Inhibitory effect of propranolol on the vasoconstrictor response to sympathetic nerve stimulation

New trends in pharmacology

Among the factors influencing the arterial blood pressure the neural control through the autonomic nervous system is of paramount significance. The sympathetic division of this system holds a key position in this regard and its neural transmission mechanisms represent a well established target for pharmacological interference aimed at lowering blood pressure. This presentation will first deal with the morphological and physiological basis for blood pressure control through the sympathetic nervous system and the various possibilities for antihypertensive drug action through this system. Special attention will be drawn to recent advances in catecholamine research which may offer new leads in the development of blood pressure lowering agents. A second topic in this review will be some remarks on other pharmacological principles for interference with vascular control apart from the sympathetic system, particularly the vasodilator principle.

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.

Adrenergic control of lacrimal secretion in rabbits

Stimulation of the preganglionic trunk of the ipsilateral superior cervical sympathetic ganglion with square wave electrical pulses produced secretion from the main excretory duct of the rabbit lacrimal gland. The secretion was inhibited by propranolol (1 mg/kg) and metoprolol (10 mg/kg) but not by phenoxybenzamine (2 mg/kg) and atropine (25 micrograms/kg). The results indicate that the sympathetically induced secretion in the rabbit lacrimal gland occurs through an adrenergic mechanism. The adrenergic receptors in the lacrimal gland is most likely of beta 1-type.

Treatment of hypertension associated with head injury

Arterial hypertension that occurs after severe head injury is characterized by elevation of systolic blood pressure, tachycardia, increased cardiac output, normal or decreased peripheral vascular resistance, and increased circulating catecholamines. The effects of two drugs used in the management of hypertension, propranolol and hydralazine, on these indices of cardiovascular function were examined in six head-injured patients. Both drugs effectively normalized blood pressure. However, hydralazine increased heart rate by 30%, cardiac index by 49%, left cardiac work by 21%, and pulmonary venous admixture by 53%, and was responsible for an increase in intracranial pressure or decreased compliance in two patients. Hydralazine produced no consistent change in arterial catecholamines. In contrast, propranolol decreased heart rate by 21%, cardiac index by 26%, left cardiac work by 35%, pulmonary venous admixture by 15%, and oxygen consumption by 18%. Propranolol decreased arterial epinephrine levels by 48% and norepinephrine levels by 28%. Propranolol appears to be a useful antihypertensive drug in the hyperdynamic head-injured patient because it normalizes blood pressure and the underlying hemodynamic abnormalities both by its beta-adrenergic blocking action and by decreasing circulating levels of catecholamines.

The influence of vascular beta-adrenoceptors on the position and mobility of the rabbit incisor tooth

The influence of vascular beta-adrenoceptors on the tooth-supporting functions of the periodontal ligament were studied in 30 anesthetized rabbits. Propranolol and the beta 2-adrenoceptor antagonists ICI 118,551 and H 35/25 induced a marked extrusion of the incisor, probably due to reduced vasodilator tone in periodontal postcapillary blood vessels. Isoprenaline reversed the movement. beta 1- and beta 2-receptor antagonists decreased the intrusive responses to sympathetic nerve stimulation in teeth close to control position, but had no effect on responses in fully extruded teeth. Intrusive mobility, as tested by intermittent loading with 5-10 g, remained unaffected by beta-adrenoceptor blockade. The results suggest that vascular beta 2-adrenoceptors are important in the regulation of tooth position by their control of periodontal postcapillary resistance. Prejunctional beta-receptors enhancing the vasoconstrictor--and intrusive--responses to sympathetic nerve stimulation are probably a mixture of beta 1- and beta 2-receptors. It would seem that the periodontal vasculature is important for the position of the rabbit tooth but, due to rapid adaptive mechanisms of the tooth-supporting tissues, the vasculature contributes little to the resistance against load-induced intrusive movements of the tooth.

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.

Mechanisms and implications of vasodilator tolerance in the treatment of congestive heart failure

Vasodilators play an important role in the treatment of the patient with severe heart failure and increased systemic vascular resistance. However, there are both clinical data and theoretic reasons to anticipate that some degree of tolerance may develop during the long-term use of most agents. The cause of the increased vascular resistance of heart failure is not completely understood, but it appears to be related to a number of neuroendocrine, molecular and physical mechanisms including increased activity of the sympathetic nervous and renin-angiotensin systems, and increased vascular stiffness due to intra- and extracellular sodium and fluid accumulation. Not surprisingly, a lowering of systemic vascular resistance either by direct smooth muscle relaxers or by blockade of specific neuroendocrine systems may result in a number of compensatory responses at the neuroendocrine and/or molecular level. The over-all effectiveness of a particular vasodilator is the net sum of its direct pharmacologic action, and the neuroendocrine and molecular responses to the drug. The specific compensatory mechanisms activated depend on several factors including the type of vasodilator used, the dose employed, the baseline neuroendocrine status of the patient, the severity of heart failure and the functional integrity of various reflex systems. Although not directly applicable to patients with heart failure, much information derived from the use of these agents to treat patients with hypertension and angina pectoris suggests several potential mechanisms by which tolerance may develop to virtually all classes of vasodilators. The major types of vasodilators are discussed with regard to their potential mechanisms of tolerance. Finally, the evidence currently available from long-term studies is reviewed in order to assess the potential relevance of vasodilator tolerance to the clinical management of the patient with heart failure.

Central cardiovascular effects of gamma-hydroxybutyric acid: interactions with noradrenaline, serotonin, dopamine and acetylcholine transmission

Gamma-hydroxybutyric acid (GHBA) increased arterial blood pressure and heart rate dose-dependently following intraperitoneal and intracerebroventricular administration to conscious rats. Cardiovascular responses to GHBA were completely prevented after pretreatment with reserpine or phenoxybenzamine and after a prehypothalamic brain transection. Intraperitoneal GHBA increased brain noradrenaline (NA) synthesis and utilization, particularly in the neocortex. Selective central NA depletion in combination with NA synthesis inhibition prevented the cardiovascular effects of GHBA. No overt interactions were observed with agents influencing 5-hydroxytryptamine (5-HT), dopamine (DA), or acetylcholine (Ach) transmission. Pentobarbitone but not diazepam or diphenylhydantoin abolished the hypertensive properties of GHBA. We conclude that the cardiovascular effects of GHBA are of central origin and that they are mediated via the peripheral sympathetic nervous system. They are dependent on an intact central NA transmission while there are few indications for a significant role of central DA-, 5-HT or Ach-mechanisms.

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.

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.

Pre- and postjunctional beta-adrenoceptor mediated effects on transmitter release and effector response in the isolated rat portal vein

Pre- and postjunctional control mechanisms of the portal vein of spontaneously hypertensive rats (SHR) were characterized. Emphasis was placed on the influence of the presynaptic beta-adrenoceptor mediated mechanism for regulation of neuronal noradrenaline (NA) release (studied as tritium overflow) and its consequences for the contractile response under in vitro conditions. It was found that isoprenaline increased, whereas dl-propranolol decreased the release of neuronal NA during transmural nerve stimulation, while effector responses remained unaltered. d-Propranolol and the beta-1 selective adrenoceptor antagonist, metoprolol, did not affect these two variables. It is concluded that the presynaptic beta-adrenoceptors in the rat portal vein are mainly of the beta-2 type and mediate facilitation of neuronal transmitter release and that concomitant changes of the effector responses of this tissue are below the level of detection under the present experimental conditions.

Increased noradrenaline release in rat portal vein during sympathetic nerve stimulation due to activation of presynaptic beta-adrenoceptors by noradrenaline and adrenaline

With the aim of investigating whether exogenous noradrenaline (NA) and adrenaline (A) can modulate transmitter release via the stimulation of presynaptic beta-adrenoceptors, 3H-release from isolated portal veins was studied after pretreatment with 3H-1-NA, phenoxybenzamine, desipramine and normetanephrine. NA (10 muM) and A (0.05 muM) increased the fractional 3H-release elicited by sympathetic nerve stimulation by 30%. This effect could be blocked by d, 1-propranolol which per se reduced the release by 10%. It is concluded that NA can facilitate its own release via a presynaptic beta-adrenoceptor-mediated positive feed-back mechanism and that adrenaline can stimulate this beta-adrenoceptor-mediated mechanism.

The second Lilly Prize Lecture, University of Newcastle, July 1977. beta-Adrenergic receptor blockade in hypertension, past, present and future

All beta-adrenoceptor blocking drugs that have been described share the common property of being competitive inhibitors. They differ in their associated properties, the presence or absence of cardioselectivity, membrane stabilizing activity, and partial agonist activity. Recently some beta-adrenoceptor blocking drugs have been reported which also possess alpha-adrenoceptor blocking activity. The associated properties have been used as a basis for classifying beta-adrenoceptor blocking drugs (Fitzgerald, 1969, 1972). The presence or absence of cardioselectivity is most useful for dividing beta-adrenoceptor blocking drugs. The non-selective drugs (Division I) can be further divided according to the presence or absence of intrinsic sympathomimetic activity (ISA) and membrane stabilizing activity (Fitzgerald's groups I-IV). Group I possess both membrane activity and ISA, e.g. alprenolol, oxprenolol, group II just membrane action, e.g. propanolol, group III ISA but no membrane action, e.g. pindolol. Fitzgerald placed pindolol in group I but should be placed in group III as it possesses a high degree of beta-adrenoceptor blocking potency in relation to its membrane activity (Prichard, 1974). Finally drugs in group IV have neither ISA nor membrane action, e.g. sotalol, timolol. The cardioselective drugs (Division II) can be similarly sub-divided into groups I-IV according to the presence or absence of ISA or membrane action (Fitzgerald grouped all these together as group V). Lastly there are new beta-adrenergic receptor blocking drugs which in addition have alpha- adrenergic receptor blocking properties (Division III).

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.