THE CHRONIC EFFECTS OF β-ADRENORECEPTOR ANTAGONISTS ON THE EFFLUX OF TRITIATED NORADRENALINE FROM SYMPATHETIC NERVES OF THE PITHED RAT

• 1 The effect of chronic administration of two β-adrenoreceptor blocking drugs, propranolol and timolol, on transmitter noradrenaline release from sympathetic nerves has been investigated in vivo using the pithed rat preparation.
• 2 Oral treatment for 4 weeks with either propranolol (46.3 mg/kg/day) or timolol (7.1 mg/kg/day) significantly raised the stimulation frequency threshold for release of radioactivity on stimulation of the whole spinal outflow of the pithed rat after i.v. injection of ³H-NA.
• 3 No differences in the stimulation-evoked rise in mean arterial pressure were observed between control or treated rats nor was the heart rate response to stimulation altered after timolol treatment. However, in propranolol treated rats the mean rises in heart rate were significantly higher with 3 and 30 Hz stimulation than in control rats.
• 4 Timolol treatment significantly increased the blood concentration and lowered the heart content of ³H-NA whilst propranolol treatment did not significantly change either blood or heart levels.
• 5 Log dose-response curves for mean rises in heart rate after i.v. isoprenaline were not shifted to the right in either propranolol or timolol treated pithed rats. With the lowest doses of isoprenaline (1 and 25 ng), the mean rises in heart rate in timolol treated rats were significantly greater than in the controls.
• 6 Thus chronic administration of propranolol or timolol decreased the stimulation-induced increase in plasma ³H-NA but this change in release was not related to reduced rises in blood pressure or heart rate.

     

β-ADRENORECEPTOR ANTAGONISTS WITH MULTIPLE PHARMACOPHORES: PERSISTENT INHIBITION OF RAT HEART ADENYLATE CYCLASE

• 1 β-Adrenoreceptor antagonists containing several pharmacophores, so called alprenolol-Jeffamines, were studied. These compounds are derived from Jeffamine^R NH₂-CH-(CH₃)-CH₂-[-O-CH₂-CH(CH₃)-]_n- NH₂₉n = 2.6 ave. by substitution of nitrogen with 3-(2-allylphenoxy)-2-hydroxypropyl groups, which are part of the Alprenolol^R pharmacophore.
• 2 Alprenolol-Jeffamines inhibited (-)isoprenaline stimulated adenylate cyclase activity; the derivative with one pharmacophore was about 4-fold and the derivative with two pharmacophores was about 17-fold less potent than (+)alprenolol; the trisubstituted derivative which has one complete and two partial pharmacophores was ineffective.
• 3 The ratio of K_i's for inhibition of (-)³H-DHA binding and for inhibition of adenylate cyclase were approximately one for each derivative.
• 4 When (+)alprenolol-Jeffamine derivatives were injected intraperitoneally into rats and heart membranes or homogenates were prepared 18–20 h afterwards, the mono, di, and trisubstituted derivatives, but not (+)alprenolol, inhibited bindng of (-)³H-DHA. This persistency pattern is different from that observed in vitro, where only di and trisubstituted derivatives are persistent. Slow metabolism/slow excretion of the monosubstituted derivative may be a source of the increased persistency in vivo.
• 5 In similarly prepared animals, the dose-response curve for (-)isoprenaiine stimulated adenylate cyclase was shifted to the right 3-to-4 fold for mono and disubstituted derivatives but was unaffected by (+) alprenolol and the trisubstituted derivative.
• 6 The results suggest that these derivatives interact with physiologically important β-adrenoreceptors in vitro, and that, in vivo, they persistently block β-adrenoreceptors and inhibit isoprenaline stimulated adenylate cyclase.

     

MECHANISMS OF THE EFFECTS OF ENDOTHELIN ON RESPONSES TO NORADRENALINE AND SYMPATHETIC NERVE STIMULATION

1. The effects of endothelin (0.1-1 nmol/L) and the phorbol ester, phorbol 12-myristate 13-acetate (PMA, 0.1-100 nmol/L) have been studied on vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline in the rabbit isolated ear artery. 2. Endothelin (0.1 nmol/L) significantly enhanced the biphasic response of the arteries to a prolonged period of sympathetic nerve stimulation (2 Hz for 60 s). Both the component of the response attributable to mobilization of intracellular calcium (phasic response) and the component due to the influx of extracellular calcium (tonic response) were enhanced. 3. PMA (0.1 and 0.3 nmol/L), which activates protein kinase C, enhanced vasoconstrictor responses to noradrenaline but a higher concentration of PMA (100 nmol/L) inhibited responses to noradrenaline. 4. It is concluded that the enhancement of responses to sympathetic nerve stimulation and noradrenaline produced by endothelin may be due to facilitation of the influx of extracellular calcium and to the activation of protein kinase C.     

IMPAIRMENT BY NIFEDIPINE OF VASOPRESSOR RESPONSES TO STIMULATION OF POSTSYNAPTIC α2-ADRENORECEPTORS IN GANGLION-BLOCKED RABBITS. FURTHER EVIDENCE FOR THE SELECTIVE INHIBITION OF POSTSYNAPTIC α2-ADRENORECEPTOR-INDUCED PRESSOR RESPONSES BY CALCIUM ANTAGONISTS

• 1 After ganglionic blockade and bilateral vagotomy, vasopressor responses induced by activation of postsynaptic α₁- and α₂-adrenoreceptors were elicited in the intact circulatory system of rabbits.
• 2 The hypertensive effects of the selective stimulating agents methoxamine (α₁-agonist) and B-HT 920 (α₂-agonist) were effectively antagonized by the adrenoreceptor antagonists prazosin and yohimbine, respectively. These findings confirm the existence of two types of postsynaptic α-adrenoreceptors (α₁-and α₂-type) in vascular smooth muscle of rabbits.
• 3 The calcium antagonistic drug nifedipine did not affect the maximal increase in diastolic pressure brought about by methoxamine, whereas it strongly inhibited the hypertensive effects of B-HT 920.
• 4 It is concluded that this confirmation of the selective inhibition of postjunctional α₂-adrenoreceptor-mediated vasopressor responses by a calcium antagonistic drug, such as nifedipine, indicates that this activity constitutes a general phenomenon. This finding supports the hypothesis that an influx of extracellular calcium is necessary for the vasoconstriction mediated by postsynaptic α₂-adrenoreceptors.

     

γ-AMINOBUTYRIC ACID AND POSTGANGLIONIC SYMPATHETIC TRANSMISSION IN THE PULMONARY ARTERY OF THE RABBIT

• 1 The effect of γ-aminobutyric acid (GABA) on postganglionic sympathetic neurotransmission was studied in strips of the rabbit pulmonary artery. The strips were preincubated with ³H-noradrenaline and then superfused with ³H-amine-free medium. They were stimulated either electrically at 2 Hz, or by 60 mM potassium, or by 1 μM tyramine.
• 2 GABA (1 – 1000 μM) did not change the basal outflow of tritium, but decreased the electrically evoked overflow as well as the contractile response. GABA 1 μM decreased the evoked overflow by 12%, and GABA 1000 μM, by 42%. The effect of GABA was not changed by yohimbine, propranolol, cocaine, corticosterone, or indomethacin. It was not antagonized by picrotoxin or bicuculline methiodide. GABA 100 μM also slightly reduced the potassium-evoked overflow of tritium but did not change the tyramineevoked overflow.
• 3 The results show that, in the pulmonary artery of the rabbit, GABA inhibits the release of noradrenaline. Its effect is independent of α- and β-adrenoreceptors and is not mediated by prostaglandins. The effect may be due to activation of presynaptic receptors which appear to differ from conventional GABA receptors inasmuch as they are insensitive to blockade by either picrotoxin or bicuculline.

     

DIFFERENTIAL INHIBITION OF SERINE PROTEINASES BY RABBIT α 1-PROTEINASE INHIBITORS F AND S

Inhibition of six serine proteinases (bovine trypsin and chymotrypsin, equine leucocyte proteinases type 1 and 2A, porcine pancreatic elastase type III and rabbit plasmin) by rabbit α₁-proteinase inhibitors F and S was studied. In each case examined, the F form reacted more rapidly. The number of moles of an enzyme inhibited by one mole of α₁ -proteinase inhibitor in a complete reaction (molar inhibitory capacity) ranged from 0.26 (leucocyte proteinase type 1) to 1.01 (trypsin). More significantly, however, the molar inhibitory capacities of both α₁-proteinase inhibitors differed for the same enzymes. The highest F/S inhibitory ratio was recorded with chymotrypsin (1.88), and the lowest with elastase (0.69). These differences in molar inhibitory capacities are likely to reflect the dual nature of the reaction between the inhibitor and a proteinase, that is, either complex formation or inactivation of α₁-proteinase inhibitor without enzyme inhibition. No evidence was obtained to suggest that differential reactivity and differential inhibitory capacity are interdependent. The observations are consistent with the view that rabbit α₁-proteinase inhibitors F and S are closely related yet functionally distinct proteins.     

THE ACTIONS OF SOME α-ADRENORECEPTOR AGONISTS AND ANTAGONISTS IN AN ANTINOCICEPTIVE TEST IN MICE

1. It has been shown that a number of sympathomimetic drugs, administered subcutaneously, have potent antinociceptive activity in the mouse abdominal constriction test. These drugs were found to be equieffective antagonists of the nociceptive action of acetic acid and acetylcholine. Of the drugs tested, clonidine was the most potent, being almost 60 times more active than morphine, whilst noradrenaline and oxymetazoline were approximately three and five times less active respectively than clonidine. Phenylephrine was without effect after doses of 2 mg/kg. 2. The regression lines relating the magnitude of the antinociceptive effect of noradrenaline and oxymetazoline to log dose were moved to the right in an approximately parallel manner after subcutaneous administration of piperoxane, 8 and 16 mg/kg. The antinociceptive action of clonidine was also antagonised by piperoxane, but to a lesser extent. Phentolamine had no antagonistic effect on any of these three a-agonists after subcutaneous doses of 16 mg/kg. 3. Oxymetazoline and clonidine, when given by intracisternal injection, were approximately eight-three and fifty times more potent respectively than by subcutaneous administration, and their antinociceptive action was not antagonized by piperoxane, 16 mg/kg subcutaneously, or 50 μg/kg intracisternally. 4. It is postulated that the antinociceptive action of the a-agonists is due to an effect on α-adrenoreceptors located on sensory nerve endings in the peritoneum, and that the affinities of these receptors for the α-agonists and antagonists is different from that shown by either pre- or postsynaptic α-adrenoreceptors. It also appears likely that the antinociceptive action of clonidine and oxymetazoline when given by intracisternal injection involves different mechanisms from their peripheral effects.     

BLOCKADE OF THE CENTRAL HYPOTENSIVE EFFECT OF CLONIDINE BY α-ADRENORECEPTOR ANTAGONISTS IN RATS, RABBITS AND DOGS

SUMMARY 1. Clonidine and three α-adrenoreceptor blocking agents were injected into the cisterna magna of rats, rabbits and dogs. Clonidine (1 μg. kg⁻¹) induced a fall in blood pressure in the three species.
2. Phentolamine, tolazoline and phenoxybenzamine (100 μg. kg⁻¹) induced a fall in blood pressure in rats, but only phentolamine was effective in rabbits, and none of these drugs significantly altered blood pressure in dogs when given intracisternally.
3. A subsequent administration of clonidine after recovery of blood pressure no longer decreased blood pressure in rats and rabbits.
4. In dogs the effects of clonidine were antagonized by tolazoline and phentolamine, but not by phenoxybenzamine.
5. These results indicate that clonidine stimulates central α-adrenoreceptors to produce a decrease in blood pressure, but the structure of these receptors may vary according to animal species.     

国产乙酰普马嗪的药理作用研究——加强作用及对交感神经末梢介质释放的影响

(1)采用热水刺激小白鼠尾巴法試驗鎮痛作用,乙酰普馬嗪0.5毫克/公斤可使“疼痛”反应出現时間延迟,較盐酸嗎啡(2毫克/公斤)为弱;二者均以半量合并应用,鎮痛作用强度虽未見加强,但鎮痛时間則延长。(2)脑室內注射乙鮂振R嗪,可立即引起小白鼠安靜,并使其体溫明显下降;皮下注射同剂量时,安靜及降溫作用均不显著,但可明显加强安替比林及水合氯醛的降溫作用。(3)乙酰普馬嗪局部滴药及皮下注射均可产生角膜麻醉,并可加强普魯卡因的表面麻醉作用,尤以皮下注射法为强。(4)乙酰普馬嗪2.5微克/公斤即可減弱腎上腺素的升压作用;5微克/公斤时,使后者作用翻轉,并使电刺激交感神經节前纤維及注射腎上腺素所引起的瞬膜收縮反应減弱。(5)在离体兔神經——迴腸标本,电刺激交感神經节后纤維引起腸张力下降,运动減弱,乙酰普馬嗪及氯丙嗪均可加强此交感反应。     

HETEROGENEITY OF β-ADRENORECEPTOR SUBTYPES IN THE HUMAN PLACENTA

• 1 The human placenta is a rich source of β-adrenoreceptors. However, previous work has provided conflicting evidence as to the predominant receptor subtype present. This study was designed to clarify this situation.
• 2 The radioligand ³H-dihydroalprenolol (³H-DHA) was used to identify β-adrenoreceptor binding sites in membranes prepared from human placentae obtained immediately post partum from normal full term pregnancies.
• 3 ³H-DHA binding to human placental membranes was saturable, of high affinity and exhibited the pharmacological characteristics of a β-adrenoreceptor. Displacement by β-adrenoreceptor agonists suggested a predominant β₁-subtype with a hierarchy of potency isoprenaline > adrenaline > noradrenaline. Analysis of the displacement of the specific binding of ³H-DHA by β-adrenoreceptor antagonists yielded Hill plots with an apparent slope (nH) of one for the non selective antagonist (?)propranolol, and of less than one for the highly selective antagonists practolol (β₁), and ICI 118,551 (β₂) indicating a possible heterogeneity of binding sites.
• 4 A direct comparison of the displacement of ³H-DHA binding by selective antagonists in the placenta with that occurring in rat and rabbit lung, further established that a mixture of β₁- and β₂-adrenoreceptor binding sites was present.
• 5 Graphical analysis by Hofstee plot and computer assisted iterative curve fitting was used to further evaluate the displacement by the selective agents and established the presence of an heterogeneous population of β-adrenoreceptor subtypes in the human placenta with approximately 65%β₁ and 35%β₂.

     

STUDIES ON THE PRE- AND POSTJUNCTIONAL ACTIVITIES OF α-ADRENORECEPTOR AGONISTS AND THEIR CARDIOVASCULAR EFFECTS IN THE ANAESTHETIZED RAT

• 1 The selectivity of a number of agonists for peripheral α₁- and α₂- adrenoreceptors was determined and related to their cardiovascular effects in pentobarbitone-anaesthetized rats.
• 2 In isolated tissues, presynaptic α₂- and postsynaptic α₁-adrenoreceptor activity was determined on the rat vas deferens and anococcygeus muscle respectively. Xylazine, guanabenz, guanoxabenz and guanfacin were more selective, whilst lofexidine, tiamenidine, naphazoline, oxymetazoline and St 91 were less selective than clonidine for presynaptic α₂-adrenoreceptors.
• 3 The anococcygeus muscle of the pithed rat was used to determine the α-adrenoreceptor selectivity of the compounds in vivo. The selectivities were similar to those obtained in vitro.
• 4 Following intravenous administration to pentobarbitone-anaesthetized rats guanabenz and guanfacin caused small transient pressor responses and a prolonged secondary hypotension. In contrast oxymetazoline and St 91 produced marked initial pressor responses and the secondary reduction in blood pressure was absent. Clonidine and tiamenidine produced marked initial pressor responses followed by secondary hypotensive effects. All compounds reduced heart rate.
• 5 Intracerebroventricular (i.c.v.) administration of clonidine, guanabenz and guanfacin elicited falls in blood pressure and heart rate. In contrast blood pressure was either unaffected or elevated following i.c.v. administration of St 91, oxymetazoline and tiamenidine.
• 6 It is concluded that in the normotensive rat, central α₂-adrenoreceptors play a major role in the hypotensive effects of clonidine and related compounds.

     

COMPARISON OF THE CARDIAC EFFECTS OF β-ADRENORECEPTOR AGONISTS IN ANAESTHETISED AND CONSCIOUS DOGS

• 1 The cardiovascular effects of some β-adrenoreceptor agonists on heart rate, blood pressure and myocardial contractility (maximum rate of change of left ventricular pressure/integrated isometric tension) were measured in pentobarbitone-anaesthetised and conscious, instrumented greyhounds.
• 2 In anaesthetised dogs isoprenaline increased heart rate and myocardial contractility and reduced blood pressure. Prenalterol and RO 363, in equiactive inotropic doses, induced greater increases in heart rate than isoprenaline if blood pressure fell by less than 25 mmHg. Salbutamol had hypotensive activity at all doses and appeared to be a relatively selective inotrope.
• 3 None of the agonists caused blood pressure to fall in the conscious dogs. Prenalterol and RO 363 were more effective inotropic stimulants, producing smaller increases in heart rate and more pronounced increases in myocardial contractility. Salbutamol, however, elicited greater increases in heart rate in the conscious animals and the inotropic selectivity demonstrated in the anaesthetised animals was lost.
• 4 The direct effects of the β-adrenoreceptor agonists, without modification by reflexes could be observed in the anaesthetised animals. The differences in the actions of the agonists in the conscious animals appear to be attributable to the state of the baroreceptor reflex control system and the relatively enhanced responsiveness of the heart.