The physiological and pharmacological role of presynaptic alpha- and beta-adrenoceptors in man.

Two studies were performed each in six normal volunteers in order to find evidence of either a physiological or pharmacological role of presynaptic alpha- and presynaptic beta-adrenoceptors in man. In Study 1 subjects received a 60 min infusion of guanfacine 3 mg (alpha 2-adrenoceptor agonist) preceded by either idazoxan (alpha 2-adrenoceptor antagonist) or vehicle. Guanfacine reduced plasma noradrenaline concentration by approximately 30% and this fall was not antagonised by the alpha 2-receptor antagonist. The 30-fold increase in plasma growth hormone, measured as a marker of the central action of guanfacine, was almost completely blocked by idazoxan. A comparison of the drug concentrations of idazoxan and guanfacine, together with their relative affinities for alpha 2-adrenoceptors, suggested that the idazoxan could not block the peripheral actions of guanfacine and that these were responsible for the fall in plasma noradrenaline concentration. In Study 2 adrenaline 0.05 micrograms kg-1 min-1 was infused for 80 min preceded by either idazoxan or vehicle. After vehicle, adrenaline caused no change in plasma noradrenaline concentration whereas it rose approximately 25% after administration of idazoxan. This was probably due to unmasking of presynaptic beta-adrenoceptor stimulation by adrenaline when the opposing inhibitory autoreceptor was blocked.     

Pharmacological profile of the novel alpha-adrenoceptor antagonist KT-611 (naftopidil).

The pharmacological profile of a new alpha-adrenoceptor antagonist, KT-611 (naftopidil), was studied in vitro. In the dog mesenteric and carotid arteries and in the rabbit, guinea pig and rat thoracic aortae, KT-611 competitively inhibited alpha 1-adrenoceptor-mediated contractions induced by noradrenaline with pA2 values ranging from 6.73 to 8.15. KT-611 also inhibited the postjunctional alpha 2-adrenoceptor-mediated contractions in the dog saphenous vein (pA2 = 6.77) or dog basilar artery. However, the responses mediated through prejunctional alpha 2-adrenoceptors (rat vas deferens), beta-adrenoceptors (rat atria), muscarinic receptors (guinea pig ileum) and 5-HT2 receptors (dog mesenteric artery) were little affected by KT-611. KT-611 also inhibited the sympathetic adrenergic contraction evoked by electrical transmural stimulation in the dog mesenteric artery, and the inhibition was not relieved upon repetitive washing for 1 hour with the drug-free solution. 3H-prazosin and 3H-clonidine binding to the rat cortex membranes was inhibited by KT-611 with pKi values of 7.69 and 5.75, respectively. These results suggest that KT-611 is an alpha 1-adrenoceptor antagonist with a weak antagonistic activity to postjunctional alpha 2-adrenoceptors.     

Changes in neuromuscular transmission of guinea pig vas deferens produced by decamethrin treatment

Responses of the isolated vas deferens of guinea pig to clonidine (inhibition of contractions to field stimulation at 2.5 Hz), tyramine (inhibition of contractions to field stimulation at 10 Hz), prostaglandin E2 (inhibition of contractions to field stimulation at 10 Hz), and noradrenaline (contraction of longitudinal muscle) were determined after administration of decamethrin (18 mg kg-1, ip) once a day for 3 consecutive days. Treatment with decamethrin produced a subsensitivity of the prejunctional alpha 2-adrenoceptor system as evidenced by the fact that the alpha 2-agonist clonidine was less effective in decreasing nerve-stimulated induced contractions of the vas deferens. In addition, the presynaptic action of tyramine on postganglionic motor transmission was impaired. However, no detectable changes in the inhibition by prostaglandin E2 of twitch responses were produced by decamethrin. Decamethrin treatment had a significant effect on noradrenaline responsiveness, causing an increase in the maximum contractile response, indicative of an enhanced postreceptor mechanism. The present results suggest that decamethrin treatment reduces peripheral presynaptic adrenoceptor sensitivity. This reduction will lead subsequently to increased noradrenaline release and postsynaptic adrenoceptor upregulation.     

Histamine H3-receptors inhibit sympathetic neurotransmission in guinea pig myocardium.

The histamine H3 agonist, (R)-alpha-methylhistamine (alpha-MeHA, 10(-10) to 10(-5) M), caused a concentration-dependent inhibition of the sympathetic contractile response to electrical field stimulation of guinea pig isolated atria, but alpha-MeHA did not alter the basal tension or the contraction induced by exogenously applied norepinephrine. Blockade of H1 and H2 histamine receptors, and alpha- and beta-adrenoceptors failed to prevent the inhibitory effect of alpha-MeHA, whereas the specific H3 receptor antagonist, thioperamide, concentration dependently reversed the inhibitory effect of alpha-MeHA. At the concentration of 10(-7) M, which was effective for antagonizing the action of alpha-MeHA, thioperamide did not modify the sympathetic responses facilitated by the beta 2-adrenoceptor agonist, clenbuterol, or attenuated by the alpha 2-adrenoceptor agonist, clonidine. Our results suggest that H3 receptors exist on the cardiac sympathetic terminals, which may modulate adrenergic neurotransmission in guinea pig myocardium.     

Different mechanisms of positive chronotropic actions of isoprenaline and noradrenaline on isolated guinea pig right atria revealed by pretreatment with rilmakalim

In this study we tested the influence of activation of ATP-sensitive K+ channels (KATP) on the changes in automatism induced by isoprenaline, noradrenaline and phenylephrine. Experiments were performed on the spontaneously beating right atria isolated from guinea pig. The rate of spontaneously beating preparations was measured under different experimental conditions. Rilmakalim (formerly HOE 234) was used as an activator of KATP channels. Isoprenaline induced significant, concentration-dependent positive inotropic action. This effect was strongly attenuated only in the presence of selective blockers of beta1- (metoprolol), but not beta2-adrenoceptor subtype (ICI 11855). Pretreatment with 4 microM rilmakalim resulted in a significant increase in the described effects of isoprenaline on automatism of isolated right atria. Phenylephrine (1 to 100 microM) in the presence of 1 microM propranolol, did not cause any changes in automatism of guinea pig right atria. Slight but significant positive chronotropic action induced by noradrenaline at lower concentrations (0.1 to 10 microM) in the presence of 1 microM propranolol was significantly decreased by pretreatment with rilmakalim. However, the effects obtained at higher concentrations (30 and 100 microM) of noradrenaline were enhanced. Interactions mentioned above were prevented by addition of 3 microM glibenclamide. The results imply that positive chronotropic effect of noradrenaline in the presence of propranolol is mediated by adrenoceptor subtype different from alpha1-, beta1- and beta2-adrenoceptors.     

Increase of alpha 2-presynaptic adrenoceptor response after neuronal uptake inhibition in vivo

alpha 2-adrenoceptor antagonists, yohimbine or idazoxan (1 mg kg-1 i.p.), administered alone, did not change noradrenaline content in the central and peripheral tissues of the rat (hypothalamus, brain stem, frontal cortex and heart). The inhibition of neuronal uptake by desipramine (DMI) administered alone or prior to alpha 2-adrenoceptor antagonists did not affect the neurotransmitter content either. alpha-methyl-p-tyrosine (alpha-MT) 6 hr before sacrifice, induced a marked disappearance of the noradrenaline content, greater in central nervous tissues than in heart. When the catecholamine synthesis was inhibited by alpha-MT, neither alpha 2-adrenoceptor antagonists nor DMI at the dose used, significantly changed the disappearance rate of noradrenaline in any of the tissues studied. Under these experimental conditions, however, the combination of DMI plus alpha 2-adrenoceptor antagonists significantly decreased the neurotransmitter content in all tissues when the values were compared with the control or DMI-treated groups. The present results might suggest evidence in favour of a functional coupling between presynaptic alpha 2-autoreceptors and noradrenaline uptake mechanism.     

Contrasting effects of the imidazol(in)e alpha 2-adrenoceptor agonists, medetomidine, clonidine and UK 14,304 on extraneuronal levels of noradrenaline in the rat frontal cortex: evaluation using in vivo microdialysis and synaptosomal uptake studies.

1. In vivo microdialysis in halothane-anaesthetized rats and synaptosomal [3H]-noradrenaline uptake studies in vitro were used to evaluate the effects of imidazole (medetomidine) and imidazoline (clonidine and UK 14,304) alpha 2-adrenoceptor agonists on extraneuronal levels of noradrenaline in the frontal cortex. 2. Levels of noradrenaline in the dialysate were increased by a depolarizing concentration of K+ (60 mM for 20 min) and substantially attenuated by reducing Ca2+ supply in the perfusate. These results suggest that spontaneous efflux of noradrenaline in the cortex is regulated predominantly by cation-dependent exocytotic mechanisms. 3. At a low perfusion concentration (0.5 microM), medetomidine, clonidine and UK 14,304 all reduced the level of noradrenaline in cortical dialysates. Continuous perfusion of the selective alpha 2-adrenoceptor antagonist, atipamezole (0.5 microM) caused a sustained increase in noradrenaline efflux and reversed the inhibitory effects of medetomidine. All these changes are consistent with drug actions at presynaptic alpha 2-adrenoceptors. 4. Higher concentrations of medetomidine (5-50 microM), but not clonidine or UK 14,304, evoked a non-desensitizing increase in noradrenaline efflux. This effect was not antagonized by 0.5 microM atipamezole. 5. The tricyclic noradrenaline reuptake inhibitor, desmethylimipramine (0.5-50 microM), increased noradrenaline efflux in a concentration-dependent manner. 6. The specific uptake of [3H]-noradrenaline into cortical synaptosomes was inhibited by medetomidine and desmethylimipramine with IC50 values of approximately 7 microM and 8 microM respectively. Neither clonidine nor UK 14,304 inhibited [3H]-noradrenaline uptake.(ABSTRACT TRUNCATED AT 250 WORDS)     

DISADVANTAGES OF COCAINE AS A NEURONAL UPTAKE BLOCKING AGENT: COMPARISON WITH DESIPRAMINE IN GUINEA-PIG RIGHT ATRIUM

• 1 Cocaine and desipramine (DMI) are widely used as neuronal uptake blocking agents in studies of cardiac sympathetic transmission in isolated tissue preparations. It is generally assumed that these pharmacological tools do not alter transmitter release or postjunctional effector response. To test this assumption, we have compared the effects of cocaine and DMI on rate responses to sympathetic nerve stimulation and exogenous noradrenaline in guinea-pig isolated right atria.
• 2 Right atria were equilibrated with the irreversible α-adrenoreceptor antagonist benextramine to prevent any effect of presynaptic α-adrenoreceptors. Cumulative (–) noradrenaline concentration-response curves were shifted to the left by DMI (0.01–1 μM) without significant change in the resting or maximum rates. Cocaine (1–100 μM) also caused sensitisation to noradrenaline but caused a biphasic change in resting atrial rate. In addition there was a small but significant depression of the maximum rate at cocaine 10 and 100 μM.
• 3 Sympathetic nerve stimulation was achieved by applying trains of 1, 2 and 4 electrical field pulses delivered during one atrial refractory period. DMI caused a concentration dependent potentiation of responses to field stimulation. Cocaine (1 μM) caused significant enhancement of peak responses to field stimulation but no further enhancement and indeed depressed peak responses were observed at cocaine 10 and 100 μM respectively.
• 4 The time for atrial period to return halfway to baseline after field stimulation (t 1/2) was enhanced by cocaine in a concentration dependent manner as was observed with DMI. 5. We conclude that cocaine (but not DMI) decreases the maximum response to exogenous noradrenaline (postjunctional depression). The reduction of the peak response to sympathetic nerve stimulation in the presence of cocaine to below control responses suggests that cocaine also depresses the release of transmitter. These additional depressant properties of cocaine, which occur in a concentration range of neuronal uptake block, are important disadvantages and should discorage its use in experiments on sympathetic transmission.

     

Dysfunctional presynaptic alpha 2-adrenoceptors expose facilitatory beta 2-adrenoceptors in the vasculature of spontaneously hypertensive rats.

Previous studies on spontaneously hypertensive rats (SHR) have yielded inconsistent information about functional aberrations of the presynaptic alpha 2- and beta 2-adrenoceptor-mediated modulation of sympathetic neurotransmitter release. In the present investigation we studied the capacity of presynaptic beta 2-adrenoceptors that enhance noradrenaline (NA) release in the portal vein of freely moving, unanesthetized SHR and normotensive Wistar rats (WR) using the beta 2-selective agonist fenoterol. The results show that the presynaptic beta 2-adrenoceptor population in SHR responds to significantly lower dosages of fenoterol than that in WR. The reason for this enhanced action, however, could not be attributed to the beta 2-adrenoceptor itself, nor to a diminished neuronal uptake of NA, but to a diminished responsiveness of the presynaptic alpha 2-adrenoceptor. Stimulation of presynaptic alpha 2-adrenoceptors with oxymetazoline (45 micrograms/min) decreased basal NA levels by 46% in WR and by 3% in SHR. Blockade of alpha 2-adrenoceptors, using 0.5 mg/kg yohimbine, induced a 4.86-fold rise in the basal NA level in WR but only a 1.89-fold rise in SHR. A subsequent dose of fenoterol, however, resulted in a further 2.5- and 2.6-fold rise in WR and SHR, respectively, indicating that there is a normal presynaptic beta 2-adrenoceptor population in the vasculature of SHR.     

No evidence for reverse trans-synaptic regulation of neuronal uptake by beta-adrenoceptors in kitten atria

Kitten atria incubated with [³H]noradrenaline, 1.18 × 10^?7 M for 10 min or 3 × 10^?9 M for 30 min, actively accumulated the amine. Final tissue tritium concentrations were 2–4-fold and 8–12-fold higher, respectively, than those of the incubation fluid. Uptake was consistently greater in right than in left atria. The β₁-adrenoceptor antagonist, practolol, 10^?6 or 10^?5 M, and the β₁- and β₂-adrenoceptor antagonist, propranolol, 5 × 10^?8, 5 × 10^?7or 5 × 10^?6 M, did not affect noradrenaline uptake. Reverse trans-synaptic regulation of neuronal noradrenaline uptake by β-adrenoceptors therefore does not appear to operate in kitten atria as has been reported for rat atria and other tissues.     

Pharmacological action of SN-408, a novel long-acting selective beta 2-adrenoceptor agonist]

The bronchodilating effect and other related pharmacological properties of SN-408 were studied in comparison with those of isoproterenol, salbutamol and procaterol. SN-408 caused concentration-dependent relaxation of isolated guinea pig trachea via the beta 2-adrenoceptor. The order of relaxation potency was: procaterol greater than SN-408 greater than or equal to isoproterenol greater than salbutamol, but the duration of the action of SN-408 was far longer than those of other beta-agonists. Positive chronotropic and inotropic actions of SN-408 in isolated guinea pig right atria and left atria were less potent than those of other beta-agonists. In isolated guinea pig lung tissues, SN-408 increased cyclic AMP contents. Also in vivo, SN-408 showed dose-dependent bronchodilating action by i.v. administration in anesthetized guinea pigs and by inhalation in conscious guinea pigs. Bronchodilating actions of SN-408 were less potent than those of procaterol and isoproterenol, but the duration of action of SN-408 was far longer than those of other beta-agonists. SN-408 showed no evidence of the development of tolerance to the bronchodilating action. SN-408 caused small tachycardia in guinea pigs by i.v. and inhalation. SN-408 given i.v. suppressed vascular permeability in mice. These results indicate that SN-408 is a long-acting and selective beta 2-stimulant bronchodilator.     

Electrophysiological and antiarrhythmic actions of tocainide in isolated heart preparations of the guinea pig

Comparison of the action of tocainide in guinea pig atrial and ventricular myocardium has led to the following results: Tocainide effects a similar increase in threshold of alternating current induced arrhythmias in spontaneously beating right atria as well as stimulated left atria and papillary muscles. APD30 and APD90 (action potential duration at 30 and 90% repolarization) are decreased by tocainide in papillary muscles, especially at a low stimulation rate, but increased in left atria. On the other hand, maximum upstroke velocity of the action potential (Vmax) is decreased at a high stimulation rate (use-dependence) in both tissues. These effects are completely reversed by washing with drug-free solution within 15 min in papillary muscles and 60 min in left atria, respectively. Resting state block of Na-channels by tocainide occurs in left atria to a small degree (10-20%), but is negligible in ventricular myocardium. Recovery of Vmax following stimulus-induced inactivation is delayed by tocainide in papillary muscles and left atria. Functional refractory period is increased in left atria and in papillary muscles by tocainide. In the concentration range above 75 mumol/l the concentration response curve reaches a plateau in papillary muscles but shows a further steep increase in left atria. Quantitative analysis of the dynamic Na-channel blockade by tocainide in atrial myocardium using the modulated receptor hypothesis suggests that the drug shows a high affinity to activated and a low affinity to inactivated Na-channels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of pertussis toxin-sensitive and -insensitive mechanisms in alpha-adrenoceptor modulation of noradrenaline release from rat sympathetic neurones in tissue culture.

1. Sympathetic neurones derived from superior cervical ganglia of neonatal rats and maintained in tissue culture were used to investigate the modulation of neurotransmitter release by presynaptic receptors. Three week old cultures of neurones were loaded with [3H]-noradrenaline to label endogenous neurotransmitter stores. Release of noradrenaline was evoked by depolarization with raised extracellular K+ in the presence of desipramine and corticosterone to prevent uptake of released catecholamine. 2. Potassium (55 mmol l-1) depolarization for 30 s caused more than a four fold increase in 3H overflow from basal levels but this increase was reduced by up to 40% in the presence of exogenous noradrenaline (1 mumol l-1). The inhibition by noradrenaline of depolarization-evoked overflow was blocked by the alpha 1/alpha 2-adrenoceptor antagonist, phentolamine. Phentolamine alone did not increase K(+)-evoked 3H overflow. 3. The alpha 2-adrenoceptor antagonist, yohimbine, produced a concentration-dependent block of the inhibition by noradrenaline of K(+)-evoked overflow, while the alpha 1-adrenoceptor antagonist, prazosin, was without effect at concentrations up to 0.1 mumol l-1. 4. The beta-adrenoceptor antagonist, propranolol, neither reduced K(+)-evoked overflow nor increased the degree of inhibition caused by the addition of 1 mumol l-1 noradrenaline. 5. The alpha 2-adrenoceptor agonist, clonidine (1 mumol l-1) was less effective than noradrenaline at inhibiting K(+)-evoked overflow, while the alpha 1-adrenoceptor agonist, phenylephrine (1 mumol l-1) had no significant effect. 6. The L-channel calcium blocker, nicardipine (1 mumol l-1) significantly inhibited 3H overflow evoked by K+.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activation of alpha 2-adrenoceptors attenuates the inotropic effect of field stimulation in atria

Guinea-pig left atria were driven to contract at a rate of 0.5 Hz by stimulation with punctate electrodes. For additional field stimulation, a train of one to eight field pulses (30 V; 0.05-0.4 ms duration) was applied during each refractory period. Cholinergic effects were blocked by atropine and thus the resulting increase of the contractile force was caused by noradrenaline released from sympathetic nerve endings. Trains of several short field pulses delivered in the refractory period after each contraction produced a significantly greater inotropic effect than one single pulse of the same total duration delivered in each refractory period. Phentolamine, rauwolscine and idazoxan, by blocking prejunctional alpha 2-adrenoceptors selectively increased the inotropic effect of single field pulses. Selective blockers of alpha 1-adrenoceptors (prazosin, corynanthine) were ineffective in this respect. The effect of alpha 2-adrenoceptor blockers persisted in the presence of noradrenaline uptake blockers (cocaine or desipramine plus corticosterone) or phenylephrine, but was overcome by the alpha 2-adrenoceptor agonists, clonidine and guanabenz. It is concluded that activation of presynaptic alpha 2-adrenoceptors limits the release of noradrenaline by long-lasting single field pulses. Autoinhibition of transmitter release was not seen with trains of short field pulses.     

Reduced responsiveness to noradrenaline in isolated rat atria exposed to hyperosmotic solutions

1. The effects of hyperosmotic NaCl and sucrose solutions on the responsiveness to noradrenaline (NA) were studied in isolated rat right and left atria. 2. Sucrose caused subsensitivity to NA and isoprenaline in right atria, which was abolished by atropine. 3. NaCl caused subsensitivity only to NA in both atria, which was reversed by imipramine. 4. Both solutions decreased the maximum tension of left atria after maximal NA and Ca2+ concentrations. 5. The results suggest that hyperosmolality reduces the atrial responsiveness to NA by directly depressing contractility and increasing acetylcholine release. However, if the solute is NaCl, the main mechanism seems to be an increase of the catecholamine neuronal uptake.     

Studies with trimipramine on isolated kitten atria.

The effect of different doses of trimipramine has been studied on the force of contraction of isolated kitten atria. Trimipramine produced dose dependent increase in the force of contraction of the atria. Pretreatment of kitten with reserpine or of the isolated atria with propranolol inhibited the positive inotropic effect of trimpramine. The positive inotropic action of trimipramine is probably due to the release and /or due to blocking the uptake of spontaneously released noradrenaline. Trimipramine was also found to potentiate the positive inotropic action of noradrenaline. This potentiation not only decreased in relation to the time of exposure of the isolated atria to trimipramine but also the action of noradrenaline was antagonised.     

Evidence for beta 2-adrenoceptor-mediated facilitation of 3H-noradrenaline release from isolated guinea pig papillary muscles

The effects of beta-adrenoceptor agonists and antagonists on field-stimulated release of radioactivity from superfused guinea pig papillary muscles preincubated with 3H-noradrenaline were studied. Stimulation-evoked overflow of tritium was abolished in the absence of Ca2+ or the presence of tetrodotoxin. Isoprenaline (1 mumol/L) caused a slight facilitation of evoked overflow, whereas phentolamine (1 mumol/L) exerted a strong facilitatory action. However, when phentolamine (1 mumol/L) was present throughout superfusion, isoprenaline and the selective beta 2-adrenoceptor agonist, zinterol, caused concentration-dependent increases (half-maximal effects at 1 nmol/L). The effects of the agonists were inversely related to stimulation frequency. Furthermore, the concentration-response curve of isoprenaline was shifted to the right by the selective beta 2-adrenoceptor antagonist, ICI 118,551, but not by the selective beta 1-adrenoceptor antagonist, ICI 89,406. Schild-plot analysis revealed competitive antagonism and a pA2 value of 9.04 for ICI 118,551. Both ICI 118,551 and ICI 89,406, as well as beta-adrenoceptor antagonists with intrinsic sympathomimetic activity (pindolol and celiprolol; 1 mumol/L), had no effect on stimulation-evoked overflow of tritium (phentolamine present). It is concluded that guinea pig papillary muscles are endowed with prejunctional beta 2 adrenoceptors facilitating impulse-evoked noradrenaline release. The facilitation is markedly promoted by blockade of prejunctional alpha adrenoceptors.     

In vitro and in vivo evaluation of two ultrashort-acting beta-adrenoreceptor antagonists: ACC-9129 and ACC-9369

ACC-9129 and ACC-9369 were evaluated in guinea pig isolated atria and trachea and in anesthetized dogs and conscious rabbits for beta-blocking potency, duration of action, and intraocular pressure effects. ACC-9129 showed competitive cardioselective beta-blocking activity, with a pA₂ of 6.8 in guinea pig right atria and a pA₂ of 4.8 in guinea pig trachea. ACC-9369 was also a competitive beta blocker in these tissues, with a pA₂ of 8.0 in both atria and trachea (nonselective). Cardiodepression in guinea pig right and left atria occurred at concentrations of 1--3 × 10^?5 M and above for either ACC-9129 and ACC-9369, indicating a large therapeutic index for beta-blocking versus cardiotoxic effects of these compounds. Infusion of ACC-9129 (53 ±7 μg/kg/min for 180 min) into anesthetized dogs inhibited isoproterenol-induced tachycardia by 52 ± 4% while reducing the hypotensive response by 6 ± 3%. ACC-9369 infused at 4.3 ± 1.1 μg/kg/min (for 180 min) produced 67 ± 4% inhibition of the heart rate response and 22 ± 9% inhibition of the hypotensive response to isoproterenol. Following termination of a 3-hr infusion of each of these agents at the dose rates shown above, beta blockade was observed to decline such that there was a 50% recovery of the heart rate response to isoproterenol by 9 ± 3 min for ACC-9129 and by 20 ± 4 min (in six of nine dogs) for ACC-9369. ACC-9369, but not ACC-9129, showed some potential for intrinsic sympathomimetic activity in guinea pig atria and in anesthetized dogs. In conscious rabbits, periocular administration of ACC-9129 (3%) or timolol (0.5%), 4 hr prior to an intravenous infusion of 5% glucose, blunted the increase in intraocular pressure that occurred in the absence of drug; ACC-9369 (3%) was unable to modify the increase in intraocular pressure. These results indicate that both ACC-9129 and ACC-9369 possess competitive beta-blocking properties with a short duration of action systematically. Only ACC-9129 was effective in attenuating the increase in intraocular pressure caused by glucose infusion.     

Chronotropic effects of angiotensin I,angiotensin II,bradykinin and vasopressin in guinea pig atria

Chronotropic responses to angiotensin I and angiotensin II, vasopressin and bradykinin were measured in guinea pig isolated right atria. Angiotensin II (100–30 000 pg/ml) was slightly more potent than angiotensin I and caused a maximum tachycardia of 30–40 b/min; only 20% of the maximum response to (?)-noradrenaline. Propranolol (1 μM) or reserpine pretreatment (1 mg/kg i.p., 24 h) did not alter the response to angiotensin II or bradykinin. Converting enzyme inhibition by captopril (10 μg/ml) did not affect resting rate nor the response to angiotensin II but shifted the location of the angiotensin I curve by 40 fold to the right. Bradykinin (5–500 ng/ml) caused small increases in rate while vasopressin 1–100 ng/ml was completely without effect. These results suggest that angiotensin II has a small positive chronotropic effect that is not dependent on tissue noradrenaline release or β-adrenoceptors and that tissue converting enzyme is active in right atria. Relatively high concentrations of angiotensin and bradykinin were required to directly stimulate the sino-atrial node compared with plasma levels measured during physiological stimuli. Therefore these effects on atria are probably of little physiological significance for peptide concentrations in plasma but may be important in relation to local tissue generation of angiotensin II.     

Structure-activity relationships among benextramine-related tetraamine disulfides at peripheral alpha-adrenoreceptors

Several N,N'-(dithiodi-2,1-ethanediyl)bis[N'-(arylmethyl)-1,6-hex anediamines] were prepared and evaluated for their blocking activity on postsynaptic alpha 1-adrenoreceptors in the isolated rat vas deferens. The results were compared with those obtained for benextramine (1). N,N'-(Dithiodi-2,1-ethanediyl)bis[N'-(pyrrol-2-ylmethyl)-1, 6 -hexanediamine] (pyrextramine, 29) was the most potent among the tetraamine disulfides investigated. Thus, it was selected for further pharmacological evaluation to assess its receptor specificity. At a concentration of 10 microM it did not affect the responses elicited by 5-hydroxytryptamine and histamine in guinea pig ileum and by isoproterenol in guinea pig atria and tracheal chain. Furthermore, it was more specific than benextramine (1) toward the muscarinic receptor, being significantly less potent in inhibiting the carbachol-induced response in rat jejunum. These results show that pyrextramine (29) is an irreversible alpha-blocking agent that is more potent and specific than benextramine (1). In conclusion, 29 may be a useful tool in the elucidation and characterization of the peripheral alpha 1-adrenoreceptor.