An investigation of presynaptic alpha-adrenoceptor subtypes in the pithed rat heart and in the rat isolated vas deferens

Investigation of neurotransmission in vas deferens from alpha(2A/D)-adrenoceptor knockout mice

1. We have investigated pre- and post-junctional responsiveness in vas deferens from wild-type and alpha(2A/D)-adrenoceptor knockout mice. The response to a single stimulus was not significantly different between wild-type and knock-out mice. The isometric contraction to 10 Hz stimulation for 4 s was significantly larger in vas deferens from knockout as compared with wild-type. 2. The maximum potentiation of 10 Hz stimulation-evoked contractions by yohimbine was to 206.2+/-38.0% of control in wild-type but to 135.8+/-13.6% of control in knockout. The alpha(2A/D)-adrenoceptor selective antagonist BRL 44408 significantly increased the 10 Hz stimulation-evoked contraction in wild-type but not knockout, and the reverse was true for the alpha(2C)-adrenoceptor selective antagonist spiroxatrine. The alpha(2B)-adrenoceptor antagonist imiloxan had no effect on the evoked contraction except at high concentrations, and only in wild-type. Following cocaine (3 microM) and BRL 44408 (1 microM), 10 Hz responses were similar in shape and maximum between wild-type and knock-out. 3. The alpha(2)-adrenoceptor agonist xylazine virtually abolished the early component of the contraction to 10 Hz stimulation in the presence of nifedipine (10 microM) in vas deferens from knockout mice in a way consistent with a change of receptor subtype but without clear evidence for a reduced receptor number. However, the late component of the contraction to 10 Hz stimulation was significantly potentiated by xylazine in tissues from knock-out mice. 4. It is concluded that, although non-alpha(2A/D)-adrenoceptors replace alpha(2D)-adrenoceptors in this knockout, the alpha(2)-adrenoceptor agonist and antagonist data are contradictory. The antagonist data suggest a major loss of prejunctional alpha(2)-adrenoceptors, but this is not necessarily supported by the agonist data.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.

The interaction between atrial natriuretic peptide and cardiac parasympathetic function

We have demonstrated previously that atrial natriuretic peptide (ANP) inhibits hypotension-induced reflex tachycardia via a parasympathetic mechanism. The present study further defines that parasympathetic mechanism. We tested the hypothesis that ANP, during vagus nerve stimulation, acts as a physiological antagonist to interfere with alpha 1-adrenoceptor modulation of efferent cardiac vagal action. Sprague Dawley rats were divided into five groups, each group receiving a different infusion. Infusates included one of vehicle (Ringer's solution; RS), an alpha 1-adrenoceptor agonist (phenylephrine; PE), a combination of agonist and either a known alpha 1-adrenoceptor antagonist (prazosin; PE+PRZ) or the putative physiologic antagonist, ANP (PE+ANP). The fifth group received all three drugs, PE+PRZ+ANP. Under Inactin anesthesia (100 mg/kg i.p.), efferent autonomic input to the heart was surgically interrupted. Animals were also adrenalectomized to limit the effects of circulating catecholamines. We then monitored each group for the change in heart rate (delta HR) in response to efferent vagus nerve stimulation at various frequencies (2 Hz, 5 Hz, 10 Hz). Infusion of PE significantly (P < 0.01 by ANOVA) attenuated the magnitude of delta HR when compared to the RS group. This attenuation of vagally-induced bradycardia was eliminated by the addition of the alpha 1-adrenoceptor antagonist, prazosin (PE+PRZ group). The PE+ANP group responded with results similar to those of the PE+PRZ group. There was no difference between delta HR responses of the PE+PRZ+ANP group and the PE+PRZ group.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization of presynaptic alpha 2-autoreceptors in rat submaxillary gland and heart atrium

1. The pharmacological properties of presynaptic alpha 2-autoreceptors were studied in rat isolated submaxillary glands and atria. Tissue pieces were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine, and stimulated electrically. In one series of experiments, pEC30 values of 12 alpha-adrenoceptor antagonists were determined, i.e., negative logarithms of concentrations that increased the electrically evoked overflow of tritium by 30%. In another series, pKD values of 9 alpha-adrenoceptor antagonists against the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), and of 3 antagonists against the release-inhibiting effect of methoxamine, were determined. 2. In submaxillary glands, the pEC30 values of the antagonists correlated well with their pKD values against UK 14304 (r = 0.93). The same was true for atria (r = 0.92). 3. In submaxillary glands, the pKD values of 3 antagonists against UK14304 were very similar to their pKD values against methoxamine, with a maximal difference of 0.4. The same was true for atria where the maximal difference was 0.3. 4. The pEC30 values obtained in submaxillary glands correlated significantly with those obtained in atria (r = 0.81). The same was true for the pKD values (r = 0.79). However, the pEC30 and pKD values also indicated consistent differences between the two tissues. 5. It is concluded that the sites of action of the imidazoline UK 14304 (alpha 2-selective), the phenylethylamine noradrenaline, and the phenylethylamine methoxamine (alpha 1-selective) are exclusively alpha 2-adrenoceptors. There is no indication for presynaptic alpha 1-adrenoceptors or for an effect of UK 14304 mediated by presynaptic imidazoline receptors.The 02-autoreceptor population in the submaxillary gland differs from that in the atrium.6. Comparison with studies from the literature indicates that the submaxillary autoreceptors are closely similar to the a2D radioligand binding site found in the bovine pineal gland and probably the rat submaxillary gland. The atrial autoreceptors also conform best to this site, but the agreement is more limited; the atrial autoreceptors may represent a type related to, but distinct from, the a2D site, or a mixture of different types.

Interactions between the effects of endothelin-1, clonidine and yohimbine on electrically-induced contractions in rat vas deferens

1. The relationship between endothelin-1(ET-1)-induced effects on the contractile responses of epididymal portion of rat vas deferens elicited by field electrical stimulation (FES: 80 V, 1 msec, 0.1 Hz) and the effects of the alpha 2-adrenoceptor agonist clonidine and the alpha 2-adrenoceptor antagonist yohimbine were studied. 2. ET-1 (0.01 nM-0.1 microM) concentration-dependently increased the FES-induced contractions. 3. ET-1 (0.1 nM-0.1 microM) reversed the inhibitory effect of clonidine on the FES-evoked contractions whereas ET-1 applied before clonidine exerted a dual effect on the clonidine-induced inhibition of the FES-evoked contractions. 4. The ET-1-induced enhancement of FES-induced contractions was potentiated in the presence of 1 microM yohimbine and was not observed at all in the presence of 10 microM yohimbine. Yohimbine, applied at concentrations of 1 and 10 microM exerted similar blocking effects on the alpha 1-adrenoceptor agonistic effects of phenylephrine. However, yohimbine at a concentration of 10 microM markedly potentiated the contractile effect of exogenous adenosine 5'-triphosphate (ATP), 30 microM. Tetrodotoxin abolished this effect of yohimbine. 5. The results presented here suggest the existence of modulating interactions between the ET-1-evoked increase of FES-induced contractions of rat vas deferens and the alpha 2-adrenoceptor drugs clonidine and yohimbine.

Activation of α1- and α2-adrenoceptors inhibits noradrenaline release in rabbit renal arteries: effects of pertussis toxin and N-ethylmaleimide

1. The effects of selective alpha 1- and alpha 2-adrenoceptor agonists and antagonists on the stimulation-induced (S-I) outflow of radioactivity at 2 Hz were investigated in superfused rabbit renal arteries incubated with [3H]-noradrenaline. 2. The alpha 1-adrenoceptor agonist methoxamine (10 microM) inhibited S-I outflow of radioactivity and this effect was abolished by the alpha 1-adrenoceptor antagonist prazosin (0.1 microM) but not by the alpha 2-adrenoceptor antagonist rauwolscine (1 microM). Neither the prostaglandin synthesis inhibitor indomethacin (10 microM) nor the adenosine receptor antagonist 8-phenyl-theophylline (1 microM) prevented the inhibitory effect of methoxamine. 3. The alpha 2-adrenoceptor agonists clonidine (0.1 microM) and UK 14304 (0.1 microM) both inhibited S-I outflow of radioactivity. The inhibitory effect of clonidine was blocked by rauwolscine but not by prazosin. The inhibitory effect of UK 14304 was markedly reduced by rauwolscine. 4. Prazosin (0.1 microM) alone did not enhance the S-I outflow of radioactivity at 2 Hz and slightly enhanced S-I outflow at 4 Hz. Rauwolscine (1 microM) alone markedly enhanced S-I outflow of radioactivity at 2 and 4 Hz. 5. Pretreatment of the arteries with pertussis toxin (1 microgram ml-1) did not significantly alter the inhibitory effects of methoxamine or UK 14304 or the potentiation by rauwolscine. However, pretreatment of the arteries with a higher concentration of pertussis toxin (5 micrograms ml-1) prevented the inhibitory effect of methoxamine but still did not affect the responses to UK 14304 and rauwolscine. 6. Pretreatment of the arteries with N-ethylmaleimide (NEM, 10 microM) for 30 min did not alter the inhibitory effect of methoxamine but markedly attenuated the inhibitory effect of UK 14304 and the facilitatory effect of rauwolscine. 7. The results suggest that both alpha 1- and alpha 2-adrenoceptors take part in the modulation of noradrenaline release from sympathetic nerves in rabbit renal arteries. Alpha 1-adrenoceptor mediated inhibition may be coupled to G-proteins which are pertussis toxin sensitive and alpha 2-adrenoceptor mediated inhibition to G-proteins which are NEM-sensitive.

Characterization of the subtype of presynaptic alpha 2-adrenoceptors modulating noradrenaline release in cat and bovine cerebral arteries

The possible existence of a heterogeneous population of alpha 2-adrenoceptors (alpha 2A and alpha 2B, demonstrated by binding studies) in adrenergic nerve endings of cat and bovine cerebral arteries modulating noradrenaline release was investigated. Electrical field stimulation elicited an increase of tritium secretion from these vessels preincubated with (+/-)-[3H]noradrenaline, which was reduced by the alpha 2-agonists, clonidine (1 microM) and B-HT 920 (0.01 and 0.1 microM), in cat cerebral arteries but only by B-HT 920 in bovine cerebral arteries. This reduction was inhibited by the antagonist of the alpha 2B-subtype, prazosin, and the antagonists of alpha 2A- and alpha 2B-subtypes yohimbine and particularly rauwolscine. The effect of B-HT 920 was partially inhibited by clonidine in bovine, but not in cat cerebral arteries. In both types of arteries, prazosin, yohimbine and the alpha 1-agonist methoxamine (all at 1 microM) failed to modify the stimulated radioactivity liberation, whereas it was increased by 1 microM rauwolscine, and by yohimbine plus prazosin in cat cerebral arteries. The basal tritium release was enhanced by rauwolscine and prazosin in cat cerebral arteries but only by the latter in bovine cerebral arteries. These results suggest: (1) the existence of presynaptic alpha 2-adrenoceptors, mainly of the alpha 2B-subtype, in these vessels negatively modulating noradrenaline release, their activity being greater in cat than in bovine cerebral arteries, and (2) clonidine has no agonistic but a weak antagonistic action in the latter vessels.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in prejunctional potency of B-HT 933 during aging in the rat vas deferens

1. Age-related changes in prejunctional alpha 2-adrenoceptors were examined in the rat vas deferens using pharmacological techniques. 2. B-HT 933 (1 x 10(-8) - 1 x 10(-6) mol/L) caused a concentration-dependent inhibition of isometric contractions (tetrodotoxin-sensitive) induced by stimulation with single field-stimulus pulses, in both the epididymal and prostatic regions of rat vas deferens. The concentration-response curve to B-HT 933 was shifted to the right with age in the prostatic regions of the vas deferens. 3. In high concentrations (10(-6) - 3 x 10(-4) mol/L), B-HT 933 caused concentration-dependent enhancement of the contractile response to stimulation and evoked spontaneous contractile activity. No significant difference in this postjunctional activity occurred with age in either the prostatic or epididymal regions of the vas deferens. 4. Schild analysis revealed no significant differences in pA2 values for the antagonisms of the prejunctional inhibitory effect of B-HT 933 by rauwolscine in either the prostatic or epididymal regions of vas deferens between young and old rats. 5. These results could be interpreted as a decrease in alpha 2-adrenoceptor number with age. The more marked decrease in the prejunctional inhibitor potency of B-HT 933 in prostatic regions of vas deferens with aging may be due to a smaller receptor reserve in this region of the vas deferens.

The effects of ageing on prejunctional 5-hydroxytryptamine receptors in the rat vas deferens

The prejunctional inhibitory effects of a series of 5-HT1 receptor agonists were examined against the isometric contraction of epididymal portions of rat vas deferens evoked by single stimulus pulses in the presence of nifedipine (10 mumol/l). The 5-HT1A ligand flesinoxan produced inhibition of contractions which was not inhibited by cyanopindolol or yohimbine. However, the prejunctional inhibitory concentration response curve for the 5-HT1 agonist 5-carboxamidotryptamine (5-CT) was biphasic in tissues from 1.5 month old animals but monophasic in tissues from 24 months animals. Cyanopindolol (1 mumol/l) antagonised the inhibitory effects of 5-CT in tissues from 1.5 and 3 month animals but not in tissues from 8 or 24 months animals. Inhibitory actions of 5-CT were not prevented by pretreating animals with pertussis toxin (6 micrograms/kg i.v.), a dose which abolished the negative inotropic response to acetylcholine in rat left atria. It is concluded that the nerve terminals of vas deferens from 1.5 month old animals contain both 5-HT1B and other as yet unclassified 5-HT1 receptors, but that this 5-HT1B-mediated response is lost in maturation and ageing.

No effect of pertussis toxin on peripheral prejunctional alpha 2-adrenoceptor-mediated responses and on endothelium-dependent relaxations in the rat

1. We have investigated the effects of pertussis toxin treatment on a variety of peripheral tissues in the rat. 2. Incubation with pertussis toxin (1 microgram ml-1) in vitro failed to alter the negative inotropic actions of acetylcholine in rat left atria. 3. Pretreatment with pertussis toxin (6 micrograms kg-1, i.v., 3-4 days) abolished the negative inotropic actions of acetylcholine in rat left atria. 4. Pretreatment with pertussis toxin (40 micrograms kg-1, i.v., 3-4 days) failed to alter the prejunctional inhibitory actions of the alpha 2-adrenoceptor agonist xylazine, either in terms of the isometric contraction to a single stimulus in rat vas deferens or in terms of stimulation-evoked overflow of tritium in atria pre-incubated with [3H]-noradrenaline. 5. Pretreatment with pertussis toxin (6 micrograms kg-1, i.v., 3-4 days) failed to affect, and pertussis toxin (40 micrograms kg-1, i.v., 3-4 days) potentiated endothelium-dependent relaxations of rat aorta to histamine and acetylcholine. 6. It seems unlikely that peripheral prejunctional actions of alpha 2-adrenoceptor agonists or endothelium-dependent relaxations of rat aorta involve pertussis toxin-sensitive G proteins.

Prejunctional alpha 1-adrenoceptors modify release of [3H]noradrenaline in the guinea-pig vas deferens

1. Several alpha 1- and alpha 2-adrenoceptor agonists and antagonists were examined for effects on spontaneous and stimulus-evoked release of [3H]noradrenaline from sympathetic nerves in guinea-pig vas deferens. 2. Prazosin (0.1 and 1 microM), phentolamine (30 microM) and yohimbine (10 microM) each enhanced the stimulus-evoked release of [3H]noradrenaline. 3. Prazosin and phentolamine increased the spontaneous outflow of [3H]noradrenaline, whereas yohimbine was without effect. 4. Methoxamine (10 microM) and clonidine (0.1 microM) inhibited the stimulus-evoked release of [3H]noradrenaline, whereas only methoxamine (1 microM) decreased the spontaneous outflow of [3H]noradrenaline. 5. The identity of prejunctional alpha-adrenoceptors in the guinea-pig vas deferens is discussed.

Regulation of noradrenaline release in human cerebral arteries via presynaptic alpha 2-adrenoceptors

1. Electrical stimulation induced tritium release from branches of human middle cerebral arteries preincubated with [3H]noradrenaline (NA), which was reduced by the alpha 2-adrenoceptor agonists, clonidine and B-HT 920, and not affected by the alpha 1-agonist, methoxamine. 2. The stimulated tritium release was inhibited by yohimbine (alpha 2-antagonist), and increased by phentolamine (alpha-antagonist) and prazosin (alpha 1-antagonist). 3. The inhibitory effect of clonidine was antagonized by yohimbine. 4. NA uptake was markedly reduced when the interval between the death and the autopsy was greater than 5 hr. 5. These data indicates the existence of presynaptic inhibitory alpha 2-adrenoceptors, but not alpha 1, in human cerebral arteries, and that the adrenergic nerve endings start to degenerate from 5 hr after death.

Effects of some antipsychotic drugs on cardiovascular catecholamine receptors in the rat

1. Experiments were performed to determine the activity of four antipsychotic drugs on several catecholamine receptors that control the sympathetic cardiovascular responses in rats. 2. Chlorpromazine, thioridazine (0.03 and 0.1 mg kg-1) and haloperidol (0.3 and 1 mg kg-1) inhibited methoxamine-induced diastolic blood pressure increases in the pithed rat, whereas sulpiride (1 and 3 mg kg-1) was without effect. 3. Only sulpiride (3 mg kg-1) antagonized the pressor responses induced by xylazine. 4. Xylazine inhibited the heart rate increase induced by electrical stimulation of the spinal cord (C7-Th1) in the pithed rat. This effect was partially prevented by sulpiride (1 and 3 mg kg-1) and chlorpromazine (0.3 mg kg-1). A higher dose of chlorpromazine (1 mg kg-1) abolished the inhibitory effect of xylazine. 5. Apomorphine infusion inhibited the pressor responses induced by electrical stimulation (Th5-L4) in pithed rats. This effect was reversed by sulpiride (0.01, 0.03 and 0.1 mg kg-1) and partially antagonized by haloperidol (0.1 mg kg-1). 6. The depressor response to fenoldopam in anaesthetized rats was only inhibited by the higher dose of chlorpromazine and thioridazine (3 mg kg-1). 7. Our results suggest that, in the peripheral nervous system of the rat, haloperidol and sulpiride act as antagonists of DA2 receptors while chlorpromazine and thioridazine antagonized DA1 receptors. Furthermore, thioridazine and haloperidol show alpha 1-adrenoreceptor antagonist properties, whereas sulpiride antagonizes alpha 2-adrenoreceptors. Chlorpromazine shows mixed alpha 1/alpha 2-adrenoreceptor antagonism.

Modulation of noradrenaline release in slices of rat kidney cortex through alpha 1- and alpha 2-adrenoceptors

Slices of rat kidney cortex were incubated in [3H]noradrenaline, then placed in a flow cell and subjected to electrical field stimulation. At a stimulation frequency of 5 Hz, both the alpha 2-adrenoceptor antagonist idazoxan (0.1 microM) and the alpha 1-adrenoceptor antagonist prazosin (0.1 microM) significantly enhanced the stimulation-induced (S-I) outflow of radioactivity from the slice. However, neither clonidine (0.1 microM) nor methoxamine (10 microM), alpha 2- and alpha 1-agonists respectively, affected the S-I outflow of radioactivity at this stimulation frequency. At a lower stimulation frequency (1 Hz), the S-I outflow of radioactivity was not affected by idazoxan or prazosin, but was inhibited by both clonidine and methoxamine. The effect of clonidine was prevented by idazoxan (0.1 microM), but not by prazosin (0.1 microM). The effect of methoxamine was abolished by prazosin (0.1 microM), but not by idazoxan (0.1 microM). The inhibitory effect of methoxamine was not prevented by the prostaglandin synthesis inhibitor indomethacin (10 microM) or the adenosine receptor antagonist 8-phenyltheophylline (1 microM) and thus was not mediated by either prostaglandins or adenosine. The results suggest that both prejunctional alpha 1- and alpha 2-adrenoceptors are directly involved in modulation of noradrenaline release from the renal sympathetic nerves of the rat.

Modulation of noradrenaline release from cat cerebral arteries by presynaptic alpha 2-adrenoceptors. Effect of chronic treatment with desipramine and cocaine

1. Field electrical stimulation elicited an increase of the tritium efflux over the basal level from cat cerebral arteries previously incubated with (+/-) [3H]noradrenaline ([3H]NA). 2. This efflux was: (a) reduced by clonidine, NA or B-HT 920; (b) unaffected by methoxamine, prazosin and yohimbine (10(-6) M); (c) reduced by yohimbine (5 x 10(-6) M), and (d) increased by phentolamine. 3. The effect of clonidine was blocked by yohimbine. 4. The daily treatment with the neuronal uptake blockers desipramine (10 mg/kg, i.p.) or cocaine (10 mg/kg, i.p.) [during 12 days], antagonized the inhibitory action of clonidine totally or partially, respectively. 5. These results suggest: (1) the existence of presynaptic alpha 2-adrenoceptors in these arteries, which modulate the NA release, and (2) that chronic treatment with desipramine or cocaine induces a subsensitivity of these alpha 2-receptors, which facilitates the NA release.

Reassessment of the blocking activity of prazosin at low and high concentrations on sympathetic neurotransmission in the isolated mesenteric vasculature of rats

1. The overflow of endogenous noradrenaline from the rat mesenteric arterial vasculature was determined along with the perfusion pressure response to periarterial nerve stimulation (PNS) (4-12 Hz). 2. The PNS-induced pressor responses were blocked by prazosin at 3 X 10(-8) M but not by prazosin at 10(-6) M or the combination of prazosin at 3 X 10(-8) M and yohimbine (10(-7) M). 3. The PNS-induced overflow of endogenous noradrenaline was significantly augmented by prazosin at 10(-6) M or the combination of prazosin at 3 X 10(-8) M and yohimbine (10(-7) M) but not by prazosin at 3 x 10(-8) M alone. 4. The PNS-induced pressor response and endogenous noradrenaline overflow, in the absence of prazosin, were not significantly influenced by alpha, beta-methylene-adenosine triphosphate (alpha, beta-methylene ATP) (3 X 10(-6) M). The pressor responses to PNS which remained after prazosin at 10(-6) M were abolished by pretreatment with alpha, beta-methylene ATP and therefore attributable to coreleased ATP. 5. The pressor responses to exogenous noradrenaline were abolished by prazosin at both 3 X 10(-8) M and 10(-6) M. 6. These results suggest that no functionally significant amount of ATP was coreleased with noradrenaline by PNS in the presence of prazosin at low concentrations which blocked only postjunctional alpha 1-adrenoreceptors. 7. Thus, these results indicate that the effects of prazosin at high concentrations, such as 10(-6) M, on sympathetic neurotransmission should not be ignored when considering the mechanisms of vasoconstrictor responses to PNS following high concentrations of prazosin.

Agonists at presynaptic receptors on sympathetic nerves differentially affect two phases of the contractile response in the rat vas deferens

A series of adrenoceptor agonists were investigated for their prejunctional effects on field stimulated rat vas deferens. Tissues were stimulated in 10 s trains of impulses, frequency 10 Hz, every 100 s. This produced a biphasic response comprising an initial twitch followed by a prolonged, plateau phase of contraction. The order of potency for a series of alpha 2-agonists against the twitch phase of contraction was UK14304 greater than clonidine greater than noradrenaline = alpha-methyl noradrenaline greater than B-HT920. The same order of potency was observed against the plateau phase, but approximately 10 fold higher concentrations of agonist were needed. Surprisingly, B-HT920 was inactive against the plateau phase of contraction. Characteristic differences in the slopes and maximum responses of the dose-response curves to the imidazolines (UK14304 and clonidine) and the beta-phenethylamines (noradrenaline and alpha-methyl noradrenaline) were seen against both phases of contraction. It is concluded that the two phases of contraction are influenced by activation of two distinct heterogeneous populations of prejunctional alpha 2-adrenoceptors.

Pertussis toxin and pre-junctional alpha 2-adrenoreceptors in rat heart and vas deferens

1. The effects of pretreatment with pertussis toxin (50-200 micrograms kg-1, i.p.) on the pre-junctional inhibitory actions of the alpha 2-adrenoreceptor agonist xylazine were examined by employing the pithed rat and rat isolated vas deferens. 2. In pithed rats, pertussis toxin attenuated the pressor response to xylazine, but did not alter the pre-junctional cardio-inhibitory actions of xylazine. In epididymal portions of rat vas deferens, pretreatment with pertussis toxin did not alter the pre-junctional inhibitory action of xylazine. 3. These data lend no support for the view that alpha 2-adrenoreceptor-mediated prejunctional inhibition involves inhibition of adenylate cyclase through the Ni regulatory protein, at least in the tissues examined.

Pressor mechanisms linked obligatorily to spontaneous hypertension in the rat

To identify genetic factors linked obligatorily to hypertension in the rat, pithed spontaneously hypertensive rats (SHR) were compared with genetically similar (Wistar-Kyoto rats; WKY) and different (Sprague-Dawley) normotensive strains. The only variables that distinguished SHR from both WKY and Sprague-Dawley rats were a greater maximum pressor response to electrical stimulation of sympathetic outflow and decreased sensitivity to submaximal doses of the alpha 1-adrenergic agonist methoxamine (i.e., higher ED50). SHR had in common with Sprague-Dawley rats basal blood pressure after pithing plus adrenalectomy and the maximum pressor response to methoxamine; both these values were higher than those in WKY. All strains demonstrated equal sensitivity of the vasoconstrictor response to endogenous norepinephrine released by electrical simulation at submaximal frequency, even though sensitivity to the alpha 1-adrenergic receptor agonist was lower in SHR. The alpha 2-adrenergic receptor antagonist rauwolscine attenuated the pressor response to electrical stimulation in SHR and WKY but increased it in Sprague-Dawley rats. The alpha 1-adrenergic receptor antagonist prazosin attenuated the response more in SHR and WKY than in Sprague-Dawley rats. We conclude that 1) sympathetic hyperactivity is linked obligatorily to hypertension in SHR; 2) increased basal blood pressure and noradrenergic vasoconstrictor response are present in SHR, but they are not obligatorily linked to hypertension; 3) feedback inhibition of norepinephrine release is comparable in SHR or WKY and poorly developed compared with that in Sprague-Dawley rats; 4) decreased sensitivity of the pressor response to stimulation of vascular alpha 1-adrenergic receptors in SHR compensates partially for increased sympathetic activity or hyperinnervation, or both.

Investigations of cardiovascular 5-hydroxytryptamine receptor subtypes in the rat

Peripheral 5-HT receptor-mediated responses were examined in pithed spontaneously hypertensive rats and normotensive wistar rats. Responses examined were: Pressor and depressor responses, tachycardia and inhibition of stimulation-evoked tachycardia. In pithed spontaneously hypertensive rats, 5-HT, but not the 5-HT1-selective agonist 5-carboxamidotryptamine, produced pressor responses, and these were potently antagonised by the 5-HT2-selective antagonists ketanserin and LY 53857. In pithed spontaneously hypertensive rats, the tachycardia to 5-HT was abolished by a combination of the 5-HT2 receptor antagonist LY 53857 and propranolol, suggesting that the tachycardia is mediated by 5-HT2 receptors and by release of noradrenaline. In pithed spontaneously hypertensive rats, 5-carboxamidotryptamine, 5-HT, and to a lesser extent the 5-HT1 receptor agonist RU 24969, but not the 5-HT1A receptor agonist 8-OH-DPAT, produced depressor responses which were antagonised by methysergide and metitepin, but which do not clearly fit with any of the 5-HT1 ligand binding sites. In pithed normotensive wistar rat, 5-carboxamidotryptamine was approximately 100 times more potent than 5-HT and 8-OH-DPAT at inhibiting the cardio-acceleration produced by single pulse electrical stimulation and this inhibition was antagonised by metitepin, so that the response is mediated by 5-HT1 receptors.

Pre- and postjunctional alpha-adrenoceptor-mediated effects of prazosin, methoxamine and 6-fluoronoradrenaline in blood-perfused canine skeletal muscle in situ

Pre- and postjunctional effects of the alpha 1-selective adrenoceptor antagonist prazosin and the alpha 1- and alpha 2-selective adrenoceptor agonists methoxamine and 6-fluoronoradrenaline, respectively, were studied in skeletal muscle in situ. Prazosin reduced the vasoconstriction and enhanced the overflow of endogenous noradrenaline elicited by sympathetic nerve stimulation (1-4 Hz, 2 min); the threshold concentration was 10-100 times lower for postjunctional than for prejunctional alpha-adrenoceptor blockade. The enhancement of noradrenaline overflow by prazosin was not inversely frequency-dependent, as shown elsewhere for alpha 2-adrenoceptor antagonists. Thus, different mechanisms may be involved. Inhibition of prostaglandin synthesis by diclofenac did not alter the stimulation-evoked noradrenaline overflow, indicating a minor importance of prostaglandin-mediated transjunctional mechanisms in the modulation of noradrenaline overflow. Methoxamine and 6-fluoronoradrenaline elevated the basal vascular tone and, at higher concentrations, reduced the stimulation-evoked noradrenaline overflow. Methoxamine was 20 times more selective than 6-fluoronoradrenaline for postjunctional receptors. Our results are compatible with a pre- and postjunctional localization of alpha 2-adrenoceptors and a predominantly, but not exclusively, postjunctional localization of alpha 1-adrenoceptors. The postjunctional selectivity for prazosin was less marked than previously reported from in vitro studies. Hence, care should be taken when extrapolating in vitro findings to the more complex in vivo situation.

Alpha-adrenoceptor responsiveness in the aged rat

The effects of ageing on vascular alpha 1- and alpha 2-adrenoceptors were examined using anaesthetised and pithed young (3-7 months) and old (21-24 months) Sprague-Dawley rats. In pithed animals, the pressor and cardioinhibitory effects of the alpha 2-adrenoceptor agonist xylazine were significantly reduced in old animals (8- and 6-fold shift), but the pressor effects of the alpha 1-adrenoceptor agonist amidephrine were not significantly altered by ageing. In anaesthetised rats, the pressor response to the mixed alpha-adrenoceptor agonist noradrenaline (NA) was not significantly altered, and the pressor potency of the alpha 1-adrenoceptor agonist amidephrine was only slightly reduced (1.7-fold shift) in old animals. In the presence of cocaine (1 mg kg-1) the pressor potency of NA was markedly reduced (13-fold shift) in old animals. In the presence of prazosin (1 mg kg-1) to eliminate alpha 1-adrenoceptor-mediated responses, the pressor potency of NA was markedly reduced in old as animals as compared to young animals (16-fold shift). The neuronal uptake blocker cocaine (1 mg kg-1) significantly potentiated the pressor response to NA only in young. In summary, we have found a reduced alpha 2-adrenoceptor-mediated pressor and cardioinhibitory responsiveness and a reduced neuronal uptake of NA in old animals with little change in alpha 1-adrenoceptor-mediated vascular responsiveness.

An investigation of alpha-adrenoceptor responsiveness in the vas deferens of spontaneously hypertensive rats

alpha-Adrenoceptor-mediated responses were investigated in isolated vasa deferentia from spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NWR). There was no significant difference between NWR and SHR in the inhibition of the isometric contraction to single pulse field stimulation by alpha 2-selective agonists in prostatic portions, nor by alpha 2-selective agonists and the alpha 1-selective agonist, amidephrine in epididymal portions in the presence of nifedipine to prevent postjunctional actions of alpha-1-selective agonists. There was no significant difference between NWR and SHR in the potency of amidephrine in causing a postjunctionally mediated potentiation of the isometric contraction to single pulse field stimulation in prostatic portions but the maximum potentiation was significantly reduced in SHR. However, the maximum potentiation of the isometric contraction by the calcium entry facilitator, Bay K 8644, was not significantly different between NWR and SHR. The maximum direct contraction to amidephrine, but not to Bay K 8644, was also significantly reduced in SHR. The irreversible alpha 1-adrenoceptor antagonist, phenoxybenzamine was more potent in SHR than NWR in reducing the maximum potentiation by amidephrine of the stimulation-evoked isometric contraction and in reducing the maximum direct contraction to amidephrine. It is concluded that there is a reduced postjunctional alpha 1-mediated responsiveness in the vas deferens of SHR due probably to a reduction in receptor number.

No evidence for differences between pre- and post-junctional alpha 2-adrenoceptors

We have examined the pre- and post-junctional effects of a series of alpha-adrenoceptor agonists and antagonists at alpha 2-adrenoceptors in the pithed rat preparation and the human isolated saphenous vein. In the pithed rat, there was no difference in relative agonist and antagonist potencies between pre- and post-junctional alpha 2-adrenoceptors but the absolute potencies of antagonists differed: antagonists were more potent prejunctionally. In the human saphenous vein, the alpha 2-adrenoceptor antagonist yohimbine had pre- and post-junctional actions over the same concentration range. We have no evidence to suggest differences between pre- and post-junctional alpha 2-adrenoceptors: differences in absolute antagonist potencies in the pithed rat may be due to non-equilibrium conditions.

Further examination of the inhibitory actions of alpha 1-adrenoceptor agonists in rat vas deferens

The inhibitory actions of alpha 1-adrenoceptor agonists were examined in the isolated bisected vas deferens of the rat. The calcium entry facilitator Bay K 8644 markedly potentiated the isometric contraction to a single stimulus pulse in epididymal portions of rat vas deferens: subsequent amidephrine produced an inhibition which was antagonised by the alpha 1-adrenoceptor antagonist, prazosin, but not by the alpha 2-adrenoceptor antagonist, yohimbine. The alpha 1-adrenoceptor agonists amidephrine and cirazoline failed to inhibit the transmitter overflow to trains of pulses at a frequency of 2 Hz in epididymal portions, but also failed to abolish the nifedipine-resistant adrenergic contraction to trains of pulses at 2 Hz in epididymal portions. It is concluded that alpha 1-adrenoceptor agonists have inhibitory effects which may be by action at presynaptic alpha 1-adrenoceptors.

Acetylcholine release from rat atria can be regulated through an alpha 1-adrenergic receptor

Isolated superfused rat atria release [3H]acetylcholine when depolarized with 57 mM potassium. The depolarization-induced overflow of [3H]acetylcholine is markedly inhibited by micromolar concentrations of epinephrine and norepinephrine. The alpha 1-selective adrenergic agonist methoxamine also inhibits tritium overflow, but the alpha 2-selective adrenergic agonist clonidine and the beta-adrenergic agonist isoproterenol do not. Prazosin, an selective alpha 1-adrenergic antagonist, blocks adrenergic inhibition of [3H]acetylcholine overflow with a Ki of approximately 0.4 nM. Yohimbine has approximately one-hundredth the potency of prazosin for blocking adrenergic inhibition of [3H]acetylcholine overflow. [3H]Norepinephrine overflow from isolated rat atria is also inhibited by norepinephrine, but this effect is antagonized by yohimbine and not by prazosin. We suggest that the release of acetylcholine from cardiac parasympathetic neurons can be regulated through an alpha 1-adrenergic receptor, and that this mechanism may underly, at least in part, the relative lack of effects of prazosin on heart rate.

Further examination of the effects of ageing on the adrenoceptor responsiveness of the rat vas deferens

Age-related changes in presynaptic alpha 2-adrenoceptors were investigated in prostatic and epididymal portions of vasa deferentia from young adult (2-3 months) and old (24-29 months) Sprague-Dawley rats, using the alpha 2-selective agonists xylazine and clonidine. In prostatic portions the inhibitory effects of clonidine against the isometric contractions to single pulse field stimulation were complicated by a postsynaptic action in old animals, but in epididymal portions in the presence of nifedipine both xylazine and clonidine were 3 times less potent in old rats. However, there were no significant differences between young and old in the potency of xylazine at inhibiting the overflow of tritium or the isometric contraction evoked by 5 Hz stimulation for 3 min in tissues pre-incubated with [3H]noradrenaline. It is suggested that there is reduced responsiveness of presynaptic alpha 2-adrenoceptors in the vas deferens of old rats, but that this can only be demonstrated using a sensitive measure of the presynaptic potency of agonists: the isometric contraction to a single stimulus.

An investigation of the actions of the calcium entry facilitator Bay K 8644 on the rat vas deferens

The pre- and postsynaptic actions of the calcium entry facilitator Bay K 8644 were investigated in the rat isolated vas deferens. Bay K 8644 had no presynaptic effect on adrenergic neurotransmission in the epididymal portion. Bay K 8644 and alpha 1-adrenoceptor agonists potentiated, and the calcium entry blocker nifedipine abolished, the non-adrenergic contraction to a single stimulus in prostatic portions. Bay K 8644 sensitised the prostatic portion to, but did not increase the maximum height of, spontaneous direct contractions produced by alpha-agonists.