Adrenergic transmission in the dog mesenteric vein and its modulation by alpha-adrenoceptor antagonists

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Novel localization of CD38 in perivascular sympathetic nerve terminals

Using high performance liquid chromatography fraction analysis we have recently established that numerous smooth muscle preparations, including the canine mesenteric artery and vein, release beta-nicotinamide adenine dinucleotide upon short-pulse electrical field stimulation in tetrodotoxin- and omega-conotoxin GVIA-sensitive manners [ Release of beta-nicotinamide adenine dinucleotide upon stimulation of postganglionic nerve terminals in blood vessels and urinary bladder. J Biol Chem 279:48893-48903.]. The beta-nicotinamide adenine dinucleotide metabolites ADP-ribose and cyclic ADP-ribose are also present in the tissue superfusates. CD38 is a multifunctional enzyme involved in the degradation of beta-nicotinamide adenine dinucleotide to ADP-ribose and cyclic ADP-ribose. Western immunoblot analysis revealed that CD38 is expressed in both artery and vein. Confocal laser scanning microscopy established colocalization of CD38 with tyrosine hydroxylase, synaptotagmin and synaptic vesicle protein in both blood vessels. High performance liquid chromatography with fluorescence detection demonstrated that whole tissue segments metabolize 1,N(6)-etheno-nicotinamide adenine dinucleotide to 1,N(6)-etheno-ADP-ribose and nicotinamide-guanine dinucleotide to cyclic GDP-ribose, suggesting the presence of both nicotinamide adenine dinucleotide-glycohydrolase and ADP-ribosyl cyclase activities in these blood vessels. Both enzymes appear to be associated with the membrane fraction, and therefore might be attributed to CD38. These data demonstrate a previously uncharacterized localization of CD38 in perivascular autonomic nerve terminals. Therefore, the beta-nicotinamide adenine dinucleotide/CD38 system may provide new mechanisms in autonomic neurovascular control.

PI3K and PKC contribute to membrane depolarization mediated by alpha2-adrenoceptors in the canine isolated mesenteric vein

BACKGROUND

Norepinephrine (NE), a classic neurotransmitter in the sympathetic nervous system, induces vasoconstriction of canine isolated mesenteric vein that is accompanied by a sustained membrane depolarization. The mechanisms underlying the NE-elicited membrane depolarization remain undefined. In the present study we hypothesized that phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) are involved in the electrical field stimulation (EFS)-induced slow membrane depolarization (SMD) in canine isolated mesenteric vein. EFS (0.1-2 Hz, 0.1 ms, 15V, 10 s)-induced changes in the membrane potential were recorded with a conventional intracellular microelectrode technique and evaluated in the absence and presence of inhibitors of neuronal activity, alpha-adrenoceptors, membrane ion channels, PI3K, inositol 1,4,5-triphosphate (InsP3) receptors, and PKC. Activation of PI3Kgamma and PKCzeta in response to exogenous NE and clonidine in the absence and presence of receptor and kinase inhibitors were also determined.

RESULTS

Contractile responses to NE and clonidine (0.05 - 10 microM) were significantly diminished in the presence of yohimbine (0.1 microM). Exogenous NE (0.1 microM) and clonidine (1 microM) elicited SMD. The resting membrane potential of canine mesenteric vein smooth muscle cells was -68.8 +/- 0.8 mV. EFS elicited a biphasic depolarization comprised of excitatory junction potentials and SMD that are purinergic and adrenergic in nature, respectively. The magnitude of the SMD in response to EFS at 0.5 Hz was 9.4 +/- 0.7 mV. This response was reduced by 65-98% by the fast Na+ channel inhibitor tetrodotoxin (1 microM), by the inhibitor of N-type Ca2+ channels omega-conotoxin GVIA (5 nM), the non-selective alpha-adrenoceptor blocker phentolamine (1 microM), the selective alpha2-adrenoceptor blocker yohimbine (0.1 microM), the ion channel inhibitors niflumic acid (NFA, 100 microM), 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB, 30 microM), 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 200 microM), and Gd3+ (30 microM), and the PI3K inhibitors wortmannin (100 nM) and LY-294002 (10 microM). The SMD remained unchanged in the presence of the L-type Ca2+ channel blocker nicardipine (1 microM) and the InsP3 receptor blockers 2-aminoethoxydiphenylborate (2APB, 50 microM) and xestospongin C (3 microM). The inhibitor of PKC chelerythrine (1 microM), but not calphostin C (10 microM), diminished the SMD. Exogenous NE and clonidine (1 microM each) activated both PI3Kgamma and PKCzeta, and the activation of these kinases was abolished by preincubation of tissue with the alpha2-adrenoceptor blocker yohimbine.

CONCLUSION

Neuronally-released NE stimulates smooth muscle alpha2-adrenoceptors and activates PI3K and atypical PKC in the canine mesenteric vein. Events downstream of PKC lead to SMD and vasoconstriction. This represents a novel pathway for NE-induced membrane depolarization in a vascular smooth muscle preparation.

Selective modulation of noradrenaline release by alpha 2-adrenoceptor blockade in the rat-tail artery in vitro

The effects of blocking alpha(2)-adrenoceptors on noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves have been investigated in rat-tail artery in vitro. Continuous amperometry was used to measure NA release and intracellularly recorded excitatory junction potentials (e.j.p.'s) were used to measure ATP release. Application of the alpha(2)-adrenoceptor antagonist, idazoxan (1 microm), increased the amplitude of NA-induced oxidation currents evoked by trains of 10 stimuli at 1 and 10 Hz. In cells deep in the media, idazoxan (1 microm) had no effect on the amplitude of e.j.p.'s evoked by trains of 10 stimuli at 1 and 10 Hz. In cells close to the adventitial - medial border, idazoxan produced a small increase in the amplitude of e.j.p.'s evoked at the end of trains of 10 stimuli at 1 Hz. In tissues pretreated with the neuronal NA uptake inhibitor, desmethylimpramine (0.3 microm), idazoxan (1 microm) markedly increased the amplitude of e.j.p.'s in cells deep in the media. The alpha(2)-adrenoceptor agonist, clonidine (0.5 microm), produced similar reductions in the amplitudes of both NA-induced oxidation currents and e.j.p.'s evoked by 10 stimuli at 1 Hz. These effects of clonidine were reversed by the subsequent addition of idazoxan (1 microm). The release of both NA and ATP is inhibited to a similar extent by activation of prejunctional alpha(2)-adrenoceptors by clonidine. In contrast, endogenously released NA more markedly inhibits NA release. These findings provide further support for the differential modulation of NA and ATP release.

Autoregulation of neurotransmitter release at autonomic nerve terminals: a questionable theory

1. The evidence for feedback regulation of neurotransmitter release by means of autoreceptors is questioned. 2. Autoreceptor function must meet the expectations for feedback loops. However, this concordance has not been observed in most neuroeffector systems. 3. The characteristics of per pulse transmitter release with changes in the parameters of stimulation in several autonomic systems do not support the ongoing operation of negative feedback loop mediated by locally released transmitter. Also, the effects of antagonists and agonists often do not comply with feedback expectations. 4. Evidence is provided that agonists and antagonists act at different loci to achieve their inhibiting and potentiating effects on transmitter release. 5. Future efforts should be directed to exploring the mechanism(s) of antagonist action and to a system-by-system analysis of the evidence for and against autoreceptor operation.

Differential sensitivity to hydroxyl radicals of pre- and postjunctional neurovascular transmission in the isolated canine mesenteric vein

In some pathophysiological conditions, the first target of reactive oxygen intermediates is the vascular system. Superoxide anions, when generated in the vascular circulation, may then escape into the extracellular space via an anion channel and, following dismutation to hydrogen peroxide (H(2)O(2)), form hydroxyl radicals (HO(*)). In an attempt to understand the role of HO(*) in the regulation of transmission at the sympathetic neurovascular junction, the effect of HO(*) at nerve terminals was examined by measuring the amount of noradrenaline (NA) released from isolated, spirally cut, superfused canine mesenteric vein during basal and electrical stimulation (ES; 5Hz, 2ms, 9V); tension development evoked by ES was also recorded simultaneously. HO(*) was generated from Fenton's reagent (1. 5x10(-4)M H(2)O(2) plus 10(-4)M FeSO(4)); generation of HO(*) from H(2)O(2)/FeSO(4) in the superfusate was monitored by electron spin resonance spectroscopy using the spin-trap 5, 5-dimethyl-1-pyrroline-N-oxide throughout the experimental time course. Exposure to HO(*) of the helical strips produced an irreversible decrease in tension development evoked by ES with no effect on NA release, suggesting that the observed effect is elicited postjunctionally. The susceptibility of the processes of NA-mediated contraction to HO(*) may differ greatly from that of the NA release mechanism at the prejunctional site. Exposure of the strip preparation to HO(*) leads to a substantial stimulation of basal release of NA without affecting ES-evoked NA release, possibly due to enhanced non-exocytotic Ca(2+)-independent release elicited by HO(*). A direct demonstration of this concept was obtained by showing a significant increase in the basal response of NA release in Ca(2+)-free solution. The major conclusion of the present study is that HO(*) can damage NA-mediated contraction of the vascular preparations at the postjunctional site, and may selectively induce a non-exocytotic release of NA from the prejunctional site of sympathetic neurotransmission.

Electrophysiological events during neuroeffector transmission in the spleen of guinea-pigs and rats

Intracellular recordings were made from smooth muscle cells of arterioles and the capsule of the spleen of guinea-pig and rat, and the responses to periarterial or subcapsular nerve stimulation were recorded. The innervation of the spleen was studied using fluorescence and immunohistochemical techniques. Catecholamine-containing axons were associated with smooth muscle of the splenic capsule, trabeculae, arterioles and amongst cells of the periarteriolar lymphoid sheath. Axons immunoreactive for neuropeptide Y (NPY) and tyrosine hydroxylase were distributed in an identical manner to catecholamine-containing axons, whereas axons immunoreactive for substance P or calcitonin gene-related peptide were present at a very low density in spleens from both species. In segments of arterioles, single transmural stimuli evoked excitatory junction potentials (EJPs) of 1-10 mV amplitude. EJPs facilitated during short trains of stimuli (1-10 Hz) and summated at 10 Hz, often initiating a muscle action potential. EJPs persisted in the presence of prazosin (1 microM) and idazoxan (1 microM), but were abolished by the P2x-purinoceptor antagonist suramin (1 mM). Spontaneous depolarizations were observed in smooth muscle cells of arterioles and capsule. Some events in arterioles were observed in the presence of suramin and so may originate postjunctionally independently of transmitter release. As single transmural stimuli failed to evoke a depolarization in capsular smooth muscle, spontaneous depolarizations in this tissue probably also arise postjunctionally. Short trains of high frequency stimuli (10-35 Hz) evoked biphasic depolarizations of capsular smooth muscle cells. The initial component peaked 2.5 s following the onset of stimulation; the second component peaked 15 s following the onset and decayed exponentially with a time constant of 15 s. By fitting a product of exponentials to the second component, it was possible to define the initial component, which decayed with a time constant of around 1.5 s. Neurally evoked depolarizations of capsular smooth muscle were abolished by 1 microM TTX. Blockade of alpha 1-adrenoceptors with prazosin reduced the initial component of the depolarization, whereas alpha 2-adrenoceptor blockade with idazoxan virtually abolished the second component. In some cells a small, faster depolarization persisted after alpha-adrenoceptor blockade. The slow alpha 2-adrenoceptor-mediated depolarization was identical to that recorded in the rat tail artery and in the guinea-pig mesenteric vein. The data indicate that sympathetic neuroeffector transmission from noradrenergic axons containing NPY to splenic arterial and capsular smooth muscle occur by different mechanisms.

Electrophysiological comparison of the effects of MCI-154 on smooth muscles between mesenteric artery and vein of dogs

1. Effects of MCI-154, a cardiotonic vasodilator, on smooth muscles were compared in isolated mesenteric artery and vein of dog. 2. MCI-154 depolarized the membrane, and inhibited Ca(2+)- and K(+)-contractions more in the vein than in the artery. 3. Nerve-mediated and noradrenaline-induced contractions, and the excitatory junction potential (e.j.p.) were inhibited by MCI-154 in the vein and potentiated in the artery. 4. Compound action potentials of perivascular nerves and action potentials generated on the e.j.p. were not altered by MCI-154. 5. MCI-154 predominantly dilates venous muscles, and as a result would reduce cardiac outflow and lower systemic blood pressure.

Actions of dipyridamole on endogenous and exogenous noradrenaline in the dog mesenteric vein

1. In the isolated mesenteric vein of the dog, dipyridamole inhibited both the excitatory junction potential (e.j.p.) and the slow depolarization evoked by perivascular nerve stimulation, to 60-70% of control, with no change in the postjunctional membrane potential. These inhibitory actions of dipyridamole were not modified by 8-phenyltheophylline or phentolamine, suggesting that the inhibition did not involve either the actions of endogenous adenosine or the prejunctional alpha-autoregulation mechanism. 2. Dipyridamole did not produce any detectable effects on either the facilitation process of the e.j.ps or the postjunctional membrane depolarization produced by exogenously applied noradrenaline (NA). 3. Dipyridamole reduced the outflow of both the NA and the 3,4-dihydroxyphenylglycol (DOPEG) evoked by perivascular nerve stimulation to below 10% of control, the effect being much greater than that of exogenously applied adenosine (to about 90% of the control). 4. Exogenously-added NA was degraded by incubation with a segment of the vein. Dipyridamole itself produced degradation of NA and accelerated the NA-induced degradation. By contrast, pyrogallol, but not pargyline or imipramine, prevented the NA-induced degradation. 5. It is suggested that dipyridamole degrades NA directly, and also indirectly through activation of catechol-O-methyl transferase, with no alteration of the activity of monoamine oxidase or of the uptake mechanisms of NA into nerve terminals.

Properties of sympathetic neuromuscular transmission and smooth muscle cell membranes in vascular beds

In vascular smooth muscle tissues, the cycle of contraction-relaxation is mainly regulated by the cytosolic Ca, and many other factors, such as substances released from endothelial cells and perivascular nerve terminals (mainly sympathetic nerves). In this article, we introduce regional differences in specific features of ionic channels in vascular smooth muscle membranes (mainly on features of Ca, Na and K channels) in relation to mobilization of the cytosolic Ca. In many vascular tissues, neurotransmitters released from sympathetic nerve terminals activate post-junctional receptors, and subsequently modify ion channels (receptor-activated cation channel and voltage-dependent Ca channel), whereas in some tissues, ionic channels are not modified by receptor activations (pharmaco-mechanical coupling). However, activation of receptors, with or without modulation of ionic channels, regulates the cytosolic Ca through synthesis of second messengers. In addition, receptors distributed on prejunctional nerve terminals positively or negatively regulate the release of transmitters. Roles of neurotransmitters (mainly ATP and noradrenaline) are also discussed in relation to the generation of excitatory junction potentials.

The problem with autoreceptors

There are numerous problems with the concept that antagonists enhance transmitter release by blockade of feedback. It was shown that antagonist enhancement of transmitter release does not correlate satisfactorily with the intensity of stimulation or with other indices of biophase transmitter concentration. Wide variations were shown to exist between antagonists in the amount of enhancement of release they induce. Also, antagonists enhance transmitter release or the effector response with a single stimulation pulse, a condition under which no feedback is possible. A study of agonist/antagonist relationships indicates different sites of action, and it was determined that the antagonist effect has negligible or minimal latency and that enhancement by antagonists is maximal under minimal condition of stimulation. Antagonists were shown to enhance release by a direct action, not by passive occupancy of agonist sites. Experiments were described in which acetylcholine and cold selectively antagonized antagonist but not agonist effects. Further, experiments with pulse duration shifts and with veratridine pointed to a direct action of antagonists on Na+ (also Ca++?) channel gating mechanisms, which results in a shift in the voltage dependence of activation. If antagonists, in some particular instances, enhance release by blockade of sites involved in negative feedback this is likely lost or mired in their more prominent direct actions on neurosecretion--and these must be sorted out. The acceptance of the fact that antagonists act directly to alter transmitter release (and not only as passive occupiers of presynaptic receptors), as the present study shows, both in the central nervous system and in the periphery, opens a new area for future investigation, and may be exploitable for therapeutic purposes and to gain an enriched understanding of the mechanism of neurosecretion.

Comparison of the prejunctional beta-adrenoceptor stimulating actions of adrenaline and isoprenaline in the dog mesenteric vein

1. The prejunctional beta-adrenoceptor stimulating actions of adrenaline and isoprenaline were compared by recording junction potentials from smooth muscle cells of the dog mesenteric vein. 2. The potency of adrenaline and isoprenaline on beta-adrenoceptors in the postjunctional membrane was estimated from hyperpolarization of the membrane in smooth muscle cells of the guinea-pig facial vein. In the presence of yohimbine, both agents hyperpolarized the membrane to a similar extent. 3. In the dog mesenteric vein, amplitude of the excitatory junction potential (e.j.p.) and slow depolarization was inhibited by adrenaline and potentiated by isoprenaline; the former but not the latter was accompanied by depolarization of the smooth muscle membrane. 4. In the presence of yohimbine, adrenaline inhibited the e.j.p. without depolarization of the smooth muscle membrane; the action was weaker than in the absence of yohimbine. The isoprenaline-induced potentiation of the e.j.p. was further enhanced by yohimbine. 5. It is concluded that adrenaline and isoprenaline have similar stimulating actions on postjunctional beta-adrenoceptors in the guinea-pig facial vein, but have different actions on prejunctional beta-adrenoceptors in the dog mesenteric vein; isoprenaline but not adrenaline stimulates this beta-adrenoceptor to facilitate the release of transmitter substances from perivascular noradrenergic nerves.

Bunazosin, an alpha 1-adrenoceptor blocker, differentially releases co-transmitters in dog mesenteric vessels

The effects of bunazosin on the electrical and mechanical responses of smooth muscle cells elicited by exogenously applied noradrenaline (NA) and by perivascular nerve stimulation were studied in the isolated mesenteric artery and vein of the dog. NA (above 10(-7) M in the artery and above 3 X 10(-8) M in the vein) depolarized the membrane. Perivascular nerve stimulation evoked an excitatory junction potential (e.j.p.) and slow depolarization in both vessels. Bunazosin and prazosin inhibited the NA-induced depolarization and slow depolarization in the artery but not in the vein. The NA actions in the vein were inhibited by yohimbine. Bunazosin (above 10(-6) M) increased the amplitude of the e.j.p. but decreased the outflow of NA during nerve stimulation. The amplitude and conduction velocity of the compound action potential of perivascular nerves were inhibited by higher concentrations of bunazosin (above 10(-5) M). The results provide evidence that bunazosin has selective inhibitory actions at alpha 1-adrenoceptors. This drug exerted differential effects on the release of co-transmitters which generate the e.j.p. and the slow depolarization, as bunazosin increased the former and decreased the latter. This suggests that e.j.p. is generated by a substance other than NA.

The distribution of gamma-adrenoceptors and P2 purinoceptors in mesenteric arteries and veins of the guinea-pig

1. Membrane potential changes and contractions were recorded from mesenteric arteries and veins of the guinea-pig, during perivascular nerve stimulation or application of noradrenaline or adenosine triphosphate (ATP). 2. After alpha-adrenoceptor blockade, noradrenaline activated low affinity adrenoceptors (gamma-adrenoceptors) causing depolarization and arterial contraction only in the presence of an inhibitor of catecholamine uptake. 3. Noradrenaline did not cause depolarization or contraction of the vein after alpha-adrenoceptor blockade even after catecholamine uptake was blocked. 4. Adenosine triphosphate caused depolarization and contraction of both arteries and veins. These responses were abolished by alpha-,beta-,methylene adenosine triphosphate (Me-ATP). 5. Me-ATP abolished rapid excitatory junction potentials (e.j.ps) caused by perivascular nerve stimulation of arteries but had no effect on arterial responses mediated by gamma-adrenoceptors. 6. In veins, perivascular nerve stimulation evoked slow e.j.ps which persisted in the presence of Me-ATP but were abolished after blockade of alpha-adrenoceptors. 7. The observations indicate that P2 purinoceptors are present on both mesenteric artery and vein whilst gamma-adrenoceptors are localized near the neuromuscular junction of the artery. However gamma-adrenoceptors do not appear to be directly involved in the generation of arterial e.j.ps.

Mechanisms of inhibitory noradrenergic transmission in the rabbit facial vein

In isolated buccal segment of the rabbit facial vein, electrical responses produced by perivascular nerve stimulation and exogenously applied noradrenaline (NA) were recorded from the smooth muscle cells using microelectrode. Perivascular nerve stimulation hyperpolarized the smooth muscle cell membrane. The hyperpolarization was converted to depolarization after application of the beta-adrenoceptor antagonist, propranolol, and the depolarization was blocked by alpha 2-adrenoceptor antagonists, yohimbine. These responses elicited by nerve stimulation were blocked by tetrodotoxin or guanethidine, but not by atropine. Exogenously applied NA mimicked the responses elicited by nerve stimulation. The amplitude of the beta-adrenoceptor-mediated hyperpolarization was increased in low potassium solution, decreased in high potassium solution, but unaltered by low sodium or low chloride solution, i.e., the hyperpolarization may be generated by an increase in potassium conductance of the membrane. An involvement of the apamin-sensitive (Ca-dependent) potassium channel or sodium-potassium ATPase in the hyperpolarization was ruled out.

Mechanism of action of alpha-adrenoceptor activation in single cells freshly dissociated from the rabbit portal vein

1. The action of noradrenaline was studied in freshly dispersed cells of the rabbit portal vein using microelectrode techniques. 2. In normal physiological salt solution, the ionophoretic application of noradrenaline evoked an alpha-adrenoceptor-mediated depolarization and sometimes a beta-adrenoceptor-mediated hyperpolarization. Experiments were carried out in the presence of propranolol to study the membrane mechanism associated with alpha-adrenoceptor activation. 3. In the current clamp mode of recording, the equilibrium potential of the noradrenaline-evoked depolarization was -1.9 mV. The depolarization was brought about by an increase in membrane conductance. 4. Under voltage clamp conditions, noradrenaline produced an inward current with a reversal potential of -7 +/- 3 mV (mean +/- s.e. mean). 5. The relationship between the noradrenaline-induced inward current and clamp potential was non-linear. Depolarization enhanced the conductance elicited by noradrenaline. 6. Evidence is presented which suggests that an additional conductance mechanism (probably an increase in potassium conductance) is also evoked by alpha-adrenoceptor stimulation in dispersed cells of rabbit portal vein.

Modulation by noradrenaline and yohimbine of noradrenergic transmission in the guinea-pig mesenteric artery

In the guinea-pig mesenteric artery, transmitter release modulated by noradrenaline (NA) or yohimbine was estimated from changes in amplitude of the excitatory junction potential (e.j.p.) recorded from smooth muscle cells. NA decreased the amplitude of the e.j.p. with no change in the facilitation. Yohimbine antagonized the effect of NA on the e.j.p. amplitude and enhanced the facilitation of e.j.p.; the latter action was not antagonized by NA. TTX-resistant e.j.p.s evoked by stronger intensity of stimuli were not affected by NA or yohimbine. It is concluded that NA inhibits and yohimbine enhances the release of transmitter, and that the latter event involves prejunctional alpha-adrenoceptor-dependent and -independent processes.

Heterogeneous distribution of muscarinic receptors in the rabbit saphenous artery

1. The properties of the muscarinic receptors in the rabbit saphenous artery were determined from electrical and mechanical responses of smooth muscle cells produced by acetylcholine (ACh). The inhibitory action of atropine and pirenzepine on the ACh-induced responses was also studied. 2. ACh produced a transient hyperpolarization of the membrane and inhibited the noradrenaline (NA)-induced contraction. These effects of ACh were apparent only when the endothelial cells were intact. 3. The ACh-induced transient hyperpolarization was antagonized by atropine or pirenzepine, with similar potencies (the ID50 values were about 2 x 10(-8) M for both antagonists). 4. The ACh-induced inhibition of the contraction to NA was antagonized by atropine more preferentially than by pirenzepine (the ID50 values were 2 x 10(-8) M for atropine and 10(-6) M for pirenzepine). 5. The excitatory junction potential (e.j.p.) evoked by perivascular nerve stimulation was inhibited by ACh (above 10(-8) M). The ACh-induced inhibition of the e.j.p. was antagonized by atropine more preferentially than by pirenzepine (the ID50 values were 3 x 10(-8) M for atropine and 6 x 10(-6) M for pirenzepine). 6. It is concluded that in the rabbit saphenous artery, two subtypes of muscarinic receptor (M1 and M2) are located on the endothelial cells. Stimulation of each subtype releases a different substance, i.e., a hyperpolarizing substance (M1-subtype) or a relaxant substance (M2-subtype). In prejunctional nerve terminals, the muscarinic receptors responsible for inhibiting the release of transmitter substances are of the M2-subtype.

Electrical responses of smooth muscle cells during cholinergic vasodilation in the rabbit saphenous artery

Isolated smooth muscle tissues of the rabbit saphenous artery precontracted with norepinephrine (NE) were relaxed by acetylcholine (ACh, greater than 10(-7) M) or oxotremorine (greater than 10(-7) M), through the activation of muscarinic receptors, only when the endothelial cells were intact. ACh (greater than 10(-7) M) transiently hyperpolarized the membrane (1-4 minutes) with an associated decrease in the membrane resistance, either in the presence or absence of NE, and these changes ceased during the continuous application of ACh. The ACh-induced transient hyperpolarization was not generated after mechanically removing the endothelium or by treatment with atropine. Oxotremorine (up to 10(-5) M) did not alter the membrane potential in the presence or absence of the endothelium. NE (10(-6) M) depolarized the smooth muscle membrane, which remained unchanged by additional application of ACh or oxotremorine for more than 5 minutes, or after removal of the endothelium. The excitatory junction potential generated by perivascular nerve stimulation was inhibited by ACh (greater than 10(-9) M) or oxotremorine (greater than 10(-9) M) in a concentration-dependent manner. These inhibitory actions of ACh or oxotremorine were blocked by atropine but were not affected by removal of the endothelial cells. These results suggest that the inhibitory actions of muscarinic agonists on electrical responses of smooth muscle cells of the rabbit saphenous artery were mainly indirect, i.e., a release of inhibitory substances from the endothelial cells and the inhibition of adrenergic transmission. The former required higher concentrations of ACh or oxotremorine, thereby suggesting that the latter may be more important for vasodilation related to cholinergic mechanisms.

Postjunctional alpha 2-adrenoceptors mediate venoconstriction in the hindquarters circulation of anaesthetized cats

1 A study was made of the subtypes of postjunctional alpha-adrenoceptors which mediate arterial and venous constriction in the hindquarters circulation of anaesthetized cats, as measured by changes in perfusion pressure and vena cava blood flow, respectively. 2 It was found that, while noradrenaline caused constriction in both the arterial and venous compartments, methoxamine caused only arterial constriction. Clonidine and B-HT 920 also caused arterial and venous constriction although autodesensitization to both drugs occurred. 3 The ability of either prazosin or yohimbine to antagonize the constrictor effects of noradrenaline was also examined. It was found that the combination of both antagonist drugs abolished both the arterial and venous constrictor effects of noradrenaline. However, there was a greater prazosin-resistant response to noradrenaline in the venous compartment as compared with the arterial effects of noradrenaline. Yohimbine caused approximately equal reductions in the effect of noradrenaline in both arteries and veins, which was greater than that observed with prazosin. 4 These results suggest that, in the cat hindquarters, both alpha 1- and alpha 2-adrenoceptors are present in the arterial circulation, whereas there are mainly alpha 2-adrenoceptors in the venous circulation.

Effects of bunazosin on electrical responses of smooth muscle cells of the guinea-pig mesenteric artery and vein to perivascular nerve stimulation and to noradrenaline

The noradrenaline-induced depolarization of smooth muscle cell membrane was blocked by bunazosin in the mesenteric artery but not in the mesenteric vein. Bunazosin enhanced the excitatory junction potential (e.j.p.) evoked in the mesenteric artery but did not modulate the slow depolarization evoked in the mesenteric vein. Application of noradrenaline decreased the amplitude of e.j.p. enhanced by bunazosin but not by yohimbine. It was concluded that bunazosin is a highly selective alpha 1-adrenoceptor blocker in vascular tissues.

A dual function for adenosine 5'-triphosphate in the regulation of vascular tone. Excitatory cotransmitter with noradrenaline from perivascular nerves and locally released inhibitory intravascular agent

A dual function for adenosine 5'-triphosphate in the regulation of vascular tone is considered. Adenosine 5'-triphosphate can cause vasodilation, acting via P2-purinoceptors located on vascular endothelial cells to release an endothelium-derived relaxing factor which diffuses to the vascular smooth muscle and induces vasodilation. The main source of intraluminal adenosine 5'-triphosphate is likely to be endothelial cells, and its release can be measured during pathophysiological conditions such as ischemia and hypoxia, in amounts likely to be sufficient to activate endothelial P2-purinoceptors. Adenosine 5'-triphosphate can also be released during intravascular platelet aggregation and from intact and damaged vascular smooth muscle cells, and so may play a role in the complex physiological mechanisms controlling local vascular tone under normoxic conditions and during vessel injury. Evidence is also presented for adenosine 5'-triphosphate acting as an excitatory cotransmitter with noradrenaline from sympathetic perivascular nerves, to cause vasoconstriction via excitatory P2-purinoceptors located on vascular smooth muscle. The postjunctional mechanical and electrical responses of a number of blood vessels following perivascular nerve stimulation contain a component that is resistant to blockade of the alpha-adrenoceptor. This nonadrenergic response is mimicked by adenosine 5'-triphosphate and can be blocked by selective desensitization of the P2-purinoceptor by alpha,beta-methylene adenosine 5'-triphosphate. Vesicular storage of adenosine 5'-triphosphate and its release from sympathetic perivascular nerves has also been demonstrated. The functional significance of adenosine 5'-triphosphate acting intraluminally as a vasodilator and extraluminally as a vasoconstrictor neuronal agent in the control of vascular tone is discussed.

Neuromuscular transmission in bovine mesenteric lymphatics

Neuromuscular transmission in bovine mesenteric lymphatics was investigated using the double sucrose-gap technique. Single pulses of 0.3 msec duration (35----90 V) elicited excitatory junction potentials (EJP's) with a time to peak of about 1 sec. The EJPs showed facilitation and at stimulus frequencies greater than about 0.25 Hz could summate to reach threshold for action potential firing. The action potential so produced was followed by a phasic contraction of an "all or none" type. Increasing the frequency of stimulation did not increase the force of contraction but the resulting sustained depolarization increased the probability of a second or third action potential (and thus contraction) being elicited. EJPs and their electrical and mechanical consequences could be blocked by tetrodotoxin (10(-6) M) and by phentolamine (5 X 10(-7) M) confirming their neural mediation and dependence on postjunctional alpha-receptors.

Effects of tyramine on noradrenaline outflow and electrical responses induced by field stimulation in the perfused rabbit ear artery

In the perfused rabbit ear artery the basal outflows of noradrenaline (NA) and 3,4-dihydroxyphenylglycol (DOPEG) were less than 1 ng g-1 and 1-2 ng g-1 wet weight of tissue respectively. Field stimulation increased outflows of NA and DOPEG in a frequency-dependent manner, and they reached the maximum value at frequencies over 5 Hz. Tyramine (1 X 10(-6) -1 X 10(-4) M) increased basal outflow of NA and DOPEG, in a dose-dependent manner. This effect was not blocked by tetrodotoxin (TTX, 3 X 10(-7) M), but was prevented by pretreatment with 6-hydroxydopamine (6-OHDA). Tyramine increased the field stimulation-induced outflow of NA but not that of DOPEG in a dose-dependent manner. Cocaine (1 X 10(-5) M) reduced the increased outflow of NA induced by tyramine at rest and during field stimulation, without modifying DOPEG-outflow. Guanethidine (5 X 10(-6) M), increased outflows of NA and DOPEG at rest, and reduced the NA outflow induced by field stimulation. Pretreatment with guanethidine (5 X 10(-6) M) did not block the action of tyramine on NA and DOPEG basal outflows. Additional application of guanethidine during the presence of tyramine did reduce the outflow of NA induced by field stimulation, but did not modify the outflow of NA and DOPEG at rest. Tyramine at concentrations over 1 X 10(-5) M depolarized the smooth muscle membrane of the rabbit ear artery. After chemical denervation with 6-hydroxydopamine (6-OHDA) the depolarizing action of tyramine was reduced. Tyramine-induced depolarization was attenuated by prazosin (5 X 10(-6) M) or phentolamine (5 X 10(-6) M), but not by guanethidine (5 X 10(-6) M). In 6-OHDA-denervated tissues, tyramine-induced depolarization was attenuated by phentolamine but not by prazosin. Field stimulation evoked excitatory junction potential (e.j.p.), slow depolarization and spike potential in the rabbit ear artery. Tyramine reduced, while guanethidine blocked these electrical responses. Tyramine did not alter the facilitation process of e.j.ps. In tissues pretreated with guanethidine, tyramine evoked either no electrical response or a slow depolarization during field stimulation. The slow depolarization was blocked by prazosin. Tyramine reduced the NA content of tissues in a dose-dependent manner (by 31% at 10(-4) M). Guanethidine (5 X 10(-6) M) reduced the NA content by 20%. 10 We conclude that in the rabbit ear artery, tyramine depolarizes the smooth muscle membrane indirectly by releasing neuronal NA which acts on alpha-adrenoceptors, and directly by an action on the smooth muscle cells. Two NA compartments (guanethidine-sensitive and tyramine-sensitive NA) could be identified. Field stimulation releases the former with associated generation of ej.p. and slow depolarization whilst the release of the latter is not accompanied by ej.p. generation.

Effects of amosulalol on the electrical responses of guinea-pig vascular smooth muscle to adrenoceptor activation

The effects of amosulalol, a newly synthesized sulphonamide-substituted phenylethylamine derivative, on electrical responses of smooth muscle cells of the guinea-pig vascular tissues to noradrenaline, isoprenaline and perivascular nerve stimulation were investigated. Amosulalol (10(-10) -10(-5)M) did not alter the resting membrane potential of smooth muscle cells of the mesenteric artery, the mesenteric vein, the main pulmonary artery and the portal vein. In the mesenteric artery, main pulmonary artery and portal vein, but not in the mesenteric vein, membrane depolarizations produced by noradrenaline were antagonized by amosulalol. In the portal vein, membrane hyperpolarizations produced by isoprenaline were antagonized by amosulalol. In the mesenteric artery, amosulalol (over 10(-6)M) enhanced the amplitude of excitatory junction potentials (e.j.ps) produced by perivascular nerve stimulation. Amosulalol antagonized the noradrenaline-induced decrease in the e.j.p. amplitude; this effect was much weaker than that of phentolamine. Amosulalol also antagonized the isoprenaline-induced enhancement of the e.j.p. amplitude. In the mesenteric vein, the slow depolarizations produced by perivascular nerve stimulation were depressed by amosulalol (over 10(-6)M), but the effect was much weaker than that of prazosin, yohimbine or phentolamine. Actions of amosulalol on electrical properties of vascular tissues can be summarized as follows: amosulalol blocks alpha 1- and beta-adrenoceptors. It also blocks alpha 2-adrenoceptors, though weakly.

An electrophysiological study of alpha-adrenoceptor mediated excitation-contraction coupling in the smooth muscle cells of the rat saphenous vein

The effects of perivascular nerve stimulation and application of exogenous alpha-adrenoceptor agonists on the rat saphenous vein were studied by simultaneous recordings of electrical and mechanical activities. The resting membrane potential of the saphenous vein averaged -65 mV. Perivascular nerve stimulation elicited excitatory junction potentials (e.j.ps) and slow depolarizations. Contraction was observed when either the e.j.p. or the slow depolarization reached a critical threshold of about -55 mV. Exogenously applied noradrenaline, B-HT 920 and clonidine induced depolarization and contraction similar to the slow depolarization. The responses to these agonists and the slow depolarizations were antagonized by yohimbine, but not by prazosin. The selective alpha 1-adrenoceptor agonists phenylephrine and methoxamine had very little effect on the electrical and mechanical activities of the saphenous vein. It was concluded that in the rat saphenous vein, only alpha 2-adrenoceptors are present and that these receptors mediate the slow depolarization and contraction induced by nerve stimulation.

Transmitter release modulated by alpha-adrenoceptor antagonists in the rabbit mesenteric artery: a comparison between noradrenaline outflow and electrical activity

Effects of alpha-adrenoceptor blockers (prazosin, yohimbine, phentolamine and phenoxybenzamine) on the outflow of noradrenaline (NA) and 3,4-dihydroxyphenylglycol (DOPEG) during perivascular nerve stimulation were observed in relation to electrical events in the rabbit mesenteric artery. Cocaine or imipramine increased the NA outflow and reduced the DOPEG outflow induced by nerve stimulation. In the absence of stimulation, cocaine and imipramine did not significantly modify the NA and DOPEG outflows. The alpha-adrenoceptor blockers we used enhanced the NA and DOPEG outflow during nerve stimulation, in a dose-dependent manner; the potency of the enhancement was higher for phentolamine and phenoxybenzamine than for prazosin and yohimbine. Higher concentrations (10(-5) M) of yohimbine reduced the NA and DOPEG outflows induced by nerve stimulation. Prazosin increased the DOPEG outflow in the absence of stimulation, and this effect was not inhibited by pretreatment with cocaine. Guanethidine increased the NA and DOPEG outflow in the absence of stimulation, and the NA outflow was reduced during nerve stimulation. These effects of guanethidine were prevented by pretreatment with cocaine or imipramine. Perivascular nerve stimulation evoked excitatory junction potentials (e.j.ps) and with high frequency stimulation, slow depolarization and spike potentials. Application of phentolamine, phenoxybenzamine or yohimbine enhanced, and of prazosin had no effect, on the amplitude of the e.j.p. Spike potentials were not affected by these alpha-adrenoceptor blockers. Slow depolarization ceased in the presence of prazosin, phentolamine or phenoxybenzamine, and was slightly inhibited by yohimbine. Guanethidine blocked all of these electrical responses induced by perivascular nerve stimulation. Application of prazosin, phentolamine or phenoxybenzamine did not alter the resting membrane potential of the smooth muscle cells. Depolarizations of smooth muscle membrane produced by exogenously applied NA were inhibited by prazosin, phentolamine or phenoxybenzamine. Yohimbine itself depolarized the membrane and the inhibitory effects on the NA-induced depolarization were weaker. We conclude that the smooth muscle membrane of the rabbit mesenteric artery possesses alpha 1-adrenoceptors. Increase in NA outflow by alpha-adrenoceptor antagonists during nerve stimulation was not always consistent with increase in e.j.p. amplitude, presumably due to involvement of actions other than alpha-adrenoceptor blockade with each of these antagonists.