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

Adrenergic receptors in the nucleus tractus solitarii of the rat

The nucleus tractus solitarii (NTS) is an area of the medulla in which baroreceptor afferent fibers mediating an inhibition of vasoconstriction terminate. The NTS also appears to be a site at which alpha-adrenergic receptors may be involved in blood pressure control. alpha- and beta-adrenergic agonists and antagonists were injected into the NTS via stainless steel cannulae and the effects on blood pressure determined. Norepinephrine (NE) and isoproterenol (ISO) caused dose-related decreases in mean arterial blood pressure (MABP). Phentolamine (PH) and propranolol (PR) caused dose-related increases in MABP. PH pretreatment in the NTS blocked the effects of ISO, but not NE. The effect of increased baroreceptor impulses to the NTS, produced by altering perfusion of the carotid sinus was blocked by propranolol. These results indicate the presence of beta- as well as alpha-receptors in the NTS, and also suggest the existence of presynaptic beta-receptors which facilitate neurotransmission.

The influence of drugs on the overflow of noradrenaline and the identification of receptors in guinea-pig atria

1 Salbutamol (1.0 microM) and isoprenaline (1.2 nM) significantly increased the fractional release of tritiated noradrenaline from driven left atria but phentolamine (10 microM) failed to do so. Butoxamine (4.0 microM) blocked the increase in overflow produced by isoprenaline. Isoprenaline (1.2 nM), phentolamine (10.0 microM) and salbutamol (1.0 microM) failed to increase the overflow of tritiated noradrenaline from spontaneously beating atria. 2 Spontaneously beating atria were therefore used to identify the receptors mediating chronotropism and inotropism. 3 There was no clear relationship between inotropism and chronotropism. 4 The inotropic effects of both dobutamine (0.04-4.0 microM) and isoprenaline (0.11-9.0 nM) were inhibited by practolol (4.0 microM) and by butoxamine (4.0 microM). The chronotropic effects were inhibited only by practolol (4.0 microM). 5 Both inotropic and chronotropic effects of noradrenaline (3.0-200 nM) were antagonized by practolol (4.0 microM), but not by butoxamine (4.0 microM). Thus both functions appeared to be mediated by beta 1-adrenoceptors when noradrenaline was the agonist. 6 Inotropic responses to salbutamol (0.45-7.5 microM) were inhibited by both practolol (4.0 microM) and by butoxamine (4.0 microM), but chronotropic responses were antagonized only by butoxamine (4.0 microM), Thus salbutamol acts on both beta 1-and beta 2-adrenoceptors to produce an inotropic response but only on beta 2-adrenoceptors to produce its chronotropic response. 7 It is concluded that both beta 1- and beta 2-adrenoceptors can mediate chronotropism and inotropism in guinea-pig isolated atria. Determination of the postsynaptic effects of drugs should be carried out on spontaneously beating rather than driven atria to obviate modification of the responses by noradrenaline release from sympathetic neurons.

Norepinephrine release in arteries of spontaneously hypertensive rats

The role of the sympathetic nervous system in arterial hypertension cannot be properly evaluated until we know about its activity in the vessels themselves. In this study we investigated the effect of transmural stimulation on the tail artery - labelled in vitro with 3H-norepinephrine - of 7-9 week old spontaneously hypertensive rats (SHR) and Wistar Kyoto controls (WKR). Electrical stimulation using two frequencies (2 and 10 Hz) resulted in significantly more 3H overflow in vessels from SHR than from WKR. With 10 Hz stimulation the fractional release was also greater. Column chromatographic analysis of 3H overflow revealed that transmural stimulation in arteries of SHR enhanced mainly the release of norepinephrine and not of its metabolites. Significantly, an increased release of 3H-norepinephrine on stimulation was observed in SHR before the full development of hypertension suggesting that it might be a cause rather than a consequence of high blood pressure.

Effect of propranolol on myocardial cell necroses and blood levels of catecholamines in pigs subjected to stress

The degree of myocardial cell necroses in pigs exposed to stress induced by a myorelaxant (succinylcholine) for about 12 min. was investigated. The Beta-adrenoceptor block propranolol was given in varying doses prior to the induced stress. The degree of necroses in these pigs was evaluated according to a point scale, ranging from 0 to 6 points, and compared with the changes found in untreated controls following stress. The blood levels of noradrenaline and adrenaline were assayed before, during an immediately after the period of stress. No protective effect of propranolol on the cardiac lesions was found when the drug was administered intravenously in a single dose of either 1 mg or 3 mg per kg. During the period of stress the blood levels of noradrenaline were high, indicating a high degree of activity in the sympatho-adrenal system. Propranolol in a single dose of 10 mg per kg intravenously reduced the number and size of heart lesions, while the blood levels of the catecholamines were lower under stress than after administration of 1 and 3 mg propranolol. After oral treatment with propranolol, 120 mg three times a day for 6 or 7 days, an almost complete absence of heart cell necrosis after the stress was obtained. The activity of the sympatho-adrenal system was elevated, as reflected by the high blood levels of catecholamines. The results indicate that stress-induced heart cell necrosis is mediated via the activity of the sympatho-adrenal system. The difference between a single and repeated doses of propranolol in protecting the heart suggests a complex of the adrenergic Beta-receptor blocker.

Response of blood pressure and plasma norepinephrine to propranolol, metoprolol and clonidine during isometric and dynamic exercise in hypertensive patients

The effects of metropolol (beta 1-selective), propranolol (nonselective) and clonidine (central alpha-stimulant) on plasma norepinephrine, blood pressure and heart rate were assessed at rest, during isometric work and dynamic exercise in 15 patients with moderate hypertension. Metroprolol resulted in a lower diastolic blood pressure during isometric and dynamic exercise that propranolol, which was paralleled by a lower plasma norepinephrine level during dynamic work; both beta-adrenergic blocking compounds resulted in a lower heart rate in all test situations than that obtained with clonidine; clonidine produced similar control of diastolic blood pressure to that obtained with the beta-adrenergic blocking agents, but did not clearly attenuate the systolic blood pressure response to dynamic exercise. Plasma norepinephrine concentrations tended to be lowest following clonidine, especially during dynamic work. The findings support the hypothesis that the central action of clonidine inhibits peripheral release of norepinephrine, but is insufficient to attenuate cardiac stimulation by physical exercise. The fact that propranolol caused higher plasma norepinephrine concentrations than metoprolol during exercise may explain the difference in the blood pressure responses during exercise.

Review of therapeutic modalities acting directly via central pathways

The brain influences arterial pressure through central mediation of a variety of neurotransmitters, including norepinephrine, and translates this action into changes in peripheral autonomic tone. Two opposed adrenergic systems have been described in brain: a hypothalamic pathway in which adrenergic receptor stimulation raises arterial pressure and a brainstem pathway related to the baroreflex arc in which adrenergic receptor stimulation lowers arterial pressure. Antihypertensive drugs with primarily central effects, including clonidine and alpha methyldopa, act as alpha 2 adrenoceptor agonists. The central receptor involved in their antihypertensive action is of the alpha 2 type but is located postsynaptically. Activation of this receptor by either clonidine or alpha methylnorepinephrine, a metabolite of alpha methyldopa, engages the depressor pathway in the brainstem and leads to a decrease in norepinephrine release and a reduction in peripheral sympathetic tone. Clonidine and alpha methyldopa share a similar pattern of peripheral effects, including reductions in preganglionic sympathetic nerve traffic, bradycardia, decreases in plasma renin activity, reductions in blood pressure in the supine position and adverse effects such as depression, sedation and bad dreams. Because of the frequency and severity of these side effects, there is an ongoing search for new centrally acting antihypertensive agents which might be better tolerated.

The mechanisms of antihypertensive action of beta-adrenergic receptor blocking drugs

Beta-adrenergic receptor blockers lower blood pressure in hypertensive patients and spontaneously hypertensive rats (SHR). The fall in blood pressure is associated with a complex chain of hemodynamic events. The transient effects differ from long-term changes. After reviewing the dynamics of beta-blocker induced hemodynamic changes, we propose a scheme to explain both short-term and long-term effects. In this scheme it is suggested that the primary effects are (1) a fall in cardiac output through blockade of cardiac beta-adrenergic receptors and (2) a promotion of renal tubular sodium and water loss by blockade of renal tubular beta-adrenergic receptors. The initial reactions of the cardiovascular control system to these effects as well as the final adaptations are discussed. It is suggested that for future developments in the field of beta-adrenergic control of blood pressure further analysis of the renal tubular sympathetic control of sodium and water excretion is necessary.

Failure of exogenous serotonin to inhibit the release of 3H-serotonin induced by electrical field stimulation in superfused goldfish brain slices

1. Goldfish brain slices preloaded with 3H-serotonin were superfused with a solution of Krebs-Ringer phosphate at 30 degrees C in the presence of 1 microM fluoxetine, a serotonin uptake blocker. 2. The effects of exogenous serotonin, Ca2+ absence and Ca2+ chelation on basal and electrically-evoked tritium overflow were investigated. 3. Exogenous 1 or 10 microM serotonin did not alter the stimulated tritium overflow, but this was reduced to 44% and to 31% of the control value when the superfusion was performed in Ca2+-free and in Ca2+-free-EGTA conditions. 4. Neither Ca2+ absence, Ca2+ chelation or 1 microM exogenous serotonin altered basal tritium efflux; however, the latter was significantly increased when the superfusion was performed in the presence of 10 microM serotonin.

Effects of beta-adrenoceptor antagonists on central haemodynamics in essential hypertension

1 Eleven β-adrenoceptor blocking agents (propranolol, oxprenolol, penbutolol, carteolol, bupranolol, bufetolol, pindolol, atenolol, metoprolol, acebutolol and labetalol) were orally administered to 144 patients with essential hypertension for 5 weeks to assess their antihypertensive effects. In addition haemodynamic responses were measured non-invasively using radioactive iodinated human serum albumin.

2 In a second study central haemodynamic changes in response to intravenous injection of propranolol (0.4 mg/kg) were assessed in 21 untreated patients with essential hypertension and compared with those induced by short-time therapy with propranolol, acebutolol or pindolol.

3 During short term (5 weeks) oral therapy all the antagonists with the exception of pindolol and carteolol, showed a tendency to reduce heart rate. The effects on blood pressure were not uniform and for each drug a subgroup of responders (defined as those showing 10 mmHg or more reduction of mean arterial blood pressure) was separately analysed.

4 The responders to antihypertensive therapy showed a wide spectrum of haemodynamic responses. The results would be compatible with the hypothesis that in addition to the reduction in cardiac output, the β-adrenoceptor blockers lower blood pressure by at least one, as yet unknown, mechanism leading to a reduction in total peripheral resistance.

5 The antagonists tested demonstrate a variety of ancillary properties and some differences were apparent between the haemodynamic responses to the different drugs. Cardioselective β-adrenoceptor blockers caused a significant decrease in cardiac index; however no significant change in total peripheral resistance was observed. These effects may be attributable to their weak vascular β-adrenoceptor blockade.

6 In the case of labetalol the additional α-adrenoceptor blocking activity of the drug was reflected by a fall in total peripheral resistance resulting from blockade of the vasoconstrictor effects of neuronal noradrenaline.

7 Pindolol, because of its pronounced ISA, showed qualitative differences from the other antagonists, its antihypertensive effects being accompanied by significant falls of total peripheral resistance and increases in cardiac output.

8 During short term (5 weeks) oral therapy with propranolol, acebutolol or pindolol no marked central haemodynamic changes occurred and in particular, no significant increase in cardiopulmonary blood volume was detected.

Presence and axonal transport of cholinoceptor, but not adrenoceptor sites on a cat noradrenergic neurone

1. Noradrenaline release and radioligand binding studies were carried out in the cat hypogastric nerve ligated in vito 2 cm distal to the inferior mesenteric ganglion for different time periods, and in different effector organs.2. Large quantities of noradrenaline and dopamine beta-hydroxylase (DBH) accumulated in the segments of nerve immediately proximal (P(1)) and distal (D(1)) to the ligation, with rates of about 100 and 25 mm/24 hr for the orthograde and retrograde transport, respectively.3. Nicotine evoked the release of noradrenaline from P(1) and atrial slices; the secretory response to nicotine was completely antagonized by mecamylamine. [(3)H]alpha-bungarotoxin biding to membranes from P(1) allowed the estimation of a K(D) of 2.97 nm and a B(max) of 1639 f-mole/mg protein.4. Acetylcholine inhibited the release of endogenous noradrenaline evoked by high K(+) stimulation in atrial slices, but not in P(1) segments. Similarly, carbachol decreased [(3)H]noradrenaline release induced by electrical stimulation (twenty-six shocks, 2 Hz, 5 msec) in the atrium but not in P(1).5. [(3)H]Quinuclydinilbenzylate ([(3)H]QNB) specifically binds to membranes from P(1) and vas deferens, following a saturation curve. In the case of P(1) segments taken 48 hr after ligation a K(D) of 0.35 nm and a B(max) of 129 f-mole/mg protein were found.6. The fact that the B(max) in P(1) and D(1) increased with the time of ligation suggests that orthograde and retrograde axonal transports of muscarinic binding sites exist in this nerve, with approximate rates of transport of 15 and 8 mm/24 hr, respectively.7. As far as adrenoceptors are concerned, we observed that yohimbine or phentholamine did not modify transmitter release from P(1), evoked by high K(+) or electrical stimulation. However, yohimbine enhanced the release of [(3)H]noradrenaline induced by electrical stimulation from splenic slices of the same animals.8. [(3)H]Clonidine, [(3)H]dihydroergocryptine or [(3)H]dihydroalprenolol ([(3)H]DHA) did not specifically bind to membranes from P(1), in spite of the fact that they showed typical saturation curves for specific binding in cortex and atrial membranes from the same cats.9. In conclusion, these data (a) further show that the ligated hypogastric nerve is a good model of noradrenergic nerve terminal free of effector cell; (b) provide direct evidence for the neural location of nicotinic receptors whose activation trigger noradrenaline release from noradrenergic neurones; (c) demonstrate the neural location and axonal transport of muscarinic receptor sites, but leave certain doubts about its functional role in this noradrenergic neurone; and (d) do not support the hypothesis that alpha and beta-adrenoceptors which modulate noradrenaline release from peripheral noradrenergic nerve terminals are neurally (or prejunctionally) located.

Prejunctional alpha-adrenoreceptors subserve a physiological role in cardiac noradrenergic transmission

1 The question whether prejunctional alpha-adrenoreceptors subserve a physiological role in a noradrenaline-mediated autoinhibitory feedback loop has been studied in guinea-pig isolated atria by stimulating the accelerans nerve and measuring chronotropic responses and the release of radioactivity after labelling transmitter stores with 3H-noradrenaline. 2 Phentolamine (0.3 micrometer) significantly enhanced chronotropic responses when stimulation was with 0.5 Hz for 30 s, but the increase in the release of radioactivity was too small to be measured reliably. When the frequency of stimulation was increased to 4 Hz for 30 s, phentolamine significantly increased the release of radioactivity but the chronotropic response to stimulation was near maximal and phentolamine had no significant effect on it. 3 With prolonged stimulation (12 min) at the lower frequency (0.5 Hz), both the release of radioactivity and the chronotropic response to stimulation were significantly enhanced by phentolamine (3 micrometers). 4 The results support a physiological role for prejunctional alpha-adrenoreceptors in guinea-pig isolated atria.

Differential effects of DL-propranolol on norepinephrine in cerebrospinal fluid and plasma

Acute administration of DL-propranolol (5 mg/kg, subcutaneous) to cats caused a significant increase in cerebrospinal fluid (CSF) norepinephrine and a significant decrease in plasma norepinephrine. This increase in CSF norepinephrine is probably due to a reflex increase in central noradrenergic firing after beta blockade. The decrease in plasma norepinephrine reflects decreased sympathetic outflow. These results are consistent with the centrally mediated hypotensive effects of the drug.

Acid phosphatase activity in the aorta of spontaneously hypertensive rats and the effect of various antihypertensive drugs

In an attempt to obtain information about the arterial lysosomal enzymes in hypertension, we biochemically investigated acid phosphatase (Ac-Pase) activity in the aorta of spontaneously hypertensive rats (SHR) and the effects of various antihypertensive drugs. Ac-Pase activity in SHR was always higher than that in age-matched control rats. The enzyme activity tended to increase progressively with advancing age, a tendency which was more pronounced in SHR than in control rats. The aging process expressed by the Ac-Pase activity seems to be accelerated under hypertensive conditions. Antihypertensive drugs such as reserpine, hydrochlorothiazide, hydralazine and propranolol significantly suppressed the rise of blood pressure and decreased the aortic Ac-Pase activity in SHR. In particular reserpine and propranolol lowered Ac-Pase activity more effectively than it did blood pressure. Hypertension as well as catecholamine seem to be involved in the increase in the aortic lysosomal enzyme activity in SHR.

Hypertension through adrenaline activation of prejunctional beta-adrenoceptors

1. Adrenaline can enhance the stimulation-induced release of transmitter noradrenaline in sympathetically innervated tissues by activating prejunctional beta-adrenoceptors. 2. Adrenaline incorporated into sympathetic transmitter stores by neuronal uptake can be subsequently released as a co-transmitter and can then activate prejunctional beta-adrenoceptors, thus completing a facilitatory feedback loop. 3. Rats chronically treated with adrenaline develop elevated blood pressures compared to control rats. beta-Adrenoceptor blockade prevents the rise in blood pressure. 4. Activation by adrenaline of facilitatory prejunctional beta-adrenoceptors of sympathetic nerves innervating cardiovascular effector tissues may explain adrenaline-induced rises in blood pressure.

Activation of prejunctional beta-adrenoceptors in rat atria by adrenaline applied exogenously or released as a co-transmitter

1 Adrenaline (10 nM) significantly enhanced the stimulation-induced efflux of radioactivity from rat atria previously incubated with [3H]-noradrenaline ([3H]-NA). This effect was abolished by metoprolol (.01 muM). 2 Adrenaline in a higher concentration (1 muM) and NA (1 muM) significantly reduced the stimulation-induced efflux of radioactivity. However, in the presence of phenoxybenzamine (10 muM), adrenaline (1 muM) enhanced the efflux, whereas NA (1 muM) had no effect. 3 In rat isolated atria pre-incubated with adrenaline and then incubated with NA, both catecholamines were taken up and were released by field stimulation. When pre-incubation was with adrenaline and incubation was with [3H]-NA, metoprolol decreased the stimulation-induced efflux of radioactivity. This effect did not occur if the atria were pre-incubated with NA instead of adrenaline, suggesting tht neuronally released adrenaline activates prejunctional beta-adrenoceptors. 4 In conscious rats, intravenously administered adrenaline (6.0 and 0.6 nmol/kg) was taken up and retained in the atria and could be released by field stimulation. The release was calcium-dependent from these rats up to 24 h after administration.

Presynaptic regulation of the release of catecholamines in the cat hypothalamus

The posterior hypothalamus of cats was superfused through a push-pull cannula and the release of endogenous catecholamines was determined in the superfusate. Superfusion with yohimbine, isoprenaline, salbutamol or tazolol increased, while superfusion with propranolol decreased, the release of all three catecholamines. Transection of the brain caudal to the hypothalamus inhibited 'resting' and drug-induced release. It is concluded that alpha- and beta-adrenoceptors of the hypothalamus are involved in the regulation of the release of catecholamines.

Pulmonary vasodilator responses to catecholamines and sympathetic nerve stimulation in the cat. Evidence that vascular beta-2 adrenoreceptors are innervated

We investigated the effects of catecholamines and sympathetic nerve stimulation in the feline pulmonary vascular bed under conditions of controlled pulmonary blood flow. Norepinephrine and nerve stimulation caused dose- and stimulus frequency-dependent increases in pulmonary vascular resistance. However, when pulmonary vascular tone was enhanced and alpha receptors blocked, norepinephrine and nerve stimulation caused dose- and frequency-dependent decreases in pulmonary vascular resistance. The decreases in pulmonary vascular resistance were blocked with propranolol and were of greater magnitude than were constrictor responses observed under basal conditions. Vasodilator responses to nerve stimulation were not modified by atropine. Epinephrine and isoproterenol had marked vasodilator activity in the pulmonary vascular bed when pulmonary vascular tone was elevated. When alpha receptors were blocked, isoproterenol and epinephrine had similar vasodilator activity, and when beta receptors were blocked, epinephrine and norepinephrine had marked vasoconstrictor activity. Selective beta-1 receptors antagonists had little effect on vasodilator responses to isoproterenol, whereas responses to this substance were blocked by propranolol. These results suggest that presence of alpha-and beta-2 adrenoreceptors in the feline pulmonary vascular bed and that both types of adrenergic receptors are innervated by the sympathetic nervous system.

An interaction between prejunctional alpha-adrenoceptors and prejunctional beta-adrenoceptors

The ability os isoprenaline to enhance transmitter release from sympathetic nerves in rat atria incubated with [3H]noradrenaline was assessed under three conditions of prejunctional alpha-adrenoceptor activation: in the presence of phentolamine, in the presence of noradrenaline, and in the absence of either drug. Isoprenaline-induced enhancement of transmitter release was inversely related to the degree of activation of prejunctional alpha-adrenoceptors. Thus there appears to be an interaction between the prejunctional alpha-adrenoceptor inhibitory mechanism and the prejunctional beta-adrenoceptor facilitatory mechanism. In rabbit ear arteries incubated with [3H]noradrenaline, isoprenaline facilitated transmitter release in the presence but not in the absence of phentolamine. Therefore in some tissues it may be necessary to block prejunctional alpha-adrenoceptors before prejunctional beta-adrenoceptors can be demonstrated.

Evidence that noradrenaline modulates the increase in striatal dopamine metabolism induced by muscarinic receptor stimulation

The effect of oxotremorine was studied on the concentration of homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenylethylglycol (MHPG) in the corpus striatum of rats. At a dose of 1 mg/kg oxotremorine increased HVA levels by 68% and MHPG, by 51%. MHPG was also increased in the nucl. accumbens (58%) and neocortex (42%). Pretreatment with clonidine, 0.1 mg/kg abolished the increase in MHPG in the striatum and significantly inhibited the rise in HVA. 1-Propranolol 2.5 mg/kg, but not d-propranolol, had an effect similar to that of clonidine. A lower dose of oxotremorine (0.5 mg/kg) increased striatal HVA by 36% but did not alter MHPG levels. This increase in HVA was not reduced by 1-propranolol. Eight days after bilateral lesions of the locus coeruleus, there was a reduction in the basal concentrations of noradrenaline (41%) and MHPG (57%) in the striatum. The data suggest that at higher doses of oxotremorine (1 mg/kg), but not a lower dose (0.5 mg/kg), a noradrenergic pathway is stimulated to increase the rate of metabolism of noradrenaline in the striatum. Oxotremorine also appears to increase dopamine metabolism in the striatum by at least two separate mechanisms, one of which involves the mediation of noradrenaline.

The effects of (+)- and (-)-propranolol on 3H-transmitter efflux in guinea-pig atria and the presynaptic beta-adrenoceptor hypothesis

The effects of isoprenaline and of the (+)- and (-)-isomers of propranolol on the stimulation-induced overflow of 3H-transmitter was assessed in guinea-pig atria to evaluate the hypothesis of presynaptic beta-adrenoceptors. 2 Isoprenaline (1.2 x 10(-8) M) enhanced the efflux of tritium at 2 and 5 Hz with 100 pulses and did so to a similar extent at both frequencies. 3 The (-)-isomer of propranolol (1.0 x 10(-7) M) blocked the enhancing effect of isoprenaline but did not by itself modify transmitter efflux. 4 The (+)-isomer of propranolol, almost devoid of beta-adrenoceptor blocking properties, was also effective at 1.0 x 10(-7) M in blocking the enhancement of tritium efflux by isoprenaline. 5 The (-)-isomer of propranolol (1.0 x 10(-7) M) blocked almost entirely the inotropic response to isoprenaline (3 x 10(-7) M) but even 3.0 x 10(-6) M (+)-propranolol was ineffective in antagonizing the beta-adrenoceptor-mediated contractile responses to the catecholamine. 6 It is concluded that the presynaptic site of isoprenaline action does not show the requisite stereo-specificity of beta-adrenoceptors and that a 'non-specific' action of the antagonist probably accounts for its reduction of the effect of isoprenaline.

Facilitation of sympathetic transmission by prejunctional beta-adrenoceptors in cardiac and vascular tissues

1. In rat isolated tail arteries and atria in which transmitter stores are labelled with (3H)-noradrenaline, isoprenaline in low concentrations facilitates the stimulation-induced efflux of tritium. 2. This effect is dependent on the frequency and duration of electrical stimulation, being observed only at low frequency (2 Hz) and short duration (10 s in atria and 30 s in arteries) or at high frequency (5 Hz) and long duration (1 min in atria and 4 min in arteries). 3. In atria, phenotlamine enchances tritium efflux with stimulation at 5 Hz for 1 min but not at 2 Hz for 10 s. Conversely, propranolol reduces tritium efflux with stimulation at 2 Hz for 10 s but not at 4 Hz for 1 min. 4. The results can be interpreted in terms of the selective operation, depending on the frequency and duration of electrical stimulation, of inhibitory and facilitatory feedback loops for transmitter release involving prejunctional alpha -and beta-adrenoceptors, respectively.

Antagonism of an indirectly acting agonist: block by propranolol and sotalol of the action of tyramine on rat heart

Some agonists act indirectly in the sense that they cause the release of a second substance that brings about the response finally observed. An antagonist which competitively inhibits the action of the intermediate substance will also reduce the response to the indirectly acting agonist, provided that the receptors are freely accessible. A simple mass-law model for indirect antagonism of this kind is presented, and its predictions are compared with the results obtained in an experimental study of the influence of propranolol and sotalol on the inotropic response of isolated rat atria to tyramine. While there is reasonable qualitative agreement, the fit is not exact and reasons for this are discussed.

Recent developments in noradrenergic neurotransmission and its relevance to the mechanism of action of certain antihypertensive agents

This report reviews a number of significant developments in the fields of noradrenergic transmission and adrenergic receptors which suggest that, in addition to the classical postsynaptic adrenoceptors, there are also presynaptic adrenoceptors that help modulate the release of norepinephrine (NE) from peripheral as well as central noradrenergic nerve endings during nerve stimulation. In particular, stimulation of presynaptic alpha-adrenoceptors reduces this release of transmitter and the reverse is observed after blockade of these receptors. Clearcut pharmacological differences exist between the postsynaptic alpha 1-adrenoceptors that mediate the responses of certain organs and the presynaptic alpha 2-adrenoceptors that modulate the NE release during nerve stimulation. Therefore, subclassification of alpha-adrenoceptors into alpha 1 and alpha 2 subtypes is warranted but must be considered to be independent of the anatomical location of these receptors. Some noradrenergic nerve endings have also been shown to possess beta-adrenergic receptors, the stimulation of which increases the quantity of transmitter released by nerve impulses. Physiologically, these receptors could be activated by circulating epinephrine (E) and be involved in essential hypertension. A third type of catecholamine receptor found at the noradrenergic nerve ending is the inhibitory dopamine (DA) receptor, which might be of significance in the development of new antihypertensive agents. Application of these new concepts of noradrenergic neurotransmission and the subclassification of alpha-adrenoceptors to the treatment of hypertension is presented. Clonidine, for example, appears to be a potent alpha 2-adrenoceptor agonist; the central receptor involved in its antihypertensive action is pharmacologically an alpha 2-type but located postsynaptically. Clonidine also induces activation of peripheral presynaptic alpha 2-adrenoceptors, which might contribute to its cardiovascular action. The antihypertensive effects of alpha-methyldopa are related to the formation of alpha-methylnorepinephrine, a preferential alpha 2-adrenoceptor agonist, which can stimulate peripheral presynaptic alpha 2-adrenoceptors leading to a decrease of NE release and a reduction in sympathetic tone. Prazosin is a new antihypertensive agent the mechanism of action of which involves a selective blockade of postsynaptic alpha 1-adrenoceptors. This drug does not antagonize several effects of clonidine that are mediated via alpha 2-adrenoceptors. The mechanisms presently considered to account for the antihypertensive activity of beta-adrenoceptor blocking agents are numerous. It is proposed that blockade of peripheral presynaptic facilitatory beta-adrenoceptors could be of significance in the antihypertensive action of these drugs.

Effects of Goniopora toxin on guinea-pig blood vessels

Effects of a marine polypeptide, Goniopora toxin (GPT) (molecular weight 12,000), were examined in isolated blood vessels guinea pigs. GPT, ranging from 10-100 nM, augmented the contractile response to electrical transmural stimulation in the thoracic aorta, portal vein, and mesenteric and femoral arteries. The effects were abolished by tetrodotoxin and bretylium, and were markedly attenuated by phentolamine. As GTP did not affect the resting tension spontaneous rhythmicity or noradrenaline-induced contraction, the toxin appears to act on the neural elements in the vascular wall rather than on the smooth muscle. In the portal vein preloaded with 3H-noradrenaline, GPT enhanced the 3H-efflux in response to electrical transmural stimulation, yet had not effect on the spontaneous efflux. The increase in stimulation-evoked 3H-efflux caused by GPT was more than 15 times larger than the increase seen with cocaine or phentolamine. Tetrodotoxin completely blocked the 3H-efflux induced by electrical transmural stimulation. These data suggest that GPT acts on nerve components in guinea pig blood vessels and increases the release of noradrenaline evoked by electrical stimulation of the nerve fibers. These effects are probably associated with prolongation of the action potential duration and repetitive discharges in the adrenergic nerve fibers.

Feedback control of noradrenaline release as a function of noradrenaline concentration in the synaptic cleft in cortical slices of the rat

Cortical slices incubated with [3H]noradrenaline (NA) were used to study quantitatively the infelucne of the NA concentration in the synaptic cleft on electrically induced release of [3H]NA from adrenergic nerve terminals. Stimulation-induced [3H]-overflow was regarded to be proportional to the NA concentration in the synaptic cleft. High concentrations of piperoxan or clonidine were used to block, or maximally stimulate, respectively, the presynaptic alpha-receptors and thus to eliminate feedback control. These two extreme conditions were thought to delineate maximal feedback range. With alpha-receptors not artifically manipulated, [3H]overflow increased with stimulus intensity, yet not in proportion to the [3H]-overflow when the presynaptic alpha-receptors were blocked, demonstrating increasing feedback inhibition with increasing NA concentrations in the synaptic cleft. Feedback inhibition of NA release was shown to depend in an exponential fashion on NA concentration in the synaptic cleft. The slope of the regression line indicated that maximal inhibition of NA release occurred with NA concentrations in the synaptic cleft 65,000--700,000 times higher than threshold concentration. Data from experiments with different stimulus rates also supported the notion of feedback control of NA release.

Influence of beta-adrenoceptor blocking agents on the turnover rate of cardiac and splenic noradrenaline in rats

1 The effects of (+) and (+/-)-propranolol, pindolol, alprenolol, practolol, acebutolol and bretylium were studied on the turnover rate of noradrenaline in heart and spleen of rats. 2 Bretylium (8 mg/kg) greatly reduced the turnover rate of noradrenaline in both organs. 3 (+)-Propranolol (4 and 10 mg/kg) also diminished the turnover rate of noradrenaline, but its effects were smaller than those of (+/-)-propranolol (4 and 10 mg/kg). 4 Pindolol (300 micrograms/kg) greatly increased the turnover rate of noradrenaline; this effect was especially important in the spleen. 5 Alprenolol (4 and 10 mg/kg) acebutolol (20 and 40 mg/kg) practolol (10 mg/kg) did not produce any significant change. 6 These effects are compatible with the view that beta-adrenoceptor blocking agents may affect noradrenaline release in different manners: anaesthetic properties of some of these drugs and blockade of beta 2 prejunctional adrenoceptors produce a diminished release of transmitter, whereas the intrinsic sympathomimetic action of pindolol causes the opposite effect.

Some mechanism which may modulate noradrenaline release in guinea-pig isolated ileum

Guinea-pig isolated ileum pre-incubated with [3H]noradrenaline, released 3H-label during coaxial stimulation at 3.2-12.8 Hz. This evoked 3H-overflow seems due to nerve stimulation, and may be frequency-dependent. The 3H-overflow evoked by coaxial stimulation was increased with 2.6 muM phentolamine, reduced at threshold stimulation by 0.56 muM prostaglandin E1, but the tendency for 2.8 muM indomethacin to increase the evoked 3H-overflow was not statistically significant. The results indicate that the release of noradrenaline from guinea-pig ileum may be modulated by activation of presynaptic alpha-adrenoceptors, whereas prostaglandins act mainly at a postsynaptic site to modulate the sympathetic response.

Chronic propranolol treatment inhibits sympathetic nerve activity and keeps blood pressure from rising in spontaneously hypertensive rats

When D,L-propranolol, 100 mg/kg/day, was added to the drinking water of spontaneously hypertensive rats (SHR), systolic pressures measured with the tail-cuff method fell significantly within 1 month and were almost the same as those in normotensive controls (KNR) by the end of 3 months. This antihypertensive effect was later confirmed by direct recording of phasic aortic pressures from indwelling catheters. Blood pressure was lowered selectively only in SHR and not in KNR; by contrast, body weight, fluid intake, and heart rate always decreased whether the rats were hypertensive or not. Because pressor responses to hypothalamic stimulation in SHR treated with propranolol were reduced while those to injected norepinephrine were unaltered, a peripheral inhibition of cardiovascular reactivity was considered unlikely. Supporting the interpretation that diminished pressor responsiveness was caused by concurrent reduction of sympathetic vasomotor activity, frequency of spike potentials recorded from abdominal sympathetic nerves was appreciably lessened in propranolol-treated SHR, as was the vasodepression resulting when autonomic ganglia were pharmacologically blocked with pentolinium. These findings are consistent with the conclusion that prolonged oral administration of propranolol prevents development of spontaneous hypertension in rats by reducing sympathetic vasomotor tons.

Propranolol decreases sympathetic nervous activity reflected by plasma catecholamines during evolution of myocardial infarction in man

Plasma 1-norepinephrine and epinephrine contents were strikingly elevated in 70 patients during evolution of myocardial infarction. Propranolol or placebo, 0.1 mg/kg i.v., was administered randomly an average of 10 h after infarction and continued orally for 3 d. Propranolol, but not placebo, acutely decreased 1-norepinephrine contents from 2.24 +/- 1.33 (mean +/- SD) to 1.31 +/- 0.74 microgram/liter, P less than 0.001, and epinephrine contents from 0.97 +/- 0.42 to 0.74 +/- 0.42 microgram/liter, P less than 0.02. Decreases in 1-norepinephrine contents were related to the initial plasma concentrations, r = 0.85, P less than 0.001. A similar, but less strong relationship was observed between the initial epinephrine contents and propranolol-induced changes, r = -0.51, P less than 0.01. Propranolol reduced plasma-free fatty acid contents from 1,121 +/- 315 to 943 +/- 274 mumol/liter, P less than 0.001. Decreases in plasma contents of free fatty acids were related to decreases in epinephrine, r = 0.66, P less than 0.001. Propranolol did not cause significant additional changes in plasma catecholamine contents during the subsequent 3 d. In the placebo group 1-norepinephrine contents had decreased 24 h after infarction from 1.92 +/- 0.99 to 1.37 +/- 0.93 microgram/liter, P less than 0.02. Plasma epinephrine contents did not change. Heart rate remained below the control values during the entire study period in the propranolol, but increased in the placebo group. The data indicate that sympathetic hyperactivity, indirectly reflected by plasma catecholamine contents, is acutely reduced by propranolol during evolution of myocardial infarction.

Plasma catecholamines and cardiac, renal and peripheral vascular adrenoceptor-mediated responses in different age groups of normal and hypertensive subjects

The role of the sympathetic nervous system in cardiac, renal and peripheral vascular adrenoceptor-mediated responses was investigated in patients with essential hypertension and age-matched normotensive subjects. Regardless of age plasma adrenaline was significantly higher in hypertensive when compared with normotensive subjects. This suggests a sympatho-adrenal factor in essential hypertension. Plasma noradrenaline tended to increase with age but its similarity between normotensive and hypertensive subjects points to similar postganglionic neural activity and/or similar overflow of noradrenaline into the circulation. On the other hand, beta-adrenoceptor-mediated tachycardia in response to exercise and intravenous isoproterenol as well as the forearm vasodilator response to intraarterial isoproterenol decreased in normal subjects with older age. In hypertensives this age-dependent beta-receptor-related effect tends to be enhanced as judged from the greater reduction of cardiac isoproterenol sensitivity and the blunted renin response to exercise stimulation. The dilator response to alpha-adrenoceptor blockade with phentolamine was not different in both groups. Therefore a qualitative rather than quantitative derangement of sympathetic control of vascular resistance - in which beta-dilator effects are reduced and alpha-constrictor mechanisms prevail - may contribute to the maintenance of established hypertension.

Inhibition of angiotensin II potentiation of sympathetic nerve activity by beta-adrenergic antagonists

Since beta-adrenergic blockers are effective in the therapy of hypertension by a mechanism related to the degree of activation of the renin-angiotensin system, the effect of eight beta blockers was examined on angiotensin II potentiation of nerve stimulation (NS) in isolated perfused rat mesenteric vessels. The vasoconstrictor response to periarterial NS was obtained by monitoring changes in perfusion pressure while a beta blocker or a beta blocker and angiotensin II (3 ng/ml) were added to the perfusate. Although each beta blocker tended to decrease responses to NS, in the concentrations used, only metoprolol significantly inhibited responses to NS. Angiotensin II, when infused alone, potentiated the responses to NS by 63% (p less than 0.01). These enhanced responses following angiotensin II were inhibited in a dose-related manner (10--300 ng/ml) by beta 1, beta 2, and mixed beta blockers. At the 100 ng/ml concentration, DL-propranolol, timolol, metoprolol, practolol, butoxamine, and H35/25 inhibited the angiotensin II potentiation of NS by 83%, 76%, 77%, 59%, 72%, and 41% respectively. The order of potency for this action was as follows: timolol = metoprolol = butoxamine greater than propranolol greater than practolol greater than H35/25. Administration of D- and L-propranolol also reduced the responses by 75%. The vasoconstrictor responses to injected norepinephrine (NE), in the presence and absence of angiotensin II, were not altered by DL-propranolol or timolol. In conclusion, beta-adrenergic blockers were found to interfere with the effect of angiotensin II on the sympathetic neuron, a property that could contribute to the antihypertensive action of these drugs.

Adrenaline activation of prejunctional beta-adrenoceptors in guinea-pig atria

1. Adrenaline in a concentration of 1.0 microM depressed the stimulation-induced efflux of tritium from the guinea-pig atria incubated with [3H]-noradrenaline, whereas adrenaline in a concentration of 0.5 nM significantly enhanced the stimulation-induced efflux of tritium. This enhancement was blocked by metoprolol (0.1 microM) and thus appears to be mediated by beta-adrenoceptors. 2. In guinea-pig atria incubated with unlabelled adrenaline and then with [3H]-noradrenaline, both catecholamines were released by field stimulation. In such atria metoprolol, practolol, oxprenolol or propranolol decreased the stimulation-induced efflux of tritium. These effects did not occur if the atria were incubated with unlabelled noradrenaline and then with [3H]-noradrenaline, suggesting that neuronally released adrenaline activates prejunctional beta-adrenoceptors. 3. The effect of oxprenolol in decreasing the release of tritium from guinea-pig atria, incubated with unlabelled adrenaline and then with [3H]-noradrenaline was greater in the presence of phentolamine. This may reflect the alpha-adrenoceptor blocking activity of oxprenolol.