Noradrenergic neuromuscular transmission with special reference to arterial smooth muscle

Autoinhibition, sympathetic cotransmission and biphasic contractile responses to trains of nerve stimulation in the rodent vas deferens

1. The present review critically discusses the evidence for and against the various hypotheses that have been proposed to explain the biphasic contractile response of the rodent vas deferens to trains of electrical field stimulation (EFS). 2. It is widely accepted that the initial component of the biphasic response of the rodent isolated vas deferens to trains of EFS is mediated by ATP and the second slower tonic contractions is mediated by noradrenaline (NA). This theory is based on the ability of antagonists of the post-junctional receptors for these neurotransmitters to inhibit the respective components of the biphasic response and on the ability of exogenous application of either ATP or NA to mimic the responses of each phase. 3. Prejunctional autoinhibition has also been proposed as the cause of the biphasic response. This is based primarily on the ability of alpha 2-adrenoceptor antagonists to transform responses from biphasic to monophasic and on the ability of neuronal NA uptake inhibitors to accentuate the separation of the two phases. 4. Atypical or extrajunctional NA receptors have also been proposed to be the mediators of the component of the response to nerve stimulation that is resistant to the traditional alpha-adrenoceptor antagonists. 5. Different contractile mechanisms and/or sources of calcium have also been postulated to cause the biphasic response. Blockers of intracellular Ca2+ mobilization are able to block the initial component, while blockers of extracellular Ca2+ entry inhibit the second tonic phase. 6. It is concluded that because alpha 1-adrenoceptor antagonists and blockers of P2 purinoceptors have also been shown to block both phases of the response to trains of EFS, prejunctional auto-inhibitory mechanisms perhaps provide the most sound explanation for the phenomenon of the biphasic contractile response to trains of EFS.

Patterns of excitation-contraction coupling in arterioles: dependence on time and concentration

We sought to understand the excitation-contraction coupling process in arterioles. KCl or phenylephrine (PE) was applied via the superfusion solution or by brief pulsatile ejections from a micropipette onto unpressurized arterioles (in vitro) from either the guinea pig small intestine or hamster cheek pouch. With either mode of application, KCl caused depolarizations that were tightly and predictably correlated with subsequent constrictions (electromechanical coupling). In contrast, the relationship between membrane potential and vasoconstriction in response to phenylephrine was dependent on both stimulus duration and agonist concentration. Application of short pulses of PE (< 1 s) produced mechanical responses that were dominated by pharmacomechanical coupling (i.e., they were not associated with changes in membrane potential). With longer PE stimuli, electromechanical coupling became more important and dominated microvessel responses. We conclude that adequate understanding of the signaling process in microvessels requires a consideration of both concentration and duration of application. Both the mode and duration of agonist application affect the relative degree of electromechanical or pharmacomechanical coupling in response to a vasomotor stimulus. These observations have important implications for intracellular and intercellular signaling.

Actions of vasodilator nerves on arteriolar smooth muscle and neurotransmitter release from sympathetic nerves in the guinea-pig small intestine

1. Brief constrictions of arterioles of the isolated submucosa of the guinea-pig small intestine were evoked by stimulation of the perivascular sympathetic nerves. Prior stimulation of vasodilator neurones in the submucosal nerve plexus greatly reduced the constrictor response to sympathetic stimulation. 2. Vasodilator nerve stimulation reduced both the amplitude and rate of decay of the excitatory junction potential (EJP) evoked in the arteriolar smooth muscle by sympathetic nerve stimulation. 3. Computer simulation of the effect of membrane resistance changes on the EJP amplitude indicated that the change in amplitude could not be explained by the fall in membrane resistance alone, suggesting that vasodilator nerve activity reduced neurotransmitter release from the sympathetic nerves.

Unusual effects of SCN and lyotropic anions on contractility of vascular smooth muscle from female rats

Replacement of extracellular chloride ions by thiocyanate anions (SCN-) followed by washout in normal chloride-containing solution produced contractions in isolated rat aortas and portal veins of female rats followed by slow relaxation; these contractions consisted of fast and slow phases. These SCN(-)-induced biphasic contractions were also noted in rat aortas precontracted by 80 mM KCl and 100 microM noradrenaline. No differences were noted between isolated aortic precontracted by 80 mM KCl and 100 microM noradrenaline. No differences were noted between isolated aortic strips versus intact ring preparations. The SCN(-)-induced contractions in both the aorta and portal vein were inhibited markedly by denervation with 6-hydroxydopamine. Use of prazosin, rauwolscine, propranolol, atropine, methysergide, diphenydramine, indomethacin or procaine (10(-3) M) failed to alter the SCN(-)-induced responses. However, use of phentolamine at 10(-5) M, but not at lower concentrations of the drug, resulted in complete inhibition of SCN(-)-induced contractions. Treatment of the vascular tissues with EGTA (5 mM) or incubation in Ca(2+)-free media abolished the SCN(-)-induced contractile responses. Treatment with verapamil (10(-6) M) or washing in Ca(2+)-free Krebs Ringer solution after incubation with SCN(-)-Krebs Ringer selectively inhibited the slow phases of the aortic contractions. Replacement of SCN- anions with other foreign monovalent anions or with sucrose modified the amplitude of the SCN(-)-induced contractions. These foreign anions seemed to follow a relative order of potency similar to that for a lyotropic series of anions, where acetate greater than isethionate greater than chloride greater than bromide greater than nitrate greater than iodide ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a decrease in sympathetic control of intestinal function in the aged rat

The ability of carbachol and isoprenaline to contract and relax respectively the longitudinal layer of ileal smooth muscle has been compared in rats aged six and twenty-four months. The concentration response curve to carbachol did not vary with the age of the animal. In contrast, the ability of isoprenaline to relax longitudinal smooth muscle precontracted with carbachol was significantly (p less than 0.02) reduced in the twenty-four month age group. This reduced response was due to a decrease in the maximal relaxation induced by isoprenaline rather than by a shift to the right of its dose-response curve. These results are discussed in the context of previous histochemical and microscopical studies which have shown a marked reduction in the density of the sympathetic innervation of the rat small intestine in old age.

Differential effects of calcium antagonists and Bay K 8644 on contractile responses to exogenous noradrenaline and adrenergic nerve stimulation in the rabbit ear artery

1. The effects of three calcium antagonists (nifedipine, verapamil, diltiazem) and the calcium agonist Bay K 8644 were compared on contractile responses of similar amplitude elicited by noradrenaline (NA) and electrical nerve stimulation (ENS) in the rabbit isolated ear artery. 2. Contractions induced by both NA (3 x 10(-7) M) and ENS (10 Hz, 10s) were almost exclusively mediated by alpha 1-adrenoceptors, since 10(-7) M prazosin abolished (NA) or almost abolished (ENS) the responses, and prazosin was more than three orders of magnitude more potent than rauwolscine on both types of response. 3. ENS-induced contractions were considerably less inhibited by nifedipine, verapamil and diltiazem than were those elicited by NA. Bay K 8644 enhanced responses to NA more than those to ENS. 4. The inhibitory effect of nifedipine and Ca2+ deprivation on NA-induced contractions decreased with increasing NA concentration. Reduction of the NA response by prazosin or phenoxybenzamine increased the nifedipine inhibition. 5. Reduction of the ENS-induced contractions by prazosin or phenoxybenzamine, or by use of a lower stimulation frequency did not increase the inhibitory effect of nifedipine. 6. In conclusion, the differential effects of the calcium antagonists on NA- and ENS-induced contractions were not related to differences in alpha-adrenoceptor subtype (alpha 1/alpha 2), receptor reserve or response amplitude, but may rather reflect temporal and spatial differences in alpha-adrenoceptor activation between the responses.

Responses of blood vessels in the rabbit knee to electrical stimulation of the joint capsule

1. An in vitro preparation of the rabbit knee joint, perfused with oxygenated Locke's solution, was used to study the response of articular blood vessels to electrical stimulation of the joint capsule. 2. Using trains of stimulus pulses of different durations, frequency-response curves were obtained. Electrical stimulation always produced vasoconstriction of joint blood vessels, which increased as a function of both frequency and pulse width. 3. This vasoconstrictor response was neurally mediated as it was markedly inhibited after addition to both bath and perfusate of tetrodotoxin. In addition, the response to field stimulation of the capsule was virtually abolished in animals pretreated with reserpine which depletes sympathetic nerve endings of noradrenaline. 4. The response to electrical stimulation was substantially reduced by the alpha-adrenergic antagonist phenoxybenzamine (10(-5) M), the alpha 1-blocker prazosin (10(-6) M), and by guanethidine (10(-5) M) which inhibits the release of noradrenaline, ATP and neuropeptide Y from sympathetic nerve endings. 5. The attenuation of the vasoconstrictor response to field stimulation by prazosin (10(-6) M) was little altered by addition of the alpha 2-adrenoceptor blocker rauwolscine (10(-6) M) to the perfusate. 6. alpha, beta-Methylene ATP (10(-6) M), a P2-purinoceptor desensitizer, had no effect on the vasoconstrictor response to electrical stimulation. 7. These results indicate that the vasoconstrictor response to electrical stimulation of the rabbit knee joint capsule is mediated via noradrenaline acting upon alpha 1-adrenoceptors.

Actions of neuropeptide Y on arterioles of the guinea-pig small intestine are not mediated by smooth muscle depolarization

Neuropeptide Y was applied to arterioles of the submucosa of the guinea-pig small intestine while arteriole diameter and smooth muscle membrane potential were monitored. Neuropeptide Y (50 nM-1 microM) caused no smooth muscle depolarization, and caused a small constriction in only 15 out of 38 arterioles studied. 50 nM Neuropeptide Y increased the amplitude of constriction caused by noradrenaline or brief trains of nerve stimulation, showing that it potentiated the effects of vasoconstrictors as it does in other arteries. The factor by which the amplitude was increased was greatest for small constrictions. Neuropeptide Y reduced the amplitude of the excitatory junction potential, suggesting that it decreased neurotransmitter release. These results show that the potentiating action of Neuropeptide Y does not depend on smooth muscle depolarization.

Influence of the endothelium and alpha-adrenoreceptor antagonists on responses to noradrenaline in the rabbit basilar artery

1. Noradrenaline (10(-6)-10(-2) M) produced slow, concentration-dependent depolarization of smooth muscle cells in the rabbit basilar artery, which preceded the onset of contraction by around 8 s (n = 18). 2. With concentrations greater than 10(-4) M, noradrenaline produced action potentials and fast rhythmic depolarizations superimposed on the slow depolarization. Each fast event was followed by a clear increase in the rate of smooth muscle contraction. The selective alpha 1-adrenoreceptor agonist phenylephrine produced very similar membrane and contractile responses. 3. Action potentials were not produced in artery segments where the endothelium had been removed. In these segments, the amplitude of both contraction and slow depolarization to noradrenaline was similar to that observed in segments with an intact endothelium, but the tension increased more slowly; 84 s compared to the 52 s required to produce 50% of total contraction when the endothelium was functional. 4. The selective alpha 1-antagonist prazosin (10(-6) M) either abolished or significantly reduced both the slow depolarization (with concentrations less than 10(-3) M-noradrenaline) and smooth muscle contraction to noradrenaline. When prazosin was present, action potentials with 10(-3) M-noradrenaline were only produced in 50% of the cells studied. 5. Irreversible blockade of alpha-adrenoreceptors with benextramine (10(-5) M for 20 min) abolished action potentials and both the depolarization and contraction produced with all but the highest concentrations of noradrenaline. With 10(-3) M-noradrenaline, depolarization was produced but it was significantly reduced and usually not associated with smooth muscle contraction. 6. The results show that smooth muscle depolarization, contraction and possibly endothelium-dependent action potentials are produced by alpha-adrenoreceptor stimulation. They also show that noradrenaline-induced action potentials produce smooth muscle contraction, and that slow depolarization is an important, but not absolute requirement for contraction. The fact that action potentials were produced in response to high concentrations of noradrenaline in the presence of prazosin, but not after benextramine, suggests that these concentrations of noradrenaline can surmount competitive antagonism with prazosin.

Noradrenaline induces the polyploidization of smooth muscle cells: the synergism of second messengers

The effect of noradrenaline (NA) on DNA replication of cultured smooth muscle cells (SMC) isolated from rat aorta was examined. It was found that 10 microM NA significantly increased (approximately by twofold) the frequency of tetraploid cells. Cultivation of 4C cells isolated by flow cytometric cell sorting revealed that they were true polyploid cells. This receptor-mediated effect of NA was blocked only by simultaneous action of alpha- and beta-adrenoreceptor antagonists. SMC polyploidization was also stimulated by simultaneous application of direct activators of "second messenger" systems forskolin and phorbolmyristate-acetate. Thus, NA may be one of mediators of the "hypertensive" response of vessel wall SMC, which probably occurs due to synergism of two second messenger systems.

A study of the phasic response of arterioles of the guinea pig small intestine to prolonged exposure to norepinephrine

Arterioles of the guinea pig small intestine constricted in response to norepinephrine, but the constriction was not maintained. The duration of constriction was reduced after pretreatment by theophylline, 3-isobutyl-1-methyl-xanthine, bromo-cAMP, or bromo-cGMP, suggesting that the relaxation was related to an increase in cyclic nucleotide levels in the cell. Forskolin also reduced the duration of constriction, suggesting the involvement of cAMP. The duration of constriction was not affected by propranolol or isoprenaline, indicating no involvement of beta adrenoceptors. A scheme to explain these observations, in which alpha 1-adrenoceptor activation stimulates adenylate cyclase, leading to a rise in cAMP and an increased rate of intracellular calcium sequestration is proposed. The resulting fall in intracellular calcium leads to repolarization and relaxation.

Aminergic histofluorescence and contractile responses to transmural electrical field stimulation and norepinephrine of human middle cerebral arteries obtained promptly after death

The responses of cerebral arteries to catecholamines and sympathetic nerve stimulation show wide variation between animal species. We examined the catecholaminergic histofluorescence and the contractile responses elicited by transmural electrical field stimulation and norepinephrine (NE) in proximal segments of human middle cerebral artery (MCA) obtained during autopsy. Twenty-four percent of the specimens were obtained within 2 hours and 76% within 4 hours of death. A moderately dense catecholaminergic histofluorescence was seen in all segments of human MCA using the glyoxylic acid technique, counterstained with pontamine sky blue. However, only seven of 35 (20%) MCA segments tested showed tetrodotoxin-blocked transmural electrical field stimulation contractions, and all of these were harvested within 4 hours of death. The responses were mostly seen in the most proximal MCA segments and, at 32 Hz, only achieved 6 +/- 1% of the maximal tissue contraction. NE caused two distinct responses in human MCA segments. At low concentrations, it acts via an alpha-like adrenoreceptor to cause contractions 20 +/- 3% of the maximal tissue response. The NE ED50s for the three successive segments were not different from each other; the value for the most-proximal segment was 7.9 +/- 0.2 x 10(-7) M. At concentrations above 10(-5) M, this catecholamine acts on low-affinity sites resistant to alpha-adrenergic antagonists causing contractions that at 10(-3) M reach 52 +/- 5% of the maximal tissue response.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenaline (gamma) and ATP responses of innervated and non-innervated rat cerebral arteries

1. The distribution of sympathetic adrenergic nerves on the rat middle cerebral artery and on the arterioles which originated from it was determined by use of gloxylic histochemistry. 2. Whereas the middle cerebral artery and proximal arterioles arising from this artery received a sympathetic innervation, the distal regions of the same arterioles were devoid of innervation. 3. The arteries and arterioles which were innervated were depolarized by noradrenaline in the combined presence of alpha- and beta-adrenoceptor antagonists. Those which were not innervated were not depolarized by noradrenaline. 4. ATP depolarized all arteries and arterioles examined. 5. These observations are discussed with respect to the similarities and differences between gamma-adrenoceptors and P2 purinoceptors.

Evidence that neuropeptide Y released from noradrenergic axons causes prolonged contraction of the guinea-pig uterine artery

The participation of neuropeptide Y (NPY) in transmission from sympathetic vasoconstrictor neurons was investigated in the guinea-pig uterine artery, where NPY has been demonstrated immunohistochemically in noradrenergic axons. Exogenous NPY produced long-lasting contractions of isolated arterial segments at low resting tone. Low concentrations of NPY (10(-8)-3 X 10(-7) mol.l-1) were more potent than equimolar concentrations of noradrenaline (NA). NPY produced concentration-dependent desensitization to further application of NPY, but did not affect the magnitude of NA contractions. Trypsin (1.4-2 micrograms.ml-1) reduced NPY-induced contractions by 80-100%, but did not alter NA-induced contractions. Transmural electrical stimulation of arterial segments, after surgical removal of vasodilator axons, produced biphasic contractions which were abolished by guanethidine. Prazosin abolished the fast phase of the neurogenic contraction, leaving a slow contraction with a time course similar to that produced by a low concentration of NPY. The slow contraction was more pronounced at higher frequencies of stimulation (15-20 Hz) than at lower frequencies, and was selectively reduced after desensitization produced by NPY (10(-5) mol.l-1), or after exposure to trypsin. These results suggest that sympathetic vasoconstriction of the guinea-pig uterine artery is produced by release of both NA and NPY from noradrenergic axons.

New alpha 1-adrenergic receptor antagonists for the treatment of hypertension: role of vascular alpha receptors in the control of peripheral resistance

The pharmacology, clinical efficacy and safety of new alpha-adrenergic receptor antagonists for the treatment of hypertension was reviewed (Table XIV). Although all these agents block alpha 1 receptors, some of them have additional effects on histamine, serotonin, dopamine, and alpha 2 receptors. These other actions account for the differences in the side effect profiles observed, i.e., increased incidence of central nervous system side effects found with indoramin, ketanserin, and urapidil, as well as for some additional beneficial effects of ketanserin (i.e., antiplatelet aggregation activity). The magnitude of BP reduction observed with antagonists of alpha 1-adrenergic receptors is modest. In most studies, the degree of BP reduction is comparable to that of prazosin, but less than that achieved with thiazide diuretics, beta-receptor antagonists, or methyldopa. Studies on the comparative efficacy and safety of new alpha 1 antagonists with converting enzyme inhibitors or calcium-channel blockers are not available. In general, alpha 1 antagonists produce greater reductions in standing than in supine BP, an effect due to the venodilatory action of these drugs. New alpha 1 antagonists appear to have equal efficacy in black and white hypertensive individuals. Their comparative efficacy and safety in young vs elderly hypertensive individuals requires further investigation. No information about the possible development of tolerance during treatment with new alpha 1 blockers was encountered. The effects of alpha 1 antagonists on HR are variable and depend on how long after the oral dose the measurements were obtained. In most studies, no significant HR changes are noticed for readings obtained 24 hours post dose; whereas tachycardia has been observed at the time of peak hypotension. Since alpha 1 antagonist-induced tachycardia is most likely of reflex nature, i.e., mediated to an increase in sympathetic activity, the increased HR may be associated with increases in myocardial contractility and in myocardial oxygen consumption. Consequently, a 24-hour HR monitoring during treatment with alpha 1 antagonists should be required for evaluation of new agents. The hemodynamic, humoral, and hormonal effects of the newer alpha 1-receptor antagonists are comparable to those of prazosin. The most consistent finding is a reduction in total peripheral resistance associated with either no change or with only small increases in cardiac index. These agents have been shown either not to change or to increase renal blood flow.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptide Y and reserpine-resistant vasoconstriction evoked by sympathetic nerve stimulation in the dog skeletal muscle

1. The effects of sympathetic nerve stimulation (evoked by recordings of authentic irregular vasoconstrictor nerve fibre discharge with average frequencies of 0.59, 2.0 and 6.9 Hz) on the perfusion pressure and the overflow of noradrenaline (NA) and neuropeptide Y-like immunoreactivity (NPY-LI) were investigated in the blood-perfused gracilis muscle of the dog in situ. 2. Nerve stimulation in the untreated control group evoked a frequency-dependent increase in perfusion pressure and overflow of NA. A significant overflow of NPY-LI was found at the highest frequency only. 3. In a separate group of animals, the sympathetic supply was unilaterally interrupted by preganglionic decentralization before the administration of reserpine (1 mgkg-1 i.v.) 24 h before the experiment. Reserpine reduced the NA content of the intact and decentralized gracilis and gastrocnemius muscle by 98-99%. Reserpine also induced a marked (80%) reduction of the muscular content of NPY-LI. The depletion of NPY-LI was, in contrast to that of NA, prevented by the decentralization, suggesting that nerve impulse activity was of primary importance for the reserpine-induced depletion of NPY-LI. 4. A slowly developing and long-lasting perfusion pressure increase was evoked by nerve stimulation, at 2.0 and 6.9 Hz after reserpine treatment. These responses were larger in the decentralized, as compared to the intact gracilis muscle and correlated with the nerve stimulation evoked overflow of NPY-LI (r = 0.79, P less than 0.001). Stimulation at 0.59 Hz caused vasoconstriction in the decentralized but not in the intact gracilis. 5. Administration of alpha,beta,-methylene adenosine triphosphate did not evoke an increase in perfusion pressure in the gracilis muscle of reserpine-treated animals. 6. In conclusion, a large perfusion pressure increase to sympathetic nerve stimulation occurs in the reserpine-pretreated skeletal muscle vasculature of the dog in vivo, providing that preganglionic decentralization has been performed. It is suggested that the released NPY-LI may mediate this vasoconstrictor response.

Actions of neuropeptide Y on innervated and denervated rat tail arteries

1. Neuropeptide Y caused a dose-dependent contraction and depolarization of the smooth muscle of the rat tail artery. 2. 30 nM-neuropeptide Y increased the contraction caused by either nerve-released noradrenaline or smooth muscle action potentials. 3. 30 nM-neuropeptide Y did not change the amplitude or rate of rise of the smooth muscle action potential. It did not change the amplitude of small excitatory junction potentials, suggesting that it did not affect neurotransmitter release. 4. 30 nM-neuropeptide Y increased the contraction caused by exogenous noradrenaline, 5-hydroxytryptamine and K in concentrations that gave submaximal contractions. It did not affect the response to higher concentrations that gave maximal or near-maximal contractions.

Sympathetic sprouting in the central nervous system: a model for studies of axonal growth in the mature mammalian brain

Sympathetic fibers innervate many peripheral tissues but are normally confined to extracerebral structures within the cranial cavity, e.g. blood vessels. The invasion of the central nervous system by vascular sympathetic axons is a unique example of neuronal plasticity which provides new information concerning the regulation and mechanisms of neuronal sprouting in both the peripheral and central nervous systems. In this paper, the principal findings concerning the conditions under which such sprouting occurs, the mechanisms which may be involved, and the question of its possible function are reviewed. Of special interest is the fact that a nerve growth factor-like brain factor may be involved in this growth response. The principles gleaned from studies of this sprouting phenomenon may be applicable to other models of neuronal plasticity and may have clinical relevance.

Effects of denervation on the responses of the rat tail artery to alpha beta-methylene ATP

1. One microM alpha beta-methylene ATP (alpha beta meATP) caused transient contraction and depolarization of the smooth muscle of the rat tail artery. 2. The peak contraction was greater in denervated arteries; the peak depolarization was the same in normal and denervated arteries. 3. This suggests that alpha beta meATP receptors do not increase following denervation. 4. In denervated arteries electrical stimulation produced depolarizing responses that were not due to neurotransmitter release but which were blocked by alpha beta meATP. 5. It is suggested that alpha beta meATP may be a blocker of cation channels.

Junctional transmission in renin-containing and smooth muscle cells of the afferent arteriole

Intracellular recordings were done in renin-containing juxtaglomerular (JG) and vascular smooth muscle (VSM) cells of the mouse kidney afferent arteriole. Both cell types exhibited a membrane potential around -75 mV and spontaneous depolarizing transients resembling spontaneous excitatory junction potentials (SEJPs) in the arterioles of other organs. The amplitude distribution of these randomly occurring transients was skewed in both cell types with a modal value of 1.2-1.9 mV. Activation of presumably postjunctional alpha 1-, P2-, ANG II- and AVP-receptors depolarized JG and VSM cells. Application of the P1-purinoceptor agonist 2-chloroadenosine strongly increased frequency and amplitude of the SEJP-like events, whereas these transients were abolished by the P1-purinoceptor antagonist 8-phenyltheophylline, both substances presumably acting on prejunctional receptors. The SEJP-like events were completely depressed by reserpine treatment, but not abolished by alpha 1-, alpha 2-, and P2-antagonists. At present, it cannot be decided, whether norepinephrine is the sole transmitter in the afferent arteriole, acting on specialized junctional adrenoceptors with the P2-purinoceptors being irrelevant for junctional transmission, or whether both substances are co-transmitters. Except norepinephrine and ATP, all other transmitter candidates tested were ruled out for various reasons.

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.

Effects of alpha beta methylene ATP on membrane potential, neuromuscular transmission and smooth muscle contraction in the rat tail artery

alpha beta methylene ATP (meATP, 400 nM-1 microM) caused depolarization and constriction of the smooth muscle of the rat tail artery, and block of the excitatory junction potential (e.j.p.). A similar depolarization caused by 25 mM K did not block the e.j.p. Contractions caused by nerve-released noradrenaline acting on alpha-adrenoceptors were potentiated by meATP but this was probably due to the depolarization as 25 mM K had a similar effect. MeATP blocked e.j.p.s recorded in the presence of 3 microM tetrodotoxin, suggesting that meATP was not blocking e.j.p.s by a local anaesthetic action.

Analysis of the alpha-adrenoceptor-mediated, and other, components in the sympathetic vasopressor responses of the pithed rat

The vascular receptors activated following sympatho-adrenal stimulation were determined by analysing the effects of 'selective' antagonists on the vasopressor response to spinal sympathetic nerve activation in the pithed rat. The net vascular response to adrenal stimulation was a balance between alpha-adrenoceptor-mediated vasoconstriction and beta-adrenoceptor-mediated vasodepression. Part of the alpha-adrenoceptor-mediated response was 'prazosin-sensitive' (alpha 1) and the remainder was abolished by rauwolscine (alpha 2). As with adrenal stimulation, direct sympathetic nerve stimulation of the vasculature evoked pressor responses which were partly resistant to prazosin. Rauwolscine only partly blocked the prazosin-sensitive component. Reserpine pretreatment led to smaller responses than prazosin plus rauwolscine. Thus, the response resistant to alpha-adrenoceptor antagonists could be mediated, in part, by adrenoceptors distinct from alpha-adrenoceptors, as currently defined. alpha, beta-Methylene ATP reduced the nerve-mediated pressor response after alpha-adrenoceptor blockade or reserpine pretreatment but not in drug-free controls. The results suggest that stimulation of the adrenal medulla can produce a vasopressor response which consists of summating alpha 1- and alpha 2-adrenoceptor-mediated components, and is identical to the effect of injected adrenaline. In contrast, the response to vasopressor nerve stimulation appears to be essentially mediated by alpha 1-adrenoceptors, with a facilitatory influence from alpha 2-adrenoceptors. A further response obtained after alpha-adrenoceptor blockade may contain a purinergic component and another which is adrenergic but not mediated by stimulation of alpha-adrenoceptors.

Measurements of the membrane potential of arterial smooth muscle in anesthetized animals and its relationship to changes in artery diameter

Intracellular recordings of membrane potential were made from the smooth muscle of small (90-200 micron i.d.) arteries in the ears of anaesthetized guinea pigs and rabbits. Perivascular nerve stimulation evoked excitatory junction potentials and action potentials in the smooth muscle. No spontaneous muscle action potentials were recorded. Local block of perivascular nerve activity with tetrodotoxin caused a vasodilatation and a small hyperpolarization of the smooth muscle. Prazosin caused a smaller vasodilatation and no hyperpolarization. It was concluded that some of the nerve-mediated arterial tone may be caused by noradrenaline acting alpha 1-receptors, and some by some other mechanism.

Is there a basis for distinguishing two types of P2-purinoceptor?

It is suggested that the P2-purinoceptor may be separated into two subtypes largely on the basis of the rank order of agonist potency of structural analogues of ATP and also on the activity of antagonists at the P2-purinoceptor: Subtype 1 (designated P2X), potency order: alpha, beta-methyleneATP, beta, gamma-methyleneATP greater than ATP = 2 methylthioATP; antagonism by ANAPP3 and selectively desensitisation following administration of alpha, beta-methyleneATP; present in the vas deferens and urinary bladder of guinea-pig and rat, frog and rat ventricle, and also in the smooth muscle of the rat femoral artery and rabbit central ear artery, where they mediate excitation. Subtype 2 (designated P2Y), potency order: 2-methylthioATP much greater than ATP greater than alpha, beta-methyleneATP, beta, gamma-methyleneATP; weak antagonism by ANAPP3 and desensitisation following administration of alpha, beta-methyleneATP; present in the guinea-pig taenia coli and the longitudinal muscle layer of the rabbit portal vein, where they mediate relaxation and also on the vascular endothelial cells of the rat femoral artery and pig aorta (where occupation leads to the production of endothelium-derived relaxing factor). Differences in the structure of the P2-purinoceptor in various tissues may be useful in the development of drugs for the treatment of vascular, gastrointestinal and urinoglenital disorders.

Two-component responses to sympathetic nerve stimulation in the rat tail artery

Membrane potential and tension were recorded simultaneously from the smooth muscle of the rat tail artery. A single stimulus to the perivascular nerves caused a tension transient. The tension transient had two components, one due to a muscle action potential and one due to alpha-adrenoceptor activation. During trains of stimuli most of the tension was due to alpha-receptor activation, even when every stimulus caused a smooth muscle action potential.

Perivascular nerves with immunoreactivity to vasoactive intestinal polypeptide in cephalic arteries of the cat: distribution, possible origins and functional implications

The distribution of nerves containing vasoactive intestinal polypeptide(VIP)-immunoreactive material was examined in the cephalic arteries and cranial nerves of cats using an indirect immunofluorescence procedure on whole mounts. Perivascular VIP-immunoreactive nerves were widely distributed in arteries and arterioles supplying glands, muscles and mucous membranes of the face. Within the cerebral circulation, perivascular VIP-immunoreactive nerves were most abundant in the circle of Willis and the proximal portions of the major cerebral arteries and their proximal branches supplying the rostral brainstem and ventral areas of the cerebral cortex. Nerves containing VIP-immunoreactive material were absent from distal portions of arteries supplying the posterior brainstem, cerebellum and dorsal cerebral cortex. Cerebral perivascular VIP-immunoreactive nerves had extracerebral origins probably from VIP-immunoreactive perikarya within microganglia in the cavernous plexus and external rete. Extracerebral perivascular VIP-immunoreactive nerves probably arose from VIP-immunoreactive perikarya in microganglia associated with the tympanic plexus, chorda tympani, lingual nerve and Vidian nerve as well as from cells in the otic, sphenopalatine, submandibular and sublingual ganglia. Therefore, it seems likely that each major segment of the cephalic circulation is supplied by local VIP-immunoreactive neurons. If the VIP-immunoreactive nerves cause vasodilation, they are well placed to allow redistribution of arterial blood flow within the head. During heat stress, neurogenic vasodilation of the appropriate beds would permit efficient cooling of cerebral blood, particularly that supplying the rostral brainstem and surrounding areas of the cerebral cortex.

Contraction-mediating alpha-adrenoreceptors in isolated human omental, temporal and pial arteries

The alpha-adrenoreceptors mediating contraction in human omental (OA), temporal (TA), and pial (PA) arteries obtained during surgery, were characterized by means of subtype selective agonists and antagonists. In the OAs and TAs, prazosin concentration-dependently shifted the noradrenaline (NA) concentration-response (cr) curve towards higher concentrations, without depression of maximum. The corresponding Schild plots had slopes close to unity. Also rauwolscine caused a rightward displacement of the NA cr-curve in both OAs and TAs, without affecting the maximum response. In the TAs, OAs, rauwolscine 3 x 10(-8) M shifted the curve and the Schild plot seemed to be biphasic. Oxymetazoline, but not clonidine, produced contractile responses in the TAs and OAs, and phenylephrine was a full agonist in both types of vessel. The PAs showed a pronounced inter- and intra-individual variation in the response to NA, and often exhibited spontaneous activity. Prazosin was considerably more effective than rauwolscine and yohimbine to inhibit NA-induced responses. Clonidine had no contractant effect, whereas, oxymetazoline was more, and phenylephrine less potent than NA. It is concluded that in human OAs, TAs and PAs, the alpha-adrenoreceptor mediating contraction is mainly of the alpha 1-type.

Assessment of vascular smooth-muscle mechanisms using isolated segments of the vessel wall

Isolated segments of the blood vessel wall in the form of strips or rings offer advantages for the assessment of vascular smooth-muscle contractile function and its contribution to the properties of blood vessels. Several criteria must be met to obtain these advantages. These requirements are outlined in the context of an analytical framework based on the sliding filament/crossbridge model. The applicability of this model to smooth muscle is discussed, and the concept of a "functional unit" in the vascular wall is introduced. A functional unit is defined as all the smooth-muscle cells that are mechanically linked in series and bear equal stress. Several factors contribute to heterogeneity among functional units. The constraints imposed on the estimation of the static and dynamic mechanical properties of the smooth muscle are discussed. Various experimental strategies to minimize the effects of inhomogeneities in functional units can enhance the value of mechanical measurements.

Identification and regulation of adrenergic receptors in target cells

The application of radioligand binding techniques has recently led to increased understanding of adrenergic receptors, the recognition sites for catecholamines on target cells. Radioligand binding is a simple technique to conduct, although data obtained from binding studies are not always as easy to interpret as they are to generate. Nevertheless, the goal of these studies is to help define the molecular physiology of catecholamine action. In this article, I address five principal questions that largely dominate current research on adrenergic receptors. 1) How many types and subtypes of adrenergic receptors exist? 2) What is the structure of adrenergic receptors? 3) How is occupancy of adrenergic receptors translated to changes in target cell response? 4) How do target cells regulate their complement of adrenergic receptors? 5) Do alterations in adrenergic receptors mediate altered pathophysiology in disease? Although only preliminary answers are available, the use of radioligand binding methodology, together with other biochemical, cell biological, immunological, and molecular biological approaches should provide the tools to obtain definitive answers to those questions within the near future.

The relation between the structure and innervation of small arteries and arterioles and the smooth muscle membrane potential changes expected at different levels of sympathetic nerve activity

The membrane potential changes in arterial smooth muscle due to natural sympathetic nerve activity have been calculated. The electrical properties of the smooth muscle syncytium were taken into account and various assumptions made concerning the release of noradrenaline by the perivascular nerves. The depolarization that would result from asynchronous nerve activity at various mean frequencies was calculated for arterioles and small arteries of various diameters up to 150 micron. The calculations suggested that the depolarization would be irregular and that discrete excitatory junction potentials as evoked by synchronous nerve stimulation would not be recorded during natural nerve activity. The irregularity of the depolarization would be greater in small arterioles and would cause them to reach threshold for action potential generation at lower frequencies of nerve activity than larger arteries.

Sites of action of Ca2+ channel inhibitors

Ca2+ channel inhibitors are viewed as a subgroup of Ca2+ antagonists. Most of the currently used Ca2+ channel inhibitors are thought to act by reducing Ca2+ entry into the cell through Ca2+ channels. There is substantial electrophysiological evidence that the major site of action of verapamil, nifedipine and diltiazem in cardiac cells is a sarcolemmal Ca2+ channel. Cytosolic sites of action may contribute to their effects but probably only at higher than therapeutic concentrations. The recent ligand binding studies also tend to support the view that the sarcolemma is the site of action of Ca2+ channel inhibitors in smooth muscle. High affinity binding sites for 1,4-dihydropyridines without any established function are found in fast skeletal muscle and some neuronal membranes. The binding of [3H]nitrendipine to membranes from cardiac, skeletal and smooth muscle, and from brain is saturable, reversible and of high affinity; it is sensitive to cations and other drugs that interact with Ca2+ channels. Inhibition of [3H]nitrendipine binding and blockade of K+ responses in guinea pig ileum by 1,4-dihydropyridines are well correlated, supporting the view that the observed binding is to Ca2+ channel. In contrast, blockade of Ca2+ channels in cardiac and skeletal muscle and in brain synaptosomes occurs only at higher concentrations than needed to saturate the high affinity binding sites. The therapeutic success of Ca2+ channel inhibitors in the treatment of angina pectoris, hypertension, peripheral vascular diseases, and many other disease entities is based on selective inhibition of Ca2+ entry into smooth muscle cells. The specificity of some of these drugs for Ca2+ channels in different cell types, organs, or vascular beds is probably determined by receptor modulation and the effect of reflex mechanisms, which in turn determine the indications for their therapeutic use.