophasic and were abolished by propranolol. In contrast, the responses to higher concentrations (angiotensin II, 10(-6) M) were not abolished by propranolol and were biphasic (early response, 29.9 +/- 12.1 beats/min; later response, 18.6 +/- 9.0 beats/min), the early response being blocked by losartan (AT1 antagonist) but not the later one, both being blocked by saralasin (nonselective angiotensin II antagonist). In conclusion, the data suggest that angiotensin II exerts its stimulant effects on the heart through receptors located either on cardiomyocytes or neurons, depending on the agonist concentration.