Possible role of a beta-adrenoceptor in the regulation of noradrenaline release by nerve stimulation through a positive feed-back mechanism

Evidence for presynaptic beta-adrenoceptor stimulation 24 hours after a 6 hour adrenaline infusion

1. The effects of 6 h infusions of adrenaline (INF-A) and dextrose (INF-D) were compared in nine normal subjects. 2. A significant increase in systolic blood pressure was observed during INF-A compared with INF-D and, 24 h after infusion, diastolic blood pressure was higher after INF-A than after INF-D. 3. Heart rate (HR) was significantly higher during INF-A than during INF-D. 4. As expected, plasma ADR increased significantly during INF-A but, unexpectedly, remained elevated 60 min post infusion compared with INF-D. Levels during activity the next morning were somewhat higher after INF-A, but not significantly different from INF-D. 5. Plasma NA increased transiently during INF-A and decreased during INF-D. Urinary NA was significantly higher during INF-A than during INF-D, and insignificantly higher during overnight recumbency, consistent with enhanced noradrenergic transmission.

Quinolinic acid: effects on brain catecholamine and c-AMP content during L-dopa and reserpine administration

The catecholamine content in rat brain tissue was determined following the administration of quinolinic acid alone or combined either with L-dopa and decarboxylase inhibitor or reserpine. Quinolinic acid alone decreased the levels of dopamine and noradrenaline, as well as those of c-AMP, and increased those of adrenaline. Treatment with L-dopa/decarboxylase inhibitor reversed the suppressing effect of quinolinic acid on dopamine, but not on noradrenaline. Reserpine alone depleted the contents of dopamine, noradrenaline and adrenaline. It could be concluded from the effects of quinolinic acid and reserpine given together that quinolinic acid suppresses the depletion of amines induced by reserpine. It has been demonstrated that quinolinic acid leads to injuries of nerve-cell bodies in pars compacta of the substantia nigra and in the striatum. Quinolinic acid is a natural metabolite of tryptophan, normally occurring in the liver, kidney and brain (Wolfensberger et al. 1983; Moroni et al. 1984). This compound exhibits convulsant and neuron excitant properties (Stone et al. 1987). It induces a selective pattern of neuronal degeneration both at the site of intracerebral injection (Schwarcz et al. 1983; Stone et al. 1987) and after general (intracardiac) administration (Beskid and Markiewicz 1988). The ability of quinolinic acid to produce neurotoxicity was greater in the striatum than in other parts of the brain. This prompted us to study catecholamine and c-AMP levels in rat brain tissue following quinolinic acid and L-dopa administration, as well as the influence of reserpine on quinolinic acid action.

Contrasting effects of propranolol on sympathetic nerve activity and vascular resistance during orthostatic stress

BACKGROUND

Previous studies in humans advanced the concept that cardiac filling pressure and contractility, the primary determinants of ventricular mechanoreceptor discharge, are important determinants of sympathetic outflow during orthostatic stress. Thus, intravenous propranolol greatly attenuated forearm vasoconstrictor response to venous pooling with lower body negative pressure (LBNP). The aim of this study was to reevaluate the experimental support for this concept by using direct measurements of sympathetic nerve activity.

METHODS AND RESULTS

In 11 healthy humans, we recorded muscle sympathetic nerve activity (MSNA) with microelectrodes (peroneal nerve), as well as blood flow in the forearm and calf (venous occlusion plethysmography) at baseline and during graded LBNP. The same experiments were repeated after administration of propranolol (0.15 mg/kg i.v.), which is thought to decrease ventricular mechanoreceptor discharge. The major new findings are that propranolol neither increased baseline MSNA nor attenuated the increases in MSNA during graded orthostatic stress even though in the same subjects, propranolol simultaneously increased the baseline level of vascular resistance in both the forearm and calf and substantially attenuated the increases in regional vascular resistance during orthostatic stress.

CONCLUSIONS

Systemic beta-blockade causes a marked dissociation between sympathetic outflow and vascular resistance that invalidates the use of intravenous propranolol as an experimental model to examine the reflex effects of ventricular mechanoreceptors on peripheral vascular resistance in humans.

Effects of immobilization stress on renal sympathetic neurotransmission

The effects immobilization stress on renal sympathetic neurotransmission as well as on heart, spleen and adrenal catecholamine content were examined in the rat. A single 2.5 hr stress period produced significant increases in blood pressure, heart rate, plasma norepinephrine and plasma epinephrine concentrations. However, no changes in renal catecholamine content or in stimulus-induced (1 Hz, 120 pulses, supramax. V) overflow of catecholamines were observed when the isolated perfused rat kidney was studied immediately after the 2.5 hr stress period. In contrast, the single stress period produced a 3-4 fold increase in cardiac epinephrine content while no effects on spleen or adrenal catecholamine content were observed. When stress was applied for 7 daily 2.5 hr periods, the repetition of the stress failed to produce any changes in renal neurotransmitter content or stimulus-induced overflow from the isolated perfused rat kidney. The data suggest that the accumulation of epinephrine into peripheral sympathetic nerves as a result of stress-induced adrenal catecholamine release is not a phenomenon which can be generalized to all regions of the cardiovascular system.

Epinephrine and the genesis of hypertension

Several lines of evidence suggest a psychophysiological link between stress, adrenomedullary activation, and the genesis of hypertension. Experimental data support four important concepts: 1) epinephrine stimulates prejunctional beta 2-adrenergic receptors that facilitate norepinephrine release from sympathetic nerve endings; 2) epinephrine can be converted into a cotransmitter by neuronal uptake and on subsequent release augment the simultaneous discharge of norepinephrine; 3) exogenous epinephrine can induce sustained hypertension in rats; and 4) there is a period of critical sensitivity to endogenous epinephrine in a genetic model of rat hypertension. Plasma epinephrine concentrations are elevated in many young subjects with borderline or mild hypertension. The hypothesis that intermittent surges in epinephrine could initiate or promote the development of primary hypertension by amplifying peripheral neurotransmission, both directly (facilitative effect) and indirectly (cotransmitter action), is supported by reports that hemodynamic and noradrenergic responses to sympathetic activation can be augmented by increases in endogenous epinephrine or by its local or systemic (up to 30 ng/kg/min) infusion. Such responses have been documented in both normotensive and hypertensive subjects and can be blocked by propranolol. Although the weight of evidence (mostly indirect) indicates that epinephrine can augment norepinephrine release in humans, the epinephrine hypothesis, itself, remains unproven. Expression of hypertension by this mechanism may be restricted to a specific epinephrine-sensitive subset of individuals with a genetic predisposition to high blood pressure.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Novel vascular effects of isoprenaline following pacing-induced heart failure in the dog

Isoprenaline produced concentration-dependent contractions of the in vitro canine saphenous vein which were attenuated by phentolamine (10(-6) M) and pacing-induced heart failure. Both at control and peak heart failure, a biphasic response was seen in the dorsal pedal artery, consisting of an initial relaxation followed by a contraction; phentolamine and heart failure enhanced the relaxation component. In the presence of propranolol, the isoprenaline-induced contraction was sensitive to yohimbine, but resistant to prazosin. Therefore it is concluded that isoprenaline interacts not only with vascular beta-adrenoceptors, but also with alpha 2-adrenoceptors. Endothelial denudation resulted in a diminished response to isoprenaline in control saphenous vein and dorsal pedal artery but not in vessels from dogs with heart failure. The observation that the contractile response to isoprenaline diminishes in heart failure implies a specific down-regulation of peripheral vascular alpha 2-adrenoceptors.

Effects of selective adrenergic agonists and antagonists on gastric tone in the rat

The aim of the present in vivo study was to investigate in anaesthetized rats, the effects of selective adrenergic agonists and antagonists on basal gastric tone and phasic contractions by the use of a volumetric method. L-phenylephrine, an alpha-1 agonist, induced hypertension, bradycardia and a significant gastric relaxation. Clonidine, an alpha-2 agonist, caused hypotension, bradycardia and a significant gastric contraction and a reduction of the amplitude of phasic contractions. Salbutamol, a beta-2 agonist, induced a dose-dependent tachycardia and a significant inhibition of gastric tone whereas prenalterol, a beta-1 agonist, induced tachycardia without any significant influence on gastric basal tone. Yohimbine, an alpha-2 blocker, significantly decreased gastric basal tone and reversed the inhibition of phasic contractions induced by clonidine. Prazocine, a selective alpha-1 blocker, and propranolol, a non-selective beta-blocker, had no significant influence on gastric basal tone or phasic contractions. It is concluded that sympathetic inhibition of basal gastric tone in the rat is mediated by alpha-1 and beta-2 adrenergic receptors. Activation of alpha-2 adrenergic receptors significantly increased basal gastric tone and reduced the amplitude of phasic contractions. A blockade of alpha-2 receptors significantly decreased basal gastric tone and restored the amplitude of phasic contractions.

Is adrenaline released by sympathetic nerves in man?

Radiotracer methods were used to measure the rates of regional release of adrenaline and noradrenaline into plasma in man. This was done as a partial test of a theory of essential hypertension pathogenesis which envisages an important cotransmitter function for neuronally released adrenaline. In healthy resting men no release of adrenaline could be detected from the heart, lungs or liver. Adrenaline was released into the right renal vein but an adrenal medullary source is suspected. With the relatively limited activation of the cardiac sympathetic outflow which accompanied mental challenge and isometric exercise, cardiac adrenaline release remained undetectable. During supine bicycle exercise, which increased cardiac noradrenaline release 10-30 fold, to a mean value of 197 ng/min, cardiac adrenaline release averaged 2.36 ng/min. In two clinical conditions associated with persistently elevated plasma adrenaline concentrations, cardiac failure and adrenaline-secreting phaeochromocytoma, regional release of adrenaline was clearly evident. Thus, in normal man during exercise, and in patients with cardiac failure at rest, adrenaline is released from non-adrenal sources, and probably from sympathetic nerves. Whether neuronal adrenaline release of the degree found would be sufficient to facilitate noradrenaline release, augment sympathetically-mediated cardiovascular responses and contribute to the development of arterial hypertension remains to be tested.

Adrenaline release by the human heart

1. Radiotracer methods were used to measure the rates of regional release of adrenaline and noradrenaline to plasma in humans. 2. No release of adrenaline could be detected from the heart, lungs, liver and kidneys at rest. 3. With the relatively mild activation of the cardiac sympathetic outflow associated with mental challenge and isometric exercise, cardiac adrenaline release remained undetectable. 4. During supine bicycle exercise, which increased cardiac noradrenaline release 10-20 fold, to a mean value of 128 ng/min, cardiac adrenaline release averaged 1.63 ng/min. 5. Whether neuronal adrenaline release of this degree in the heart is sufficient to facilitate noradrenaline release and to augment sympathetically mediated cardiac responses remains to be tested.

Omega-conotoxin inhibits the acute activation of tyrosine hydroxylase and the stimulation of norepinephrine release by potassium depolarization of sympathetic nerve endings

Increased Ca2+ influx serves as a signal that initiates multiple biochemical and physiological events in neurons following depolarization. The most widely studied of these phenomena is the release of neurotransmitters. In sympathetic neurons, depolarization also increases the rate of synthesis of the transmitter norepinephrine (NE), via an activation of the enzyme tyrosine hydroxylase (TH), and this effect also seems to involve Ca2+ entry. We have examined whether the mechanism of Ca2+ entry relevant to TH activation is via voltage-sensitive Ca2+ channels and, if so, whether the type of Ca2+ channel involved is the same as that involved in the stimulation of NE release. We have investigated the isolated rat iris, allowing us to examine transmitter biosynthesis and release in sympathetic nerve terminals in the absence of sympathetic cell bodies and dendrites. Potassium depolarization produced a three- to fivefold increase in TH activity and an approximately 100-fold increase in NE release. Both effects were dependent on Ca2+ being present in the extracellular medium, and both were inhibited by omega-conotoxin (1 microM), which inhibits N-type voltage-sensitive Ca2+ channels. In contrast, the dihydropyridine nimodipine (1-3 microM), which blocks L-type Ca2+ channels, had no effect on either measure. These data support the hypothesis that increases in NE biosynthesis and release in sympathetic nerve terminals during periods of depolarization are both initiated by an influx of Ca2+ through voltage-sensitive Ca2+ channels and that a similar type of Ca2+ channel is involved in both processes.

Comparative effects of adrenaline and noradrenaline on the synthesis and secretion of soluble protein by isolated hepatocytes

1. Adrenaline (A) supplementation of the incubation medium of monolayer cultures of hepatocytes at 0.1, 0.2, 0.3, 1 and 10 ng/ml resulted in consistently enhanced levels of secreted and newly synthesised non-secreted proteins; supplementation with 100 and 1000 ng/ml resulted in lower or unchanged levels. These effects were most consistent 6 hr after a medium change. 2. Noradrenaline supplementation of the medium resulted in increased levels of secreted and non-secreted proteins at low concentrations (less than 2.4 ng/ml) at 6 hr after a medium change, but significantly decreased levels of both populations at high concentrations of noradrenaline (NA) (2.4 less than NA less than 1000 ng/ml), at 6-9 hr after a medium change and sustained to 24 hr, the most significant decrease being at 10 ng/ml. 3. All combinations of concentrations of A + NA resulted in non-dose-dependent decreased levels of both the secreted and non-secreted soluble protein fractions. The most significant decreases occurred at concentrations of (1 + 5) and (10 + 10) ng/ml adrenaline + noradrenaline. 4. Medium supplementation with adrenaline, noradrenaline or a combination of the two had no effect on the uptake of [3H]leucine by the cells. 5. The results are discussed in relation to receptor status on the hepatocyte membranes.

Effects of atrial natriuretic peptide and angiotensin III on the uptake and intracellular distribution of norepinephrine in medulla oblongata of the rat

1. Ten micromoles angiotensin III decreased total 3H-norepinephrine uptake in medulla oblongata of the rat and 100 nM atrial natriuretic peptide increased it. These were the threshold concentrations for the peptides to modify the uptake of the amine. 2. A threshold concentrations (1 nM) of atrial natriuretic peptide reversed the effects produced by 10 microM angiotensin III on total 3H-norepinephrine uptake, but subthreshold angiotensin III concentrations failed to alter the effects produced by 100 nM atrial natriuretic peptide. 3. Angiotensin III, as well as atrial natriuretic peptide, modified only neuronal norepinephrine uptake and did not alter non-neuronal norepinephrine uptake. 4. Angiotensin III and atrial natriuretic peptide did not modify the intracellular distribution of norepinephrine in medulla oblongata.

Effects of angiotensin II and bilateral nephrectomy on norepinephrine catabolism in central nervous system

Effects of angiotensin II (AII) on norepinephrine (NE) catabolism in hypothalamus and medulla oblongata of male rats were studied. 3H-NE uptake, 3H-NE/3H-NE metabolites ratio (NE/MET) and monoamineoxidase (MAO) activity were measured in vitro in both organs. Lack of circulating AII was elicited by means of 48 h bilateral nephrectomy. Pargyline and bilateral nephrectomy increased NE uptake and NE/MET ratio, while in nephrectomized plus pargyline treated groups and additive effect on these results was observed in both organs. All decreased the NE/MET ratio. Pargyline reversed the latter effects of AII. The peptide increased MAO activity in both organs, while bilateral nephrectomy decreased the activity of the enzyme. The results showed that AII modulates NE catabolism by means of MAO activity, eventually at the presynaptic noradrenergic ending sites in the central nervous system.

Modulation of noradrenaline release by activation of presynaptic beta-adrenoceptors in the cardiovascular system

Peripheral sympathetic nerve terminals in many tissues, but not all, are endowed with beta-adrenoceptors. Activation of these result in an enhancement of noradrenaline release evoked by electrical nerve stimulation. These so-called presynaptic beta-adrenoceptors are possibly located on the outer surface of the varicosity of the noradrenergic nerves. A postsynaptic location, however, is also a possibility. The presynaptic beta-adrenoceptors appear to be of the beta 2-adrenoceptor subtype. However, specific classification is lacking. The stereospecificity of the beta-adrenoceptors is controversial. These receptors are not activated by noradrenaline released from sympathetic nerves. Adrenaline derived from the adrenal medulla may be the physiological activator. Either circulating adrenaline or adrenaline taken up by sympathetic nerve terminals and then released as a cotransmitter with noradrenaline activates the presynaptic beta-adrenoceptors. In the latter case, a "positive" feedback" loop may be formed.

Presynaptic receptors in hypertension

The physiological status of presynaptic receptors, regulating sympathetic neurotransmitter release, remains subject to debate. Nevertheless, pharmacological techniques have shown presynaptic alpha-adrenoceptors, mediating a negative feedback inhibition of neuronal noradrenaline (NA) release, and presynaptic beta-adrenoceptors mediating a positive feedback facilitation. Decreased presynaptic alpha- or increased beta-adrenoceptor responsiveness might be expected to result in enhanced per-pulse release of NA and may contribute to hypertension development and maintenance. A potential role in hypertension development, but not its maintenance, has been established for presynaptic beta-adrenoceptors. Attempts to identify altered presynaptic adrenoceptor responsiveness in hypertension have, however, been inconclusive.

Propranolol inhibits nonexocytotic noradrenaline release in myocardial ischemia

Ischemic induces a nonexocytotic noradrenaline release in the heart, which leads to high and potentially harmful interstitial noradrenaline concentrations. The effect of beta-adrenoceptor antagonists on noradrenaline release in ischemia has been investigated in the present study. DL-Propranolol (1-100 mumol/l) concentration-dependently reduced noradrenaline release during 20 min of global and total ischemia in the perfused rat heart. Other beta-adrenoceptor blocking agents such as atenolol, metoprolol, and timolol (10 mumol/l each), however, did not share this effect. Moreover, both stereoisomers of propranolol were equipotent in suppression of ischemia-induced noradrenaline release, indicating a property of propranolol independent from interaction with beta-adrenoceptors. The well known local anesthetic action of propranolol was not likely to cause its inhibitory effect on ischemia-induced noradrenaline release, as lidocaine (10 mumol/l) did not affect noradrenaline overflow in ischemia. The effect of propranolol was further examined in cyanide intoxication, an experimental model of energy depletion. In this experimental setting the release of dihydroxyphenylethyleneglycol--the major neuronal metabolite of noradrenaline--served as indicator of increased axoplasmic noradrenaline levels which are present during nonexocytotic noradrenaline release. In cyanide intoxication DL-propranolol also reduced noradrenaline overflow but did not affect release of dihydroxyphenylethylene glycol. The latter finding suggests an interaction of propranolol with the neuronal membrane transport of noradrenaline. In ischemia and cyanide intoxication, transport of noradrenaline across the plasma membrane is known to be driven by the noradrenaline carrier (uptake1) working in reverse of its normal direction--from inside to outside. Consequently, inhibitors of the noradrenaline carrier like desipramine were shown to suppress nonexocytotic noradrenaline release in ischemia and cyanide intoxication.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of noradrenaline release in slices of rat kidney cortex through alpha 1- and alpha 2-adrenoceptors

Slices of rat kidney cortex were incubated in [3H]noradrenaline, then placed in a flow cell and subjected to electrical field stimulation. At a stimulation frequency of 5 Hz, both the alpha 2-adrenoceptor antagonist idazoxan (0.1 microM) and the alpha 1-adrenoceptor antagonist prazosin (0.1 microM) significantly enhanced the stimulation-induced (S-I) outflow of radioactivity from the slice. However, neither clonidine (0.1 microM) nor methoxamine (10 microM), alpha 2- and alpha 1-agonists respectively, affected the S-I outflow of radioactivity at this stimulation frequency. At a lower stimulation frequency (1 Hz), the S-I outflow of radioactivity was not affected by idazoxan or prazosin, but was inhibited by both clonidine and methoxamine. The effect of clonidine was prevented by idazoxan (0.1 microM), but not by prazosin (0.1 microM). The effect of methoxamine was abolished by prazosin (0.1 microM), but not by idazoxan (0.1 microM). The inhibitory effect of methoxamine was not prevented by the prostaglandin synthesis inhibitor indomethacin (10 microM) or the adenosine receptor antagonist 8-phenyltheophylline (1 microM) and thus was not mediated by either prostaglandins or adenosine. The results suggest that both prejunctional alpha 1- and alpha 2-adrenoceptors are directly involved in modulation of noradrenaline release from the renal sympathetic nerves of the rat.

Electrophysiological characterization of adrenoceptors in the rat dorsal hippocampus. III. Evidence for the physiological role of terminal alpha 2-adrenergic autoreceptors

The present electrophysiological studies were undertaken to assess the role of terminal alpha 2-adrenergic autoreceptors in regulating noradrenergic synaptic transmission in the rat CNS. The effectiveness of the electrical stimulation of the locus coeruleus (LC) in suppressing the firing activity of pyramidal neurons was determined in the dorsal hippocampus. Intravenous clonidine, an alpha 2-adrenergic agonist, decreased the effectiveness of the LC stimulation, without altering the effect of microiontophoretically applied norepinephrine. The subsequent i.v. administration of low doses of idazoxan, an alpha 2-adrenergic antagonist, reversed this effect of clonidine on the LC stimulation. To ascertain that the effect of clonidine administered i.v. was indeed attributable to its action on noradrenergic terminals, it was applied locally by microiontophoresis; it decreased the effectiveness of the LC stimulation. Another paradigm used to assess the function of terminal alpha 2-adrenoceptors was to increase the frequency of the LC stimulation from 1 to 5 Hz. This resulted in a 5-fold decrease of the effectiveness of the stimulation. That this was attributable to an enhanced activation of terminal alpha 2-adrenoceptors was suggested by the reversal of this effect of increasing the frequency of the LC stimulation by intravenous idazoxan. Furthermore, the degree of enhancement of the effectiveness of the LC stimulation by idazoxan was much greater at 5 than at 1 Hz. These results provide novel electrophysiological evidence for the potent regulatory role of terminal alpha 2-adrenoceptors on noradrenergic neurotransmission.

Evidence for tonic activation of prejunctional beta-adrenoceptors in guinea-pig pulmonary arteries by adrenaline derived from the adrenal medulla

1. The effects of (+/-)-carteolol 10(-8) M to 10(-6) M, a non-selective beta-antagonist, applied cumulatively, on stimulation-evoked 3H-release at 1 Hz were studied in pulmonary arteries isolated from guinea-pigs. The guinea-pigs were subjected to either bilateral adrenalectomy, adrenalectomy followed by injections of deoxycorticosterone acetate (DOCA) and hydrocortisone, bilateral adrenodemedullation or a sham operation, and then loaded in vitro with [3H]-noradrenaline. 2. Carteolol inhibited 3H-output in arteries from sham-operated animals in a concentration-dependent manner. This inhibitory effect was not found in pulmonary arteries from animals subjected to adrenalectomy or adrenodemedullation. However, DOCA and hydrocortisone pretreatment, did not prevent the disappearance of the carteolol-induced inhibition of 3H-release. 3. Adrenalectomy and adrenodemedullation depleted or markedly reduced the endogenous contents of adrenaline in pulmonary arteries without altering the levels of dopamine and noradrenaline. 4. It is concluded that adrenaline, mainly derived from the adrenal medulla, acts as an endogenous agonist for tonically functioning prejunctional beta-adrenoceptors in guinea-pig pulmonary arteries, probably by being taken up and co-released with noradrenaline.

Modulation of noradrenaline release by presynaptic alpha-2 and beta adrenoceptors in rat atria. Effect of pretreatment with clenbuterol

The effect of pretreatment with the beta-2-selective adrenoceptor agonist, (+/-)-clenbuterol (0.3 mg/kg, twice daily, 14 days) on prejunctional alpha-2- and beta-adrenoceptors was studied in rat atria. When atria from non-pretreated rats had been preincubated with (3H)-noradrenaline, (-)-isoprenaline (0.02 to 4.0 microM) did not affect tritium overflow evoked by stimulation of the cardioaccelerant nerves, but a higher concentration (40 microM) decreased it. Blockade of prejunctional inhibitory alpha-2-adrenoceptors by yohimbine (0.03, 0.3 and 0.8 microM) enhanced the overflow of tritium. In the presence of yohimbine, isoprenaline (1.2 microM) significantly increased stimulation-induced transmitter overflow, suggesting that in rat atria the facilitatory effect of isoprenaline mediated via prejunctional beta-adrenoceptors, is masked by the dominant influence of inhibitory alpha-2-adrenoceptors. (-)-Propranolol (0.1 microM) prevented the isoprenaline-induced increase in atrial rate and the isoprenaline-induced enhancement of transmitter release in the presence of yohimbine (0.3 microM), but did not modify by itself the stimulation-induced efflux of tritium, suggesting that neuronally released noradrenaline failed to activate facilitatory prejunctional beta-adrenoceptors. When atria from clenbuterol-pretreated rats had been preincubated with 3H-noradrenaline, the facilitatory effect of yohimbine 0.03 and 0.3 microM was markedly enhanced and, in this case, isoprenaline (1.2 and 12.0 microM) failed to cause its facilitatory effect in the presence of the alpha-2-adrenoceptor antagonist. Propranolol did not modify the facilitatory effect of yohimbine. No changes in the isoprenaline-induced increase in atrial rate were observed in clenbuterol-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic beta-adrenoceptors in rat atria: evidence for the presence of stereoselective beta 1-adrenoceptors

1. Presynaptic beta-adrenoceptor activity was studied in rat isolated atria, previously loaded with [3H]-noradrenaline. The stimulation-induced release of 3H transmitter was measured in the presence of cocaine, and adrenaline was used as a facilitatory beta-adrenoceptor agonist. 2. Adrenaline (0.1 and 2 nM) increased, by about 50%, the evoked efflux of tritium. With phenoxybenzamine present, the same activity was shown with 10 nM adrenaline. 3. The beta 2-selective adrenoceptor blocking drugs: IPS 339 and ICI 118 551 caused a concentration-dependent decrease in the activity of adrenaline. Cardioselective beta-blocking drugs: acebutolol, beta-xolol, nebivolol and its isomers (R 67 138 and R 67 145) also reduced dose-dependently the agonistic action of adrenaline. The order of potency for nebivolol and its isomers was R 67 138 greater than nebivolol greater than R 67 145. The activity of pindolol was not concentration-dependent. The inhibitory effect of acebutolol was also observed in the presence of blockade of alpha-adrenoceptors. 4. The postsynaptic beta-adrenoceptor blocking activity of nebivolol and its isomers was studied in pithed rats. They reduced isoprenaline-induced tachycardia without altering hypotensive responses. The order of potency was: R 67 138 greater than nebivolol greater than R 67 145. 5. It is concluded that in rat isolated atria, presynaptic beta 2- and beta 1-adrenoceptors coexist and that facilitatory beta 1-adrenoceptors are stereospecific.

Intracoronary injections of salbutamol demonstrate the presence of functional beta 2-adrenoceptors in the human heart

To demonstrate the presence of functional cardiac beta 2-adrenoceptors in man, we studied the responses to intracoronary injections of salbutamol in three groups of six patients. We injected salbutamol, a selective beta 2-adrenoceptor agonist, into the right coronary artery to avoid peripheral vasodilator action and to stimulate the sinoatrial node directly. Salbutamol injections caused a sinus tachycardia. The same doses of salbutamol injected into the aortic root caused no change in heart rate, ruling out a systemic effect. The mean dose required to cause an increase in heart rate of 30 beats/min (IHR30) was 2.6 micrograms in the first group of six patients. In 12 other patients salbutamol was given after beta-blockade to confirm the beta 2-selectivity of the responses. Doses of practolol (beta 1-selective blockade) and of propranolol (beta 1- and beta 2-blockade) that had equal beta 1-blocking activity were used. In six patients who were given practolol, the mean IHR30 dose was 2.1 micrograms. In six patients who were given propranolol, the mean IHR30 dose was significantly greater at 64 micrograms (p less than 0.001, practolol vs. propranolol). This study demonstrates that direct cardiac beta 2-adrenoceptor stimulation in man has a positive chronotropic effect.

Comparison of the prejunctional beta-adrenoceptor stimulating actions of adrenaline and isoprenaline in the dog mesenteric vein

1. The prejunctional beta-adrenoceptor stimulating actions of adrenaline and isoprenaline were compared by recording junction potentials from smooth muscle cells of the dog mesenteric vein. 2. The potency of adrenaline and isoprenaline on beta-adrenoceptors in the postjunctional membrane was estimated from hyperpolarization of the membrane in smooth muscle cells of the guinea-pig facial vein. In the presence of yohimbine, both agents hyperpolarized the membrane to a similar extent. 3. In the dog mesenteric vein, amplitude of the excitatory junction potential (e.j.p.) and slow depolarization was inhibited by adrenaline and potentiated by isoprenaline; the former but not the latter was accompanied by depolarization of the smooth muscle membrane. 4. In the presence of yohimbine, adrenaline inhibited the e.j.p. without depolarization of the smooth muscle membrane; the action was weaker than in the absence of yohimbine. The isoprenaline-induced potentiation of the e.j.p. was further enhanced by yohimbine. 5. It is concluded that adrenaline and isoprenaline have similar stimulating actions on postjunctional beta-adrenoceptors in the guinea-pig facial vein, but have different actions on prejunctional beta-adrenoceptors in the dog mesenteric vein; isoprenaline but not adrenaline stimulates this beta-adrenoceptor to facilitate the release of transmitter substances from perivascular noradrenergic nerves.

Adrenaline-induced enhancement of the blood pressure response to sympathetic nerve stimulation in adrenal demedullated pithed rats

The influence of the prejunctional beta-adrenoceptor-mediated control mechanism of noradrenaline release on the cardiovascular response to preganglionic nerve stimulation (PNS) and the role of adrenaline as endogenous activator of this facilitatory mechanism were studied in pithed rats. The increases in mean arterial blood pressure and heart rate responses to eight PNS (0.8 Hz, 1 ms, 75 V, for 20 s at 12 min intervals) were measured in control, adrenal demedullated and sham-operated pithed rats, respectively. Repeated stimuli of the sympathetic outflow elicited similar blood pressure responses in control and sham-operated rats, whereas the blood pressure responses were attenuated in adrenal demedullated rats. Administration of the non-selective beta-adrenoceptor antagonist propranolol (5 mg x kg-1, i.v.) decreased the blood pressure response in control, but not in adrenal demedullated rats. The increase in heart rate during PNS was abolished in both groups after administration of propranolol. Infusion of adrenaline (0.06 microgram x kg-1 x min-1) increased the circulation levels of adrenaline from 0.4 nmol x l-1 to 4.8 nmol x l-1, but did not modify the blood pressure response to PNS in control rats. In adrenal demedullated rats, however, the blood pressure response to PNS was enhanced during as well as after the infusion of adrenaline. This facilitatory effect of adrenaline on the blood pressure response in adrenal demedullated rats was blocked by pretreatment with propranolol (5 mg x kg-1, i.v.). Taken together the present findings give further support to the hypothesis that neuronal adrenaline of adrenal medullary origin is the endogenous agonist at prejunctional beta-adrenoceptors mediating a facilitation of neuronal noradrenaline release.

Presence of catecholaminergic axon-terminals containing beta-adrenergic receptor in the periventricular zone of the rat hypothalamus

The present study, using a light microscopic double-immunofluorescence method, has revealed the presence of fibers containing both tyrosine hydroxylase- and beta-adrenergic receptor-like immunoreactivities in the rat hypothalamic periventricular zone. Subsequent immunoelectron microscopic analysis demonstrated that these belong to axon terminals. These findings suggest that presynaptic beta-adrenergic receptor is present in catecholaminergic terminals.

Effects of intravenous cocaine are partially attenuated by haloperidol

We examined the effect of the dopaminergic blocking agent, haloperidol, on the subjective and physiologic response to cocaine in cocaine-using volunteers. Five subjects received cocaine (40 mg) or placebo administered intravenously 20 min following pretreatment with haloperidol (8 mg) or placebo intramuscularly in a randomized double-blind study design. Haloperidol pretreatment attenuated cocaine-induced increases in systolic and diastolic blood pressure but not heart rate. Pretreatment with haloperidol reduced subject ratings of pleasant sensations but had no effect on drug "rush." Haloperidol (8 mg) has a small and limited effect on the subjective response to cocaine when given 20 min before cocaine.

Beta-adrenoceptor-mediated facilitation of [3H]-noradrenaline release from the intramural nerves of bovine mesenteric lymphatic vessels

1. Isolated segments of bovine mesenteric lymphatic vessels were loaded with [3H]-noradrenaline and its efflux in response to field stimulation at 2 Hz (0.3 ms pulses, 1 min train) examined. 2. Isoprenaline (10(-6) M) increased evoked fractional 3H efflux (3H released as a percentage of total tissue 3H at the onset of stimulation) from its control value of 0.67 +/- 0.09 to 1.2 +/- 0.18% (s.e.mean; n = 5). 3. Propranolol (10(-6) M) alone had no effect on evoked fractional 3H efflux but blocked the increase in transmitter overflow induced by isoprenaline (10(-6) M). 4. In the presence of the alpha-adrenoceptor antagonist phentolamine (10(-6) M) the facilitatory effect of isoprenaline (10(-6) M) on transmitter efflux was enhanced. In 5 experiments isoprenaline increased evoked fractional 3H efflux from its control value of 1.07 +/- 0.17 to 2.5 +/- 0.37% when phentolamine was present. 5. Adrenaline (10(-8) M) increased evoked fractional 3H efflux from its control value of 0.76 +/- 0.09 to 1.13 +/- 0.17% (n = 7). 6. These findings are consistent with the presence of presynaptic beta-adrenoceptors in this preparation. There is no evidence that these receptors play any role in feedback regulation of transmitter release.

Support for adrenaline-hypertension hypothesis: 18 hour pressor effect after 6 hours adrenaline infusion

In a double blind, crossover study 6 h infusions of adrenaline (15 ng/kg/min; 1 ng = 5.458 pmol), noradrenaline (30 ng/kg/min; 1 ng = 5.911 pmol), and a 5% dextrose solution (5.4 ml/h), were given to ten healthy volunteers in random order 2 weeks apart. By means of intra-arterial ambulatory monitoring the haemodynamic effects were followed for 18 h after the infusions were stopped. Adrenaline, but not noradrenaline, caused a delayed and protracted pressor effect. Over the total postinfusion period systolic and diastolic arterial pressure were 6 (SEM 2)% and 7 (2)%, respectively, higher than after dextrose infusion (ANOVA, p less than 0.001). Thus, "stress" levels of adrenaline (230 pg/ml) for 6 h cause a delayed and protracted pressor effect. These findings are strong support for the adrenaline-hypertension hypothesis in man.

Characterization of presynaptic beta-adrenoceptors facilitating endogenous noradrenaline release in the portal vein of permanently cannulated, freely moving rats

We investigated the nature of presynaptic beta-adrenoceptors and the possible heterogeneity of these receptors in the portal vein nervous plexus of freely moving unanesthetized rats using the differential blockade technique with CGP 20712A as a highly beta 1-selective antagonist, ICI 118,551 as a very beta 2-selective antagonist and fenoterol and endogenous NA as beta 2- and beta 1-selective agonists respectively. The fenoterol (0.25 mg/kg)-induced increase of the basal NA level (290%) was dose dependently decreased by 0.1, 0.3 and 1.0 mg/kg ICI 118,551, but was not affected by a high dose (3.0 mg/kg) of CGP 20712A. During electrical stimulation (2 Hz, 3 ms, 5 mA) of the portal vein nervous plexus, 0.25 mg/kg fenoterol induced a 2.1-fold increase in NA overflow compared to the control stimulation value. ICI 118,551 was also able to decrease the fenoterol-induced enhancement during stimulation. During stimulation in the presence of CGP 20712A and fenoterol, the control stimulation value was not significantly decreased. Pretreatment with yohimbine (0.5 mg/kg) was used to create a strong beta 1-stimulus by raising the intra-synaptic NA level through blockade of the inhibitory alpha 2-adrenoceptors. Under these conditions, CGP 20712A did not deminish the yohimbine-induced enhancement of the stimulus evoked NA overflow, clearly indicating the absence of the beta 1-adrenoceptor subtype. ICI 118,551 (0.1 mg/kg) was also unable to influence the evoked NA overflow under these conditions, implying that, even at high concentrations, NA is not able to facilitate its own release by stimulation of the presynaptic beta 2-adrenoceptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Frequency-dependence of 3H-noradrenaline release from rabbit pulmonary artery: effect of alpha-adrenoceptor antagonists and inhibitors of transmitter inactivation

The aim of this study was to examine the modulating role of presynaptic alpha 2-adrenoceptors on transmitter release from vascular sympathetic neurones. This was done by examining the influence of removal of inactivation pathways on the effect of alpha-adrenoceptor antagonists on the release of transmitter from noradrenergic neurones. The rabbit main pulmonary artery preloaded with 3H-noradrenaline (3H-NA) was used. The artery was stimulated with 300 pulses at various frequencies (1, 3, 10 and 30 Hz). Pargyline (3 x 10(-4) M) increased the stimulation-evoked 3H-overflow at 1 and 3 Hz and decreased it at 30 Hz. U-0521 (3',4'-dihydroxy-2-methylpropiophenone; 3 x 10(-6) M) enhanced the overflow at 1 Hz and had no effect at 3-30 Hz. Corticosterone (4 x 10(-5) M) did not alter the stimulation-evoked 3H-overflow at 1-30 Hz. Cocaine (3 x 10(-6) M) enhanced the 3H-overflow slightly at 1-30 Hz. At 3 x 10(-5) M, cocaine enhanced 3H-overflow at 1 Hz and reduced it at 30 Hz. Neither corticosterone (4 x 10(-5) M) nor propranolol (10(-7) M) modified this effect of cocaine. Propranolol (10(-7) M) alone decreased the 3H-overflow at 30 Hz and had no effect at 1-10 Hz. Phenoxybenzamine (10(-6) M) and chlorpromazine (3 x 10(-6) M) potentiated the stimulation-evoked 3H-overflow at 1-30 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic and hormonal adaptations to beta-adrenoceptor blockade. A 24-hour study of acebutolol, atenolol, pindolol, and propranolol in hypertensive patients

Comparison of the hemodynamic and hormonal effects of beta-adrenoceptor antagonists with different ancillary properties may help to clarify the antihypertensive mechanism of these drugs. Under strict basal conditions, the effects of acebutolol (400 mg b.i.d.), atenolol (100 mg b.i.d.), pindolol (10 mg b.i.d.), and propranolol (80 mg t.i.d.), were studied for the first 24 hours in 40 hypertensive patients. With pindolol, mean arterial pressure was reduced (p less than 0.05) 1 hour after administration, whereas the cardiac index and the systemic vascular resistance index did not change. With the other three drugs, the fall in mean arterial pressure was delayed 2-3 hours. With these drugs, the fall in mean arterial pressure was preceded by a rise in the resistance index, which compensated for the initial fall in cardiac index. With each drug, the decrements in mean arterial pressure were associated with parallel decrements in the resistance index, and percent changes in mean arterial pressure and the resistance index were always significantly (p less than 0.001) correlated. At the end of the 24-hour period, the four drugs shared an equal antihypertensive effect, which varied 14-17%. This was associated with a return of the cardiac index toward control values by acebutolol, atenolol, and propranolol treatment and a moderately increased cardiac index above pretreatment values (13%, p less than 0.01) with pindolol. The secondary rise in the cardiac index was inversely correlated (p less than 0.001) with the fall in mean arterial pressure with all four drugs. Plasma renin was maximally suppressed 2 hours after treatment, thus before any change in mean arterial pressure had occurred with acebutolol, atenolol, and propranolol. Pretreatment values of active renin and the reduction of mean arterial pressure 24 hours after administration were not correlated in any of the four groups. Despite the "vasodilator" action of the four drugs, plasma norepinephrine did not rise. Our data show that the main hemodynamic change that occurs at the time blood pressure falls after beta-adrenoceptor antagonism is vasodilation. Neither autoregulation of blood flow nor renin suppression can explain this vasodilator action. The absence of an increase in norepinephrine, despite vasodilation, suggests that beta-adrenoceptor antagonism interferes with sympathetic vasoconstrictor nerve activity. This effect may explain the vasodilator and antihypertensive potential of beta-adrenoceptor antagonists.

Pronounced facilitation of endogenous noradrenaline release by presynaptic beta 2-adrenoceptors in the vasculature of freely moving rats

The facilitation of the noradrenaline (NA) overflow by stimulation of the presynaptic beta-adrenoceptor of the rat portal vein was investigated, using the freely moving unanesthetized permanently cannulated rat as a model. The beta 2-selective agonist fenoterol caused a maximal enhancement of about 300% of the basal NA level at a dose of 0.5 mg/kg. Following administration of cocaine (2.5 mg/kg plus 0.05 mg/kg/min) basal NA levels increased to 150% whereas combination of cocaine and fenoterol results in a dose dependant rise up to over 560% of the basal level (at a fenoterol dosage of 0.5 mg/kg). Blockade of the alpha 2-adrenoceptors with yohimbine (0.5 mg/kg) which enhances the NA level to 486%, followed by 0.125 mg/kg fenoterol results in a further 2.53-fold rise to more than 1,200% of the basal level, indicating the pronounced counterregulatory role of the presynaptic alpha 2-adrenoceptor. After ganglionic blockade with hexamethonium (3 mg/kg plus 6 mg/kg/h) the effect of yohimbine (0.5 mg/kg) alone was diminished to 162%, but the additional facilitatory effect of 0.125 mg/kg fenoterol still was 1.82-fold, to 294% of the basal level. Combination of cocaine (2.5 mg/kg plus 0.05 mg/kg/min), yohimbine (0.5 mg/kg) and fenoterol (0.125 mg/kg) induced a rise to over 9,000 pg/ml NA (about 40-fold of the basal NA level). During electrical stimulation (2 Hz, 3 ms, 5 mA) of the local portal vein nervous plexus, the role of the inhibitory alpha 2-adrenoceptor becomes even more pronounced.(ABSTRACT TRUNCATED AT 250 WORDS)

Epinephrine facilitates neurogenic vasoconstriction in humans

Numerous studies have suggested that epinephrine may facilitate neural release of NE. There have been no studies in humans that demonstrate the functional significance of this action. To determine whether epinephrine facilitates neurogenic vasoconstriction in humans, we contrasted forearm vasoconstrictor responses to a reflex stimulus (lower body negative pressure [LBNP]) and to intraarterial NE before, during, and 30 min after infusion of epinephrine (50 ng/min) or isoproterenol (10 or 25 ng/min) into a brachial artery. These doses had no systemic effects. We reasoned that if prejunctional stimulation of beta receptors by epinephrine and isoproterenol had functional significance, the vasoconstrictor response to LBNP would be potentiated in comparison to the response to NE (postjunctional mechanism). Studies were done on 23 normal male volunteers. Forearm blood flow was measured with a strain gauge plethysmograph and intraarterial pressure was recorded. The ratio of vasoconstrictor responses to LBNP/NE was used as an index of neural release of the neurotransmitter NE. This ratio increased during infusions of both epinephrine and isoproterenol. 30 min after epinephrine the vasoconstrictor response to LBNP (n = 15) was augmented from +9.9 +/- 2.2 (SE) resistance units (RU) before epinephrine to +16.4 +/- 3.2 RU (P less than 0.05); whereas the response to NE (n = 8) tended to decrease from +8.8 +/- 3.1 RU before to +4.2 +/- 1.2 RU after epinephrine (P greater than 0.05). In contrast, 30 min after isoproterenol the vasoconstrictor responses to LBNP and NE were the same as before isoproterenol. The augmented ratio of responses to LBNP/NE after epinephrine and not after isoproterenol supports the concept that epinephrine, but not isoproterenol, is taken up by the adrenergic terminal, is released subsequently during reflex stimulation, and augments the release of the neurotransmitter NE. These experiments provide the first hemodynamic evidence in humans that epinephrine and isoproterenol facilitate neurogenic vasoconstriction. The sustained effect of epinephrine in contrast to isoproterenol suggests that the late facilitation by epinephrine is related to its neural uptake and subsequent release.

Effect of isoprenaline on noradrenaline release from sympathetic neurones in rabbit isolated pulmonary artery

The aim of the present work was to demonstrate the presence of presynaptic beta-adrenoceptors in the rabbit isolated pulmonary artery by investigating the effect of isoprenaline on 3H-noradrenaline (3H-NA) release evoked by electrical field stimulation. (-)-Isoprenaline (10(-7)-10(-6) mol/l) had no effect on the 3H-overflow evoked by stimulation (3 Hz) of the pulmonary artery preloaded with 3H-NA. At 10(-5) mol/l, (-)-isoprenaline reduced the 3H-overflow by maximally 39%. (-)-Isoprenaline (10(-5) mol/l) caused an inhibition that remained almost constant with time. The same results were obtained with (-)-isoprenaline (10(-7)-3 X 10(-5) mol/l) in the presence of cocaine (3 X 10(-5) mol/l), corticosterone (4 X 10(-5) mol/l), and the catechol-O-methyltransferase inhibitor U-0521 (3',4'-dihydroxy-2-methylpropiophenone) (10(-4) mol/l). In the presence of cocaine plus corticosterone, (-)-isoprenaline (3 X 10(-10)-10(-7) mol/l) had no effect on the 3H-overflow evoked by stimulation at 1 Hz. At 10(-6) mol/l, (-)-isoprenaline slightly reduced the 3H-overflow. At 10 Hz, (-)-isoprenaline (10(-6)-3 X 10(-5) mol/l) decreased the 3H-overflow and had no effect at 10(-7) mol/l. In the presence of either rauwolscine (10(-6) mol/l), phentolamine (10(-6) mol/l) or the phosphodiesterase inhibitor ICI 63,197 (3 X 10(-5) mol/l), (-)-isoprenaline (10(-7)-10(-6) mol/l) did not enhance the stimulation-evoked 3H-overflow. (+/-)-Propranolol (10(-7)-10(-5) mol/l) did not alter the stimulation-evoked 3H-overflow.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of cyclic nucleotides in prejunctional modulation of noradrenaline release in mouse atria

1 In mouse isolated atria previously incubated with [3H]-noradrenaline, 8-bromo-cyclic AMP (3-270 microM) produced a concentration-dependent increase in the fractional stimulation-induced outflow of radioactivity. 8-Bromo-cyclic GMP induced a lesser increase in the stimulation-induced outflow. 2 The phosphodiesterase inhibitors: M&B 22948 (90 microM); ICI 63197 (30 and 90 microM) and 3-isobutyl-1-methylxanthine (90 microM) increased the fractional stimulation-induced outflow. Together these results indicate that cyclic AMP may have a modulatory effect on noradrenaline release. 3 The inhibition of the stimulation-induced outflow produced by clonidine (0.03 microM) and its facilitation produced by phentolamine (1 microM) were unaltered in the presence of 8-bromo-cyclic AMP (90 microM). However, in the presence of 8-bromo-cyclic AMP (270 microM), the facilitatory effect of phentolamine was enhanced, but the inhibitory effect of clonidine (0.03 microM) was unaltered. In the presence of ICI 63197 (30 microM) the inhibitory effect of clonidine (0.03 microM) was unaltered, but the facilitatory effect of phentolamine (1 microM) was slightly enhanced. 4 Isoprenaline (0.003-0.1 microM) enhanced the fractional stimulation-induced outflow, an effect blocked by propranolol (0.1 microM). In the presence of 8-bromo-cyclic AMP (90 microM), the facilitatory effect of isoprenaline (0.01 microM) was blocked. In the presence of ICI 63197 (30 microM) the facilitatory effect of isoprenaline (0.003 microM) was potentiated. 5 These results suggest that whereas beta-adrenoceptor-mediated enhancement of noradrenaline release is linked to the stimulation of adenylate cyclase and enhanced formation of cyclic AMP, alpha-adrenoceptor-mediated inhibition of noradrenaline release is not linked to inhibition of adenylate cyclase activity.

Method for stimulating the adrenergic system of an isolated perfused rat heart

A method is presented whereby isolated perfused rat hearts can be rapidly prepared for the stimulation of chronotropic activity by electrical pulses or exogenous noradrenaline. The mediation of the response by sympathetic nerves is demonstrated through modulation of the response by compounds with established pharmacological actions. Propranolol inhibits the increase in heart rate to both electrical stimulation and exogenous noradrenaline, whereas bretylium inhibits only electrically induced increases. Chronic pretreatment with 6-hydroxydopamine decreases the response to electrical stimulation but increases that to exogenous noradrenaline.

Facilitation of nerve stimulation evoked noradrenaline overflow by isoprenaline but not by circulating adrenaline in the dog in vivo

The influence of isoprenaline and adrenaline on the overflow of endogenous noradrenaline evoked by sympathetic nerve stimulation was studied in canine blood perfused gracilis muscle in situ. Neuronal uptake was inhibited by desipramine. Local i.a. infusions of isoprenaline enhanced stimulation evoked noradrenaline overflow by 32 +/- 10% (P less than 0.05), indicating the existence of prejunctional facilitatory beta-adrenoceptors. This effect of isoprenaline was not antagonized by beta 1-adrenoceptor blockade and does not seem to be related to the vasodilatation caused by isoprenaline. In a second series of experiments circulating adrenaline levels were raised by i.v. infusions from basal levels of 0.4 +/- 0.2 nM to 1.7 +/- 0.2 and 6.3 +/- 0.6 nM, respectively, in arterial plasma. Adrenaline elicited vasodilatation in the gracilis muscle (19 +/- 3 and 28 +/- 5% increases in vascular conductance, respectively), indicating activation of postjunctional beta 2-adrenoceptors, without influencing nerve stimulation evoked noradrenaline overflow. Thus, our results support the existence of a prejunctional beta 2-adrenoceptor mediated mechanism facilitating noradrenaline release in vivo, but provide no evidence to support the idea that physiologically relevant increases in circulating adrenaline levels enhance noradrenergic neurotransmission in skeletal muscle.

A functional response to D1 dopamine receptor stimulation in the central nervous system: inhibition of the release of [3H]-serotonin from the rat substantia nigra

The influence of dopamine receptor agonists on the calcium dependent electrically-evoked release of [3H]-serotonin was studied in the superfused rat substantia nigra. The electrically-evoked overflow of [3H]-serotonin was significantly inhibited by micromolar concentrations of exogenous dopamine and of the D1-selective dopamine receptor agonists, fenoldopam and SKF 38393. The inhibitory effects of dopamine and fenoldopam on [3H]-serotonin release were antagonized by the D1-selective dopamine receptor antagonist SCH 23390. S-sulpiride, an antagonist of D2 receptors failed to antagonize the inhibitory effects of dopamine on [3H]-serotonin release. Moreover, quinperol, the selective D2 receptor agonist did not modify the electrically evoked release of [3H]-serotonin. In the presence of nomifensine, SCH 23390 but not S-sulpiride enhanced in a concentration dependent manner the electrically-evoked release of [3H]-serotonin. Under similar experimental conditions, exposure to fenoldopam did not inhibit the electrically-evoked release of [3H]-serotonin from rat hippocampal or hypothalamic slices. These results indicate that, in rat substantia nigra, exogenous as well as endogenous dopamine causes inhibition of serotonin release by activation of dopamine receptors of the D1-subtype.

DL-propranolol inhibits the vascular changes in the rat carotid body induced by long-term hypoxia

Two groups of rats were exposed to hypoxia (10% O2) for 1 and 3 weeks with or without daily injections of DL-propranolol (0.66 mg.kg-1 ip). The structure of the carotid body was analyzed by light microscopical morphometry and the catecholamine content was assayed by high performance liquid chromatography. Exposure to hypoxia induced enlargement of the carotid body due to enhanced vascularity and hypertrophy of glomic and interstitial tissues. The dopamine and norepinephrine content were increased at both 1 and 3 weeks of hypoxia and reached levels 40-50 times those of the controls. The DL-propranolol treatment abolished the vasodilatory effect of hypoxia within the first week but did not prevent the other structural changes or the rise in catecholamine content. The data suggest that 1. the vasodilation elicited by long-term hypoxia may be controlled by beta-adrenoceptors and 2. the structural and biochemical events occurring in rat carotid body during long-term hypoxia do not influence each other and thus seem to be controlled by different mechanisms.

3H-adrenaline release from rabbit isolated aorta by electrical-field stimulation

The aim of the present work was to study the release of 3H-adrenaline (3H-A) from rabbit isolated aorta. This release was compared with that of 3H-noradrenaline (3H-NA). The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation (S1-S3). In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses (30-1000) at 3 Hz. The time course of 3H-overflows with either 3H-A or 3H-NA was compared. In the case of 3H-A, the 3H-overflow increased with time, while for 3H-NA the 3H-overflow remained unchanged both at 3 and 10 Hz. The stimulation-evoked 3H-overflow from rings preloaded with either 3H-amine was the same at frequencies from 1 to 10 Hz, whereas at 30 Hz the 3H-overflow derived from 3H-A was higher than that using 3H-NA. An analysis of the quantitative aspects of individual stimulation-evoked 3H-overflows showed that the fractional profile did not differ using either amine. In both cases, at least 96% of the 3H-overflow evoked by stimulation at 3 or 10 Hz was contained in the initial four 2-min. fractions. The ratio between stimulation-evoked 3H-overflow and spontaneous 3H-outflow was the same for both amines at 3 Hz, while the ratio was higher for 3H-NA at 10 Hz. Rauwolscine (10(-6) M) enhanced the 3H-overflow 6-fold, while (-)-propranolol (10(-7) M) had no effect at 3 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac ventricular beta 2-adrenoceptors in guinea-pigs and rats are localized on the coronary endothelium

In mammalian heart tissue beta 2-adrenoceptors are known to coexist with beta 1-adrenoceptors. In the present study, evidence that beta 2-adrenoceptors in guinea-pig and rat ventricles are primarily localized on the coronary endothelium is provided by competition binding studies with the subtype-selective beta-adrenoceptor antagonists ICI 89.406 (beta 1-selective) and ICI 118.551 (beta 2-selective) on four different plasma membrane preparations. (1) Following density gradient centrifugation of cardiac ventricular microsomes from rats or guinea-pigs, endothelial plasma membranes migrated at slightly higher density than the sarcolemmal membranes, as verified by endothelial (angiotensin converting enzyme) and sarcolemmal markers (adenylate cyclase, [3H]ouabain binding). At the activity peak of angiotensin converting enzyme, the relative amount of beta 2-adrenoceptors in guinea-pigs and rats was 25% and 65%, respectively. (2) On sarcolemmal membranes corresponding to the activity peak of adenylate cyclase, beta-adrenoceptors consisted of the beta 1-type exclusively (guinea-pig), or to at least 90% (rat). (3) Cultures of coronary endothelial cells derived from guinea-pigs revealed only beta 2-adrenoceptors. (4) Isolated guinea-pig cardiomyocytes contained only beta 1-adrenoceptors, a finding recently established in rat myocytes as well.

Effect of alpha-adrenoceptor agonists and antagonists on cholinergic transmission in guinea-pig isolated atria

Evidence was sought for the existence on cholinergic nerve terminals in guinea-pig atria of alpha-adrenoceptors subserving inhibition of acetylcholine release. The experiments were performed with atria which had been incubated with 3H-choline and transmitter release was deduced from the efflux of radioactivity elicited by field stimulation. In preparations which had been incubated with 3H-choline, field stimulation (60 pulses, 2 Hz) evoked release and radioactivity which was inhibited by 1.0 mumol/l noradrenaline, in the presence of propranolol (1.0 mumol/l), but was unaltered by clonidine (1.0 and 10.0 mumol/l). The inhibitory effect noradrenaline on the stimulation-induced efflux of radioactivity was blocked by idazoxan (0.3 mumol/l), and phentolamine (1.0 mumol/l) but not by prazosin (0.3 mumol/l). In the presence of propranolol (1.0 mumol/l), neither phentolamine (1.0 mumol/l), idazoxan (0.3 mumol/l) nor prazosin (0.3 mumol/l) had any effect on stimulation-induced efflux of radioactivity. Stimulation of the extrinsic vagus nerve of atrial preparations with trains of pulses at frequencies of 2, 4, 8, and 16 Hz produced graded decreases in the rate of atrial beating. The negative chronotropic responses to vagus stimulation were unaffected by noradrenaline (1.0 mumol/l) in the presence of propranolol (1.0 mumol/l). These findings indicate that the release of acetylcholine from the cholinergic terminals in guinea-pig atria can be inhibited by a mechanism apparently involving prejunctional alpha 2-adrenoceptors. However, under the experimental conditions used here the chronotropic responses of atria to stimulation of the extrinsic vagus nerve was not affected by activation of the prejunctional alpha 2-adrenoceptors associated with the cholinergic terminals.

The effect of adrenal demedullation on cardiovascular responses to environmental stimulation in conscious rats

Circulating plasma adrenaline has been implicated in the facilitation of neurogenic pressor responses and development of hypertension. Bilateral adrenal demedullation in rats did not affect body weight, urine output, urinary electrolyte (Na+, K+ and Cl-) excretion, nor plasma corticosterone concentration, indicating the selective nature of the demedullation procedure. Adrenal demedullation did induce significant reductions in adrenal catecholamine content, plasma adrenaline levels, resting blood pressure and heart rate in conscious rats, but did not affect alerting-induced increases in blood pressure. The adrenal medulla and circulating plasma adrenaline appear to contribute to the maintenance of resting cardiovascular parameters, but would not appear to be involved in nor facilitate the cardiovascular responses to environmental stimulation.