Prejunctional actions of some beta-adrenoreceptor antagonists in the vas deferens preparation of the guinea-pig

Renal denervation causes chronic hypotension in rats: role of beta1-adrenoceptor activity

1. Renal denervation (RDNX) chronically lowers mean arterial pressure (MAP) in normal rats but mechanisms leading to this hypotensive response remain unknown. 2. We hypothesized that this sustained decrease in arterial pressure was because of a loss of beta1-adrenoceptor mediated renin secretion. Male Sprague-Dawley rats were assigned to sham (SHAM; n = 9), unilateral (UniRDNX; n = 9), or bilateral (RDNX; n = 10) renal denervation groups and instrumented for telemetric MAP measurements, plasma renin concentration (PRC) measurements and intravenous infusion. Twenty-four h MAP, heart rate, sodium and water balances were recorded 5 days before, 3 days during and 3 days after 1-adrenoceptor blockade with atenolol. 3. The 5-day control MAP was significantly lower in RDNX (97 +/- 1 mmHg) compared to SHAM (105 +/- 2 mmHg) and UniRDNX (102 +/- 2 mmHg) rats. No significant differences in basal PRC were observed between RDNX (2.2 +/- 0.3 ngAng1/mL per h), UniRDNX (2.6 +/- 0.4 ng/Ang1/mL per h) and SHAM (2.6 +/- 0.4 ngAng1/mL per h) rats. By day 1 of atenolol, PRC was significantly lower in UniRDNX rats (1.8 +/- 0.2 ngAg1/mL per h) compared to control values, but was unchanged during atenolol infusion in the other groups. By day 3 of atenolol, MAP was significantly decreased in all groups, but the absolute levels of MAP remained statistically different between RDNX (87 +/- 1 mmHg) and SHAM (91 +/- 1 mmHg) groups. 4. We conclude that the arterial pressure lowering effect of RDNX is not solely dependent on the loss of neural control of renin release.

Effects of d,l-propranolol on apomorphine induced stereotyped behavior in rats

The effects of d,l propranolol on stereotyped behavior induced by apomorphine administration was studied quantitatively. d,l Propranolol caused a 2.5 leftward displacement of the control dose-response curve constructed to apomorphine-induced stereotypy; the ED50 for apomorphine was reduced from 1.87 +/- 0.38 to 0.74 +/- 0.17. The potentiation was dependent on both d,l-propranolol and apomorphine doses. Results are discussed in the light of a possible interference of the central noradrenergic systems with the expression of dopaminergic-induced stereotyped behavior.

The effect of beta-adrenoreceptor antagonists on the inhibition of pendular movements of rabbit ileum produced by periarterial sympathetic nerve stimulation and some sympathomimetic amines

The effects of propranolol and of the selective beta 1- and beta 2-adrenoreceptor blocking drugs atenolol and ICI 118,551 were determined on the inhibitory responses of isolated segments of rabbit ileum to noradrenaline, isoprenaline and salbutamol and to periarterial sympathetic nerve stimulation. Responses to isoprenaline (0.04-10.24 microM) and salbutamol (1.4-89.6 microM) were blocked by propranolol in concentrations up to 5.0 and 12.8 microM, respectively. Responses to sympathetic nerve stimulation were reduced but responses to noradrenaline (0.03-1.92 microM) were unaffected by propranolol in concentrations up to 10.0 and 5.0 microM, respectively. Atenolol in concentrations up to 30.0 microM blocked responses to isoprenaline (0.04-2.56 microM) but did not affect responses to noradrenaline, salbutamol or sympathetic nerve stimulation in concentrations up to 3.0, 3.0 and 1.0 microM, respectively. However, when responses to noradrenaline and sympathetic nerve stimulation were reduced by phentolamine (1.0 microM), atenolol then produced further reductions. Responses to isoprenaline (0.04-2.56 microM) and salbutamol (1.4-89.6 microM) were blocked by ICI 118,551 in concentrations up to 0.5 microM. Responses to sympathetic nerve stimulation were reduced but responses to noradrenaline were unaffected by ICI 118,551 in concentrations up to 0.01 and 0.3 microM, respectively. Salbutamol (0.1 microM) increased the inhibitory response to sympathetic nerve stimulation and this effect was blocked by ICI 118,551 (0.01 microM). It was concluded that blockade of beta 2-adrenoreceptors, presumably located on sympathetic nerve terminals, decreases the release of transmitter noradrenaline and that blockade of beta 1-adrenoreceptors, presumably located in longitudinal smooth muscle cells, reduces the response to transmitter noradrenaline when alpha-adrenoreceptors are also blocked.

Presynaptic beta-adrenoceptors

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.

Prolonged treatment with (+/-) propranolol induces supersensitivity to (L)noradrenaline in mesenteric arteries in the rat

In the isolated perfused mesenteric arteries of the rat, neither (+/-) propranolol (0.1 microM) nor (+/-)isoproterenol (0.05 microM) modified the overflow of DL-[3H]noradrenaline (DL-[3H]NA) induced by sympathetic nerve stimulation at either 5 or 10 Hz. The blockade of alpha presynaptic receptors with phentolamine (4.7 microM) increased the 3H-transmitter overflow at 5 and 10 Hz. (+/-)Propranolol (0.1 microM) failed to modify this effect. Vasoconstrictor responses to exogenous NA or sympathetic nerve stimulation were not modified by (+/-)propranolol (0.1 microM). Prolonged treatment with (+/-)propranolol (7 mg/kg) for 15 days potentiates responses to both exogenous NA and sympathetic nerve stimulation; however, the fractional release per pulse of DL-[3H]NA was not modified at either 5 or 10 Hz. These results provide no evidence to support the hypothesis that the release of NA is regulated by presynaptic beta-adrenoreceptors in the mesenteric arteries of the rat. The enhancement of vascular responses after prolonged treatment with propranolol could be caused by postsynaptic supersensitivity.

Effects of d,l-propranolol on open field behavior of rats

Rats were administered with different doses of d,l-propranolol or d,l-propranolol plus amphetamine before open-field observations. Results show that d,l-propranolol decreased locomotion and rearing frequencies and increased immobility duration in rats. An antagonism between the effects of amphetamine and d,l-propranolol on general activity of rats was also observed. Results are discussed in the light of a possible interference of the drugs with the activity of either noradrenergic neurons or mid-brain reticular formation.

Mechanisms of action and the clinical pharmacology of beta-adrenergic blocking drugs

At present more than 20 beta-adrenergic blocking drugs are commercially available in Western Europe, and six are available in the United States. The clinical indications for their usage include hypertension, arrhythmias, ischemic heart disease, thyrotoxicosis, migraine headaches, glaucoma, and anxiety states. We will review the mechanisms suggested for the antihypertensive action of beta-adrenergic blocking drugs as well as these agents' clinical pharmacologic aspects. In general, the pharmacodynamic effects of the beta blocking drugs are quite similar, yet the properties of biotransformation, including pharmacokinetics, tend to be distinguishing features.

Adrenergic vasoconstriction as a cause of inadequate hypotensive response to beta-adrenergic blockade

This overview is concerned with the causes of nonresponsiveness to the hypotensive action of beta-adrenergic blocking drugs. The overall hemodynamic response, i.e., a secondary decrease in peripheral vascular resistance, is unrelated to the primary decrease in cardiac output. The reduction in vascular resistance may be triggered by mechanisms residing in the central nervous system, the arterial baroreceptor area, or the prejunctional beta-receptor. None of these mechanisms seem to be entirely responsible. Studies in responders vs nonresponders tend to equate nonresponsiveness with alpha-adrenoceptor-mediated vasoconstriction. Although circulating catecholamines are relatively poor indices of sympathetic activity, studies focusing on the renal-neural area appear to show clearly differential profiles between responders and nonresponders. Such findings, in relation to experimental data on the renal nerves as selective neural amplifiers, may provide a renewed interpretation of the centrally mediated hypotensive mechanism of beta-adrenergic blockade.

Modulation of noradrenaline release through activation of presynaptic beta-adrenoreceptors

On peripheral noradrenergic nerve endings there exist beta-adrenoreceptors activation of which results in an enhanced release of noradrenaline in response to nerve stimulation. These presynapatic beta-adrenoreceptors do not appear to be activated by neuronally-released noradrenaline. However, adrenaline may be a physiological activator during enhanced adrenomedullary secretion. Adrenaline can also be incorporated into the noradrenergic transmitter stores and be released as a co-transmitter. Under these conditions presynaptic beta-adrenoreceptors may be activated by neuronally-released adrenaline, thus forming a 'positive feedback loop'. The release of adrenaline from the adrenal medullae may also be modulated through facilitatory beta-adrenoreceptors, but the release of noradrenaline from noradrenergic nerves in the central nervous system is not. The facilitatory presynaptic beta-adrenoreceptors appear to be in the main of the beta 2-subtype although precise receptor characterization has not been carried out. Increased activation of presynaptic beta-adrenoreceptors by adrenaline may be implicated in the development of essential hypertension. Part of the antihypertensive action of beta-adrenoreceptor blocking drugs may be due to blockade of these facilitatory presynaptic beta-adrenoreceptors.

Effect of propranolol treatment in pregnant rats on motor activity and avoidance learning of the offspring

Rats born to mothers treated with propranolol, during days 8-22 of gestation, displayed hyperactivity in the open field which lasted up to 60 days of age and an impairment of avoidance in the shuttle box which was more marked in the male rats. Females exhibited hyperactivity in the open field but developed impaired avoidance learning only when exposed prenatally to both propranolol and hypoxia. Propranolol administration during the last term of pregnancy (days 18-22) affected mostly shuttle box performance. In contrast, hyperactivity could be induced by treatment during various stages of pregnancy, (days 8-22, 8-18, or 18-22) with the duration of hyperactivity being directly related to the length of treatment of the mothers. The possible mechanism of the disruptive effect of propranolol in the fetus and newborn is discussed.

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.

Beta-adrenoceptor antagonists: studies on behaviour (delayed differentiation) in the monkey (Macaca mulatta)

1 Activity of six beta-adrenoceptor antagonists was studied on behavioural activity (delayed differentiation) in the monkey (Macaca mulatta). The drugs, three relatively lipophilic antagonists (propranolol, oxprenolol and metoprolol), and three relatively hydrophilic antagonists (acebutolol, atenolol and sotalol), were given by intraperitoneal injection (5 to 30 mg/kg).2 With atenolol (25 to 30 mg/kg), total response time was increased, but there was no effect on the number of correct responses. With acebutolol (25 to 30 mg/kg), the number of correct responses was reduced, but there was no effect on total response time. With metoprolol (25 to 30 mg/kg), there was an increase in total response time and a decrease in the number of correct responses, and correct responses were decreased 4 h after injection over the whole dose range (5 to 30 mg/kg).3 Some animals failed to respond or complete the task with 30 mg/kg oxprenolol, 25 mg/kg sotalol and 20 mg/kg propranolol. With 25 mg/kg oxprenolol, the total response time was increased and the number of correct responses was decreased. With 5-20 mg/kg sotalol, total response time was increased, but there was no effect on the number of correct responses. With 15 mg/kg of (+/-)-propranolol and its isomers, there were increases in total response time and decreases in correct responses.4 The studies suggest that lipophilic antagonists, such as propranolol, oxprenolol and metoprolol, are likely to have, at least, effects on the central nervous system, while hydrophilic antagonists may modify the peripheral nervous system. In the dose-ranges studied, propranolol had the greatest, and atenolol and acebutolol had the least effects. Atenolol and acebutolol may prove to be particularly useful in man when disturbances of the nervous system are to be avoided.

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.

Contrasting effects of acute beta blockade with propranolol on plasma catecholamines and renin in essential hypertension: a possible basis for the delayed antihypertensive response

Blood pressure, heart rate, plasma renin activity, plasma norepinephrine and plasma epinephrine were determined in 11 patients with essential hypertension at rest before and 15, 30, 45, and 60 minutes after an intravenous infusion of 0.12 mg./Kg. propranolol given over five minutes. After propranolol mean blood pressure was unchanged; heart rate decreased by 14 per cent within 15 minutes and showed no further changes. Plasma renin activity decreased progressively by 48 per cent 60 minutes after propranolol, whereas plasma norepinephrine and epinephrine were always higher after propranolol than control values. Increases in norepinephrine were statistically significant at 30, 45, and 60 minutes (respectively 49, 39, and 45 per cent, P less than 0.005 at least) and those of epinephrine even at 15 minutes (respectively 60, 82, 62, and 94 per cent P less than 0.01 for all). These results indicate that acute beta blockade with propranolol incudes increases in circulating plasma norepinephrine and epinephrine which might be consequent to rapidly induced hemodynamic changes. This augmented sympathetic activity might explain why propranolol, when acutely infused, does not decrease blood pressure despite effective cardiac and renin blockade.

Stimulation of presynaptic beta-adrenoceptors enhances [3H]-noradrenaline release druing nerve stimulation in the perfused cat spleen

1 The effects of isoprenaline, propranolol and phosphodiesterase inhibitors on (3)H-transmitter overflow elicited by low frequency nerve stimulation were determined in the isolated perfused spleen of the cat.2 (-)-Isoprenaline (0.14, 1.4, and 14 nM) produced a concentration-dependent increase in [(3)H]-transmitter overflow evoked by nerve stimulation at 1 Hz and was more effective at 1 Hz than at 2 hertz.3 A concentration of propranolol (0.1 muM), devoid of neurone blocking activity, blocked this effect of (-)-isoprenaline. These results are compatible with the presence of beta-adrenoceptors in the noradrenergic nerve endings of the cat spleen.4 (+)-Isoprenaline (140 nM) failed to increase the release of radioactivity induced by nerve stimulation, indicating that the beta-adrenoceptor mediating the facilitation of transmitter release was stereospecific.5 The increase in (3)H-transmitter overflow induced by nerve stimulation during exposure to the phosphodiesterase inhibitor, papaverine (27 muM) was more pronounced than that obtained in the presence of 3-isobutyl-1-methyl xanthine (IBMX) 0.5 mM. The facilitation in transmitter release induced by papaverine was not correlated with the granular effect produced by this drug.6 In the presence of papaverine, the concentration-effect curve for (-)-isoprenaline on transmitter release was shifted to the left and its maximum was increased. In addition, propranolol significantly reduced the enhancement in noradrenaline release obtained by exposure to papaverine under conditions in which the granular effect produced by the phosphodiesterase inhibitor was even greater than in the absence of the beta-blocker.7 It is concluded that activation of presynaptic beta-adrenoceptors in the perfused cat spleen leads to an enhancement in transmitter release which appears to be linked to an increase in cyclic adenosine 3',5'-monophosphate levels in noradrenergic nerve endings.

Beta-adrenoceptors modulate noradrenaline release from axonal sprouts in cultured rat superior cervical ganglia

Superior cervical ganglia of rats grown in organ culture were used to study the effect of beta-receptor stimulants and antagonists on 3H-noradrenaline release in response to stimulation by KC1 (75 mM). (--)-Isoprenaline 1X 10(-9)--1 X 10(-7) M) increased 20--25% the release of 3H-noradrenaline from cultured ganglia exposed to KC1. Isoprenaline did not modify either the spontaneous (non-calcium dependent) release of 3H-noradrenaline from cultured ganglia, or the KC1-stimulated release from fresh ganglia. The effect of (--)-isoprenaline was blocked by (--)-propranolol 5 X 10(-9) -- 1 X 10(-8) M and by butoxamine 10(-6) M, but not by (+)-propranolol (1 -- 5 X 10(-8) M), practolol (1 X 10(-8) -- 1 X 10(-6) M), or sotalol (1 X 10(-7) -- 1 X 10(-6) M). Isoprenaline induced augmentation of 3H-noradrenaline release and its antagonism by (--)-propranolol still occurred in the presence of DMI. It is suggested that presynaptic beta-receptors in sympathetic nerve terminals may be involved in a positive feedback of noradrenaline release.

The uptake and overflow of radiolabelled beta-adrenoceptor blocking agents by the isolated vas deferens of the rat

1. A comparison of uptake into and overflow from the isolated vas deferens of the rat has been made between [3H]-noradrenaline ([3H]-NA), [14C]-D-sorbitol and three radio-labelled beta-adrenoceptor blocking agents, [14C]-practolol, [14C]-(+/-)-propranolol and [3H]-penbutolol. 2. The accumulation of [3H]-NA after 30 min incubation was reduced by desmethylimipramine (DMI) 1 X 10(-8)M and was also reduced in vasa from rats pretreated with 6-hydroxydopamine (6-OHDA). This was not so with [14C]-D-sorbitol. 3. 6-OHDA pretreatment of the rats reduced the uptake of [3H]-penbutolol after 30 min incubation but not that of [4C]-propranolol or [14C]-practolol. DMI 1 X 10(-8)M did not alter the tissue uptake of [14C]-propranolol, [14C]-practolol or [3H]-penbutolol. 4. Electrical stimulation of vasa preloaded with [3H]-NA caused a significantly greater increase in [3H]-NA overflow than during the resting, unstimulated periods. No such increase in overflow was observed with [14C]-sorbitol or any of the three beta-adrenoceptor blocking agents use. 5. The beta-adrenoceptor blocking agent penbutolol was shown to possess adrenergic neurone blocking activity in the isolated vas deferens of the rat. 6. It is concluded that any effect that practolol or (+/-)-propranolol have on noradrenergic neurones is brought about without the need for these drugs to gain access to the interior of the neurone.

Reduction by propranolol of raised urinary output of MHPG in hyperactive rats

Prolonged isolation of rats resulted in hyperactivity in the open field and a significant increase in 24 hr urinary excretion of MHPG (3-methoxy-4-hydroxyphenylglycol). Exploratory activity of group-housed rats in open field was not associated with raised MHPG excretion, compared with that of rats remaining in home cages. Exposure of group-housed rats to 4 degrees C for 2 hr also increased urinary excretion of MHPG. Pretreatment of isolated rats with dl-, d-propranolol or practolol abolished hyperactivity of isolated rats and reduced MHPG output in these rats and in rats exposed to cold. dl-Propranolol did not reduce activity of group-housed rats in open field or their urinary excretion of MHPG. It is suggested that propranolol may have a selective inhibitory effect on stress-induced increases in noradrenaline turnover.

The effect of propranolol on sympathetic nerve stimulation in isolated vasa deferentia

(+/-)-Propranolol hydrochloride (0.5 mg kg(-1) twice daily, subcutaneously, for 3 days or approximately 2.4 mg kg(-1) daily, orally, for 21 days) failed to produce ptosis or to affect responses to transmural stimulation of isolated vasa deferentia removed from treated mice. In guinea-pig isolated vasa deferentia responses to transmural stimulation through parallel electrodes were reduced by propranolol (1 to 20 mug ml(-1); blockade was concentration dependent, fast to equilibrium (45 min), easily reversed by washing but not reversed by (+)-amphetamine sulphate (0.2 mug ml(-1). At lower concentrations (0.04 and 0.2 mug ml(-1), propranolol marginally potentiated responses to transmural stimulation. In contrast, guanethidine (0.2 mug ml(-1)) produced a slow onset blockade which was completely reversed by (+)-amphetamine. The response to electrical stimulation through concentric ring electrodes was reduced by low concentrations of propranolol but this effect is ascribed to the known local anaesthetic actions of propranolol and no evidence of true adrenergic neuron blockade was found.

Effects of beta-adrenoreceptor blocking drugs on adrenergic transmission

The peripheral actions of beta-adrenoreceptor antagonists on adrenergic transmitter mechanisms have been reviewed. In addition to receptor blockade, beta-adrenoreceptor antagonists may in high concentrations inhibit neuronal uptake of noradrenaline; inhibit monoamine oxidase; inhibit the uptake of noradrenaline into transmitter storage vesicles and inhibit the extraneuronal uptake of noradrenaline. High concentrations of beta-adrenoreceptor antagonists (threshold about 30 muM) also release noradrenaline from intraneuronal stores; however, their intrinsic sympathomimetic activity is generally attributed to their partial agonist property. Beta-adrenoreceptor antagonists possess adrenergic neurone blocking activity and quinidine-like or local anaesthetic activity. The existance of a positive feedback mechanism involving prejunctional beta-adrenoreceptors is discussed. It is suggested that bradycardia produced by beta-adrenoreceptor antagonists is due to blockade of the action of circulating catecholamines or of transmitter noradrenaline at cardiac extrajunctional beta-adrenoreceptor sites.

Some aspects of the pharmacology of beta-adrenorecptor blockers

The pharmacodynamic properties of a beta-blocker are mainly determined by its affinity to beta1 and beta2-receptors respectively and by its intrinsic activity. It is suggested that there is no absolute organ separation of the two receptor sub-types. Instead both beta1 and beta2-receptors are involved in the mediation of the same effect. The frequency distribution ratio of beta1/beta2-receptors varied markedly among various effector responses. A non-selective and a beta1-selective blocker may have different haemodynamic effects when the levels of circulating adrenaline are high, because of their markedly different potency in inhibiting the beta2-mediated vasodilator effect of adrenaline. Data are presented which suggest the existence of a presynaptic beta1-receptor mediating a positive feedback mechanism on neuronal release of noradrenaline.

The effect of propranolol on vascular responses to sympathetic nerve stimulation

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

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

1 The effects of isoprenaline, propranolol and phentolamine, were studied on tritiated noradrenaline overflow elicited by postganglionic nerve stimulation in guinea-pig isolated atria. 2 Isoprenaline (1.2 times 10-minus 8M) increased while propranolol (1.0 times 10-minus 7M) reduced the overflow of tritiated noradrenaline evoked by nerve stimulation. These effects were less than those of phentolamine (3.1 times 10-minus 6M), which increased by approximately three-fold the overflow of [3H]-noradrenaline elicited by nerve stimulation. 3 Neuronal accumulation of tritiated noradrenaline in guinea-pig atria was not affected by isoprenaline, propranolol or phentolamine at the concentration employed in this study. 4 Isoprenaline (1.2 times 10-minus 8M) induced a positive chronotropic effect of about 80 percent of the maximum. On the other hand, propranolol produced a shift to the right in the frequency-response curve to nerve stimulation and in the concentration-response curve to exogenous noradrenaline in guinea-pig atria. 5 In the isolated nictitating membrane of the cat, the frequency-response curve to nerve stimulation was not modified by propranolol, while in the presence of 3.9 times 10-minus 6M of N,-2-(2,6-dimethylphenoxy)propyl-N,N,N-trimethylammonium (beta-methyl-TM 10) there was a shift to the right and a depression of slope. Neither propranolol nor beta-methyl-TM 10 affected responses to exogenous noradrenaline. 6. The effects of isoprenaline and of propranolol on transmitter release are compatible with the view that in addition to the presynaptic negative feed-back mechanism for noradrenaline release by nerve stimulation mediated via alpha-adrenoceptors a positive feed-back mechanism exists in adrenergic nerve endings which is triggered through the activation of presynaptic beta-adrenoceptors.

Effect of acute and chronic treatment with practolol on cardiovascular responses in the pithed rat

Chronic administration of practolol to the pithed rat produced a reduction in the pressor responses to electrical stimulation of the spinal cord and potentiation of pressor responses to high doses of (—)-noradrenaline compared to control animals. Acute administration of practolol caused an increase in the pressor responses to both electrical stimulation and high doses of noradrenaline. Heart rate responses to both forms of stimulation were less than control values after both acute and chronic dosage with practolol. It is possible that practolol reduces the release of noradrenaline at the sympathetic nerve ending after chronic administration.

Further studies on the adrenergic neuron blocking activity of some -adrenoceptor antagonists and guanethidine

The effects of low (0.2) and high (20 μg ml−1) concentrations of (±)- and (+)-propranolol, (±)-, (+)- and (-)-sotalol and guanethidine were tested for their ability to reduce responses to sympathetic stimulation in the isolated vas deferens preparation from the guinea-pig. At 0.2 μf ml−1 all drugs produced a slowly developing reduction in the responses to sympathetic stimulation while responses to noradrenaline were largely unchanged. The blockade, which was similar in extent in all six compounds, was reversed by (+)-amphetamine but not by washing. With high concentrations of and (±)-propranolol and guanethidine, the block was rapid in onset and rate and responses to noradrenaline were potentiated. The block was reversed by washing and unaffected by (+)-***amphetamine. Sotalol and its isomers, which possess little non-specific depressant activity, had qualitatively similar actions at 0.2 and 20 μg ml−1. At the latter concentration responses to noradrenaline were potentiated. The results suggest that low concentrations of the β-adrenoceptor antagonists produce a blockade which is typical of guanethidine-like drugs. At high concentrations non-specific depressant (local anaesthetic) actions of propranolol and its isomers are largely responsible for the blockade. A similar mechanism may also operate when high concentrations of guanethidine are used.

Factors influencing the adrenergic neurone blocking action of propranolol

1. The reversal by propranolol of its own adrenergic neurone blocking effect in the cat can be prevented by cutting the splanchnic nerves or by ligating the adrenal veins.2. In the absence of secretion from the adrenal medulla the nerve blocking action of propranolol is more complete, but can still be reversed by repeated injections or a constant infusion of adrenaline.3. Prior treatment with adrenaline or noradrenaline also prevents the development of the blocking action of propranolol in the cat and in the isolated guinea-pig vas deferens.4. It is suggested that in the cat, propranolol stimulates the release of catecholamines from the adrenal medulla which antagonize its nerve blocking effect.

Role of adrenergic neurone blockade in the hypotensive action of propranolol

1. Propranolol, in doses of 25-100 mug/kg, blocks contractions of the nictitating membrane to nerve stimulation but not to injected noradrenaline.2. This adrenergic neurone blocking action of propranolol is antagonized by amphetamine.3. It is also reversed by raising the dose of propranolol to amounts exceeding 0.5 mg/kg.4. Still larger amounts potentiate the responses of the nictitating membrane to both submaximal stimulation of the cervical sympathetic nerve and to injected noradrenaline.5. The (+) isomer of propranolol produced adrenergic nerve blockade and some degree of hypotension without blocking cardiac beta-adrenoceptors.6. The relevance of adrenergic neurone blockade to the hypotensive effect of propranolol is discussed.

Adrenoceptor functions in the cat choledochoduodenal junction in vitro

1. The effects of alpha- and beta-adrenoceptor stimulating agents were investigated on three different kinds of preparation of the isolated sphincter of Oddi and on the duodenum of the cat.2. Adrenaline (1.5 x 10(-7)M-6.3 x 10(-7)M), noradrenaline (1.6 x 10(-7)M-6.3 x 10(-7)M), and tyramine (2.9 x 10(-6)M-5.8 x 10(-6)M) increased the activity and tonus of the sphincter musculature and decreased duodenal activity and tone. The effect on the sphincter resulted in increased resistance to flow through the sphincter. The excitatory effects on the sphincter were blocked by phenoxybenzamine (1.7 x 10(-8)M-1.7 x 10(-7)M).3. No effect was produced by tyramine in concentrations up to 4.6 x 10(-5)M on sphincters taken from reserpinized cats. It is suggested that the cat sphincter of Oddi contains adrenergic nerves of functional importance.4. Isoprenaline (1.9 x 10(-8)M-4.7 x 10(-7)M) and terbutaline (3.5 x 10(-7)M-8.8 x 10(-6)M) decreased spontaneous activity and tonus of the sphincter, and diminished resistance to flow through the sphincter. Both agents decreased spontaneous activity and tonus of the duodenum. On a molar basis, isoprenaline was 2-18 times more active than terbutaline on the sphincter and 35-90 times more active on the duodenum. The effects of isoprenaline and terbutaline were blocked by propranolol (3.9 x 10(-7)M).5. It is concluded that the cat sphincter of Oddi contains alpha-adrenoceptors active in contraction of the sphincter, and beta-adrenoceptors active in its relaxation. The beta-adrenoceptors of the sphincter differ from those in the duodenum; it is suggested that they belong to the beta(2)-group (according to Lands' classification).6. The automaticity of the isolated sphincter of Oddi resembled the sphincter activity recorded in vivo and is probably myogenic in nature, as it resisted treatment with phenoxybenzamine (1.7 x 10(-8)M-1.7 x 10(-7)M), atropine (1.4 x 10(-6)M-5.8 x 10(-6)M), hexamethonium (1.4 x 10(-5)M-1.1 x 10(-4)M) and tetrodotoxin (1 mug/ml). The activity of the sphincter has no propulsive function but prevents passage of fluid through the sphincter.