TOLERANCE TO THE EFFECTS OF Δ1-TETRAHYDROCANNABINOL ON THE PRESSOR RESPONSES TO NORADRENALINE IN RATS

1. The depressor response, but not the cardiac slowing response, to the acute intravenous administration of delta 1-THC (1 mg/kg) was significantly reduced in urethane anaesthetized rats, which had been treated with daily injections of delta 1-THC (2 mg/kg, i.p.) for 10 days (P less than 0.01). 2. No significant differences in either the depressor or the negative chronotropic effects of an acute intravenous injection of delta 1-THC (1 mg/kg) were observed in anaesthetized rats which had been treated with PVP (8 mg/kg per day, i.p.) for 10 days. 3. The depressor and cardiac slowing responses to an acute intravenous dose of delta 1-THC (1 mg/kg) were not significantly different between delta 1-THC- and PVP-treated animals which had been pithed. 4. The potentiating effects of delta 1-THC on the pressor responses to intravenously administered noradrenaline were significantly reduced (P less than 0.001) in urethane anaesthetized rats which had been treated with delta 1-THC, but not in anaesthetized PVP-treated animals. 5. Tolerance to the potentiating effect of delta 1-THC on the responses to noradrenaline has also been demonstrated in anaesthetized delta 1-THC-treated rats, but not in pithed delta 1-THC treated ones. 6. It is concluded that the development of tolerance to the depressor action of delta 1-THC, and its potentiating effect on the noradrenaline pressor responses requires the presence of an intact central nervous system.     

EFFECTS OF Δ1 AND Δ6-TETRAHYDROCANNABINOL ON THE ADENYLATE CYCLASE ACTIVITY IN VENTRICULAR TISSUE OF THE RAT HEART

Adrenaline 1 micrograms markedly elevated the activity of adenylate cyclase in ventricular tissue of rat hearts. delta 1-THC 25, 50 and 100 micrograms significantly reduced the adenylate cyclase activity with the maximum effect observed with the dose of 25 micrograms. Doses below or above this range did not produce any significant effect on the enzyme activity. Neither delta 6-THC 25 and 100 micrograms nor PVP 400 micrograms had any significant action on adenylate cyclase. The delta 1-THC-induced lowering of the cyclic AMP concentration in ventricular tissue of rat hearts can be explained, at least partly, by its ability to reduce the activity of adenylate cyclase in these tissues. It is suggested that this enzyme inhibition underlies the cardiac depressant action of delta 1-THC.     

CARDIAC β-ADRENOCEPTOR CHANGES IN EXPERIMENTAL HYPERTHYROIDISM IN DOGS

1. Triiodothyronine (T3; 1.0 mg/kg per day subcutaneously) was administered to 10 dogs for 14 days; 10 saline-treated dogs served as controls. T3-treated dogs showed the expected physiological responses of hyperthyroidism; further, chronotropic responses to isoprenaline in vivo were significantly increased in T3-treated dogs. 2. Beta-adrenoceptor subtype density was measured in membrane preparations by displacement of 125I-iodocyanopindolol binding by the selective beta 2-adrenoceptor antagonist, ICI 118, 551. T3 treatment led to a 93% increase in right atrial beta 1-adrenoceptor density and a 141% increase in left ventricular beta 1-adrenoceptor density; beta 2-adrenoceptor densities in right atrial, left ventricular and lung membranes were unchanged. 3. T3-treatment did not change basal or maximally stimulated adenylate cyclase activities in left ventricular membranes. 4. Thus, the cardiovascular changes in experimental hyperthyroidism in dogs were accompanied by an increased chronotropic response in vivo to isoprenaline and an increased beta 1-adrenoceptor density in atrial and ventricular membranes. However, there was no corresponding change in basal or maximal responsiveness of adenylate cyclase in ventricular membranes.     

INVOLVEMENT OF α1-ADRENOCEPTORS IN CHRONOTROPIC RESPONSES TO ENDOGENOUSLY RELEASED AMINES IN THE PITHED RAT

1. In pithed rats, yohimbine (1 mg/kg i.v.) enhanced the positive chronotropic responses to spinal stimulation of cardiac sympathetic nerves with eight pulses delivered at 2 or 4 Hz, indicating that auto-inhibition was operating, but did not increase responses to shorter lengths of trains of 8 pulses at 8, 16 or 32 Hz which did not allow sufficient time for auto-inhibition to come into effect. 2. The positive chronotropic response to cardiac sympathetic nerve stimulation with eight pulses at 8 Hz of about 60 beats/min was not affected by prazosin (1 mg/kg) or diltiazem (0.2 mg/kg), but was reduced to about 20% of the control value by propranolol (1 mg/kg). 3. In the presence of propranolol, the residual positive chronotropic responses to cardiac sympathetic nerve stimulation were virtually abolished by prazosin (1 mg/kg) or diltiazem (0.2 mg/kg). 4. The positive chronotropic response to tyramine (0.1 mg/kg i.v.) was reduced from 100 to 12 beats/min by propranolol (1 mg/kg), and the residual response was abolished by prazosin. 5. The findings indicate that noradrenaline released from cardiac sympathetic terminals by nerve stimulation or by tyramine acts on alpha 1-adrenoceptors to produce a positive chronotropic response that is revealed when beta-adrenoceptors are blocked.     

TIME COURSE OF RECOVERY OF β- AND α1-ADRENOCEPTORS IN EXPERIMENTAL ASTHMA

1. The time course of recovery of reduced beta-adrenoceptors caused by ovalbumin (OA) challenge was investigated using guinea-pigs. 2. The effects of prednisolone on the recovery time course were also evaluated. 3. beta- and alpha 1-receptor assays were performed using lung membranes. Adenylate cyclase activity was also measured. 4. OA challenge reduced the number of beta-adrenoceptors by 35%, and a significant decrease (13%) persisted for 7 days. The number of beta-adrenoceptor recovered after 14 days. 5. OA challenge elevated the number of alpha 1-adrenoceptors. A significant increase (24%) was observed after 7 days, and it took a further 7 days for the recovery. 6. After OA challenge there was a significant decrease in adenylate cyclase activity after 7 days, which recovered after a further 7 days. 7. Inhalation of prednisolone accelerated the recovery of beta-adrenergic responsiveness, though it did not affect the recovery of the number of alpha 1-adrenoceptors. Prednisolone inhalation also elevated beta-adrenergic responsiveness in non-asthmatic subjects. 8. It is concluded that reduced beta-adrenergic responsiveness caused by OA challenge persisted for 7 days and recovered after a further 7 days. Steroid hormone increased beta-adrenoceptors.     

EVIDENCE FOR DIRECT INTERACTION OF KETAMINE WITH α- AND β2-ADRENOCEPTORS

1. Ketamine has a number of effects that suggest that it may interact with α- and β-adrenoceptors. To date, the experimental evidence for this has been indirect and has been based on physiological studies using competitive blocking agents. In the present study we sought to determine from receptor binding studies whether ketamine binds directly to α- and β-adrenoceptors. 2. Membrane preparations o. α₁- and β₂-adrenergic binding sites were obtained from urinary bladder and urethrae of sheep. These binding sites were characterized by saturation analyses using [³H]-prazosin for α₁-adrenoceptor binding sites and [¹²⁵I]-cyanopindolol (CYP) for the β₂-adrenoceptor binding sites. The receptors were further characterized by displacement studies using selective and non-selective antagonists. 3. Studies in which ketamine was used to displac. [³H]-prazosin revealed a K_d of 3.40±1.23× 10^?3 mol/L for ketamine binding to ai-adrenoceptors. Displacement studies of [¹²⁵I]-CYP by ketamine showed a K_d of 0.35±0.03× 10^?3 mol/L for ketamine binding to β₂-adrenoceptors. 4. We conclude that ketamine interacts directly with both ai- an. β₂-adrenoceptors and that such interactions probably explain the reported effects of this agent on the vasculature and the bronchial tree.     

CARDIAC AND BRONCHIAL β-ADRENOCEPTOR ANTAGONISTIC POTENCIES OF ATENOLOL, METOPROLOL, ACEBUTOLOL, PRACTOLOL, PROPRANOLOL AND PINDOLOL IN THE ANAESTHETIZED DOG

1. The β-adrenoceptor antagonists atenolol, metoprolol, acebutolol, practolol, propranolol and pindolol have been tested for their ability to reduce isoprenaline-induced bronchodilation and tachycardia in the anaesthetized dog. 2. Atenolol, metoprolol, acebutolol and practolol all possessed a similar degree of cardioselectivity in this animal model.     

DECREASED CARDIAC β-ADRENOCEPTORS IN HYPERTENSIVE RATS

1. β-Adrenoceptors have been investigated in cardiac and renal cell membranes from hypertensive and normal rats. 2. The ¹²⁵I-labelled β-adrenoceptor antagonist l-(4-iodophenoxy)-3-isopropyl-aminopropan-2-ol was used to measure directly the number and affinity of receptors. 3. Cardiac membranes from hypertensive rats had a lower concentration of β-adrenoceptors than membranes from control normotensive animals, but the affinities of the receptors remained unchanged. Receptor concentration decreased by half and was similar in each of the three models of experimental hypertension investigated. 4. In contrast, kidney membranes from hypertensive animals had the same β-adrenoceptor concentration and affinity as membranes from normotensive rats. 5. The decrease in cardiac β-adrenoceptor concentration may reflect an increase in cardiac sympathetic drive in these experimental hypertensive models.     

FACILITATION OF SYMPATHETIC TRANSMISSION BY PREJUNCTIONAL β-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.     

THE EFFECT OF β-ADRENOCEPTOR BLOCKERS ON THE ABSORPTION AND EXCRETION OF CHLOROTHIAZIDE IN MAN

The effect of beta-adrenoceptor blockers on the absorption and elimination of the diuretic chlorothiazide was studied in healthy subjects. A week of pretreatment with either pindolol (10 mg twice daily) or propranolol (80 mg twice daily) resulted in significant reduction in 36 h mean cumulative urinary recovery of chlorothiazide in two groups of six subjects compared with a control (untreated) group. A week of pretreatment with atenolol (100 mg daily) did not significantly alter 36 h cumulative urinary excretion in another group of six subjects. None of the beta-blockers significantly changed chlorothiazide half-life. It is suggested that the non-selective (as opposed to the cardioselective) beta-blockers reduce chlorothiazide absorption by the mechanism(s) discussed.     

THE INFLUENCE OF β-ADRENOCEPTOR ANTAGONISTS ON ACCOMMODATION OF THE LENS

1. A study of the influence of single oral doses of the adrenergic β-receptor antagonists alprenolol (100 mg) and propranolol (50 mg) on the accommodation of the lens was made in six healthy male subjects. 2. There was no significant influence of either drug on lens accommodation.     

FURTHER INVESTIGATIONS ON THE α1-ADRENOCEPTOR BLOCKING PROPERTIES OF AR-C 239 IN RATS

AR-C 239, a new alpha-adrenoceptor blocking drug, appears to act selectively on alpha 1 sites in rats. At peripheral sites, this drug did not change the tachycardia induced by spinal sympathetic outflow stimulation in pithed rats, and did not antagonize the inhibitory effects of clonidine on this preparation. In addition, AR-C 239 showed predominant alpha 1-adrenoceptor blocking properties in the bisected rat vas deferens preparation. AR-C 239 did not prevent or reverse the centrally mediated hypotensive and bradycardic actions induced by clonidine, in intact animals. In conclusion, AR-C 239 seems to be a very useful tool for the characterization of peripheral and central alpha 1-adrenoceptors, in this animal species.     

THE EFFECT OF δ9-TETRAHYDROCANNABINOL (δ9-THC) ON THE TURNOVER RATE OF BRAIN SEROTONIN OF THE RAT

1. One isotopic and three non-isotopic methods were used to determine the effect of an acute intravenous dose of Δ⁹-tetrahydrocannabinol (Δ⁹-THC, 2 mg/kg) on the rat brain turnover rate of serotonin. 2. In control animals the turnover rate of serotonin was about 2 nmol/g per h. This rate was not altered by Δ⁹-THC when it was calculated from the rise of 5-hydroxyindoleacetic acid following probenecid or from the rise of serotonin following pargyline. 3. Δ⁹-THC did not alter the serotonin turnover rate when it was calculated from the conversion of ³H-tryptophan to ³H-serotonin. 4. The serotonin turnover rate was significantly increased by Δ⁹-THC when the rate was calculated from the decline of 5-hydroxyindoleacetic acid following pargyline. 5. These results suggest that Δ⁹-THC does not alter the turnover of rat brain serotonin. The previously reported Δ⁹-THC-induced changes in body temperature and increased brain levels of 5-hydroxyindoleacetic acid may be mediated by some other mechanism such as interference by Δ⁹-THC of the vesicular binding of serotonin.     

EFFECTS OF β-ADRENOCEPTOR BLOCKADE ON PROSTACYCLIN MEDIATED RENIN RELEASE IN SHEEP

1. The effect of prostacyclin (PGI₂) infusion on plasma renin concentration (PRC) was examined before and after propranolol treatment in sheep. 2. Increasing doses of prostacyclin (0.05, 0.1 or 0.3 μg/kg per min) produced dose dependent increases in PRC. 3. There was a significantly lower PRC response after propranolol at 0.3 μg/kg per min only.     

β-ADRENOCEPTOR DESENSITIZATION DURING THE DEVELOPMENT OF CANINE PACING-INDUCED HEART FAILURE

1. The goal of this review is to emphasize four major points regarding the development of catecholamine desensitization in heart failure (HF). 2. Catecholamine desensitization occurs prior to the development of HF (i.e. after 1 day of rapid pacing, physiological responses to β-adrenoceptor stimulation are depressed by over 50%, yet no evidence of HF is observed for 3-4 weeks of rapid pacing). 3. Multiple mechanisms in the β-adrenoceptor cascade are involved. In HF there are decreases in β₁-adrenoceptors, high affinity β-adrenoceptors, adenylyl cyclase activity and messenger RNA and increases in G_i. 4. Not all mechanisms appear simultaneously (i.e. early decreases occur in high affinity β-adrenoceptors and adenylyl cyclase; late increases in G_i and decreases in β-adrenoceptor density evolves). 5. Mechanisms distal to cAMP generation also play a role (i.e. alterations in ryanodine receptor binding and excitationcontraction coupling also occur).     

THE INTERACTION OF EPHEDRINE WITH β-ADRENOCEPTORS IN TRACHEAL, CARDIAC AND SKELETAL MUSCLES

Three beta-adrenoceptor mediated effects were measured in vitro on tissues from guinea-pig: (a) relaxation of the trachea (mainly beta 2), (b) increase in the force of contraction of the papillary muscle (beta 1), and (c) depression of the subtetanic contractions of the soleus muscle (beta 2). The relaxant effect of ephedrine on the trachea was weak but was resistant to reserpine pretreatment. There was no appreciable agonistic effect of ephedrine on the soleus muscle. Ephedrine per se had a marked positive inotropic effect on the papillary muscle with a pD2 of about 6.0. This effect disappeared completely after pretreatment with reserpine. Ephedrine inhibited the response to isoprenaline. The apparent pA2 was about 4.8 in all tissues studied.     

Clinical pharmacology of β3-adrenoceptors

1An atypical non β₁/β₂-adrenoceptor (AR) subtype (β₃-AR) has been identified which is selectively stimulated by a group of ligands which mediate lipolytic and thermic responses in brown and white adipose tissue. 2Molecular studies have shown that β₃-AR in man are mainly expressed in visceral adipocytes, and to a lesser extent in gall-bladder and colon. In vitro studies with β₃-AR agonists have shown activity at other sites including skeletal muscle and myocardium. 3Regulation of β₃-AR may differ from β₁/β₂-AR subtypes in that continuous agonist exposure does not result in receptor down-regulation. 4A polymorphism of the human β₃-AR gene (Trp64Arg) has been identified which is associated with obesity, insulin resistance and an earlier onset of non-insulin-dependent diabetes mellitus (NIDDM). Studies are required to establish whether expression of the mutant gene results in altered metabolic responses to β₃-AR stimulation in man. 5There is accumulating evidence to support a therapeutic role of β₃-AR agonists in NIDDM because of anti-obesity and anti-diabetic activity, as a consequence of thermogenic effects as well as increased insulin sensitivity and glucose tolerance. 6Selectivity studies with BRL35135 and isoprenaline in humans have demonstrated a β₃-AR mediated component to thermogenesis which is dissociated from β₁/β₂-mediated effects on carbohydrate and fat metabolism. Similar studies have suggested a functional β₃-AR mediating cardiac but not airway responses in humans. An evaluation of β₃-AR agonists in irritable bowel syndrome may be warranted in view of colonic antimotility properties in vitro.     

EFFECTS OF THE α2-ADRENOCEPTOR AGONIST UK14304 ON PRESSOR RESPONSES IN PITHED RATS

1. Intravenous infusions of UK14304 (0.3-10 micrograms/kg per min) in pithed rat produced dose-dependent pressor responses which were not affected by prazosin (10 micrograms/kg) but were reduced by yohimbine (0.3 mg/kg). 2. Pressor responses to noradrenaline (0.1 micrograms/kg), phenylephrine (1 micrograms/kg) and vasopressin (10 mU/kg) were enhanced during infusions of UK14304 (0.03-1 micrograms/kg per min). Likewise, pressor responses to spinal sympathetic stimulation were enhanced during infusions of low concentrations of UK14304 (0.03-0.3 microgram/kg per min) but were reduced during infusion of a higher concentration of UK14304 (10 micrograms/kg per min). 3. After administration of yohimbine (0.3 mg/kg) or the calcium channel blocking drug diltiazem (infused at 50 micrograms/kg per min), pressor responses to noradrenaline and UK14304 were reduced, and responses to noradrenaline during infusion of UK14304 were not enhanced. 4. Prazosin (10 micrograms/kg) revealed a secondary depressor component in the response to sympathetic stimulation which is due to beta-adrenoceptor activation, since it was abolished by ICI 118551 (0.3 mg/kg). In the presence of ICI 118551 plus prazosin, pressor responses to sympathetic stimulation were enhanced during infusions of UK14304. 5. The depressor response to nitroprusside and the depressor component of responses to sympathetic stimulation after prazosin were enhanced during infusions of UK14304 at concentrations that increased the blood pressure. 6. The findings show that alpha 2-adrenoceptor activation enhanced the pressor responses to sympathetic nerve stimulation, noradrenaline, phenylephrine and vasopressin in the pithed rat and beta-adrenoceptor activation produced depressor responses which increased with increasing blood pressure.     

SPECIES DIFFERENCE IN THE β1/β2-ADRENOCEPTOR SELECTIVITY OF SM220CL IN THE CAT AND GUINEA-PIG

1. The β-receptor stimulant effects of Sm220Cl, dl-N- (1,1-dimethyl-3-phenylpropyl)-2-hydroxy-2(3,4-dihydroxy-2-methoxyphenyl)ethylamine, and (-)-isoprenaline have been compared in isolated atrial (β₁) and tracheal (β₂) preparations from guinea-pigs and cats. 2. The compounds were also tested for their ability to increase the heart rate (β₁), reduce serotonin-induced increases in pulmonary resistance (β₂), and decrease soleus muscle contractility (β₂) in vivo in the two species. 3. In all experiments cumulative concentration or dose-effect curves were established, EC₅₀ or ED₅₀ values obtained and molar activity-ratios (Sm220Cl: (-)-isoprenaline) calculated. 4. Calculated selectivity ratios [activity-ratio (heart):activity-ratio (bronchial smooth muscle)] from the in vitro experiments showed that Sm220Cl possessed β₂-receptor selectivity. This was more marked in guinea-pig than in cat preparations. 5. In the anaesthetized animals this species difference was more apparent; in cats Sm220Cl was non-selective in its actions for β₁- and β₂-receptor mediated responses, while marked β₂-receptor selectivity was obtained in the guinea-pig. 6. Since in both species the activity-ratios for β₂-receptor mediated actions are similar, the differences in the β₁/β₂-receptor selectivity of Sm220Cl are caused by the divergent cardiac effects produced by the drug.