Multiple actions of cocaine on neuromuscular transmission and smooth muscle cells of the guinea-pig mesenteric artery

Cocaine Constrictor Mechanisms of the Cerebral Vasculature

Cocaine constriction of the cerebral vasculature is thought to contribute to the ischemia associated with cocaine use. However, the mechanisms whereby cocaine elicits relevant vasoconstriction remain elusive. Indeed, proposed intra- and intercellular mechanisms based on over 3 decades of ex vivo vascular studies are, for the most part, of questionable relevancy due to the generally low contractile efficacy of cocaine combined with the use of nonresistance-type vessels. Furthermore, the significance attached to mechanisms derived from in vivo animal studies may be limited by the inability to demonstrate cocaine-induced decreased cerebral blood flow, as observed in (awake) humans. Despite these apparent limitations, we surmise that the vasoconstriction relevant to cocaine-induced ischemia is elicited by inhibition of dilator and activation of constrictor pathways because of cocaine action on the neurovascular unit (neuron, astrocyte, and vessel) and on vessels outside the unit. Furthermore, previous cocaine exposure, that is, conditions present in human subjects, downregulates and sensitizes these dilator and constrictor pathways, respectively, thereby enhancing constriction to acute cocaine. Identification of specific intra- and intercellular mechanisms requires investigations in the isolated microvasculature and the neurovascular unit from species chronically exposed to cocaine and in which cocaine decreases cerebral blood flow.

Modulation of the effects of norepinephrine uptake inhibitors on the norepinephrine-induced contractile response of the porcine uterine artery during early pregnancy

The effects of norepinephrine (NE) uptake inhibitors on the porcine uterine artery's contractile response to NE and their potential alteration during early pregnancy (Day 13 postcoitus; day of coitus = Day 0) in comparison with the end of the luteal phase (Days 11-14; first day of behavioral estrus = Day 0) was investigated. This pregnancy time point is characterized by a transient increase in resting uterine blood flow dependent on the presence of blastocysts, an increased endometrial vascular permeability, and the beginning of the endometrial attachment of the blastocysts. A cumulative concentration-response curve (CCRC) to NE was produced in isometrically-mounted rings. Cocaine (5 microM) was used to inhibit neuronal NE uptake and hydrocortisone (30 microM) was used to inhibit extraneuronal NE uptake. Either drug alone induced a leftward shift of the CCRC to NE without affecting the maximal response. This shift had the same amplitude at the end of the luteal phase and in early pregnancy. However, the leftward shift induced by cocaine was larger than that induced by hydrocortisone only in early pregnancy, and the leftward shift induced by exposure to both hydrocortisone and cocaine was larger in early pregnancy than at the end of the luteal phase. These results suggest that a significant sensitization of the contractile response of the porcine uterine artery to NE is induced by neuronal and extraneuronal uptake inhibitors and that this effect is altered in early pregnancy, possibly reflecting the existence of a mechanism protecting the uterine artery against excessive sympathetic stimulation.

The effect of cocaine and desipramine on neuronal uptake of [3H]-noradrenaline and sensitivity to noradrenaline of rat mesenteric resistance arteries

1. The effects of cocaine and desipramine (DMI) on neuronal uptake (uptake1) of [3H]-noradrenaline (NA) and isometric tension development to exogenous NA were assessed in mesenteric resistance arteries of Wistar rats. 2. Both drugs concentration-dependently inhibited [3H]-NA uptake1, DMI being more potent than cocaine. The maximum inhibition produced by each drug was the same as that produced by denervation with 6-hydroxydopamine. In denervated vessels there was no effect of cocaine on [3H]-NA uptake1. 3. Cocaine, in the same concentration range which caused inhibition of uptake1, increased the sensitivity to NA, while DMI, in a concentration range which inhibited uptake1, did not increase the sensitivity to NA and at high concentrations reduced the sensitivity and maximal response to NA. Since DMI affected responses to NA but not responses to vasopressin and potassium its effect is probably related to blockade of alpha 1-adrenoceptors. 4. We conclude that the effect of cocaine on the sensitivity to NA reflects inhibition of uptake1 in rat resistance arteries, while DMI cannot be used to assess the functional effect of uptake1 in this preparation.

Nerve activity-dependent variations in clearance of released noradrenaline: regulatory roles for sympathetic neuromuscular transmission in rat tail artery

The aim of this study was to find out if clearance of noradrenaline released from sympathetic nerve terminals in rat isolated tail artery is a physiological variable and if so, to determine its role for the noradrenaline-mediated neurogenic contraction. The per pulse release of noradrenaline induced by electrical nerve stimulation and the fluctuations of the level of noradrenaline at the receptors driving the contractions were assessed from the electrochemically determined noradrenaline oxidation current at a carbon fibre electrode at the surface of the artery. Both were compared with the noradrenaline-mediated neurogenic contraction. The effects on these parameters of cocaine or desipramine, or of corticosterone, were used to assess the relative roles of neuronal and extraneuronal uptake, respectively. The effects of cocaine or desipramine, which enhance the noradrenaline level at the receptors by blocking neuronal reuptake, were compared with those of yohimbine, presumed to act exclusively by enhancing the per pulse release of noradrenaline. The results seem to support the following tentative conclusions. Clearance of released noradrenaline occurs by neuronal uptake and diffusion, while extraneuronal uptake is negligible. The noradrenaline-induced neurogenic contraction is mediated via adrenoceptors on cells near the plane of the nerve plexus; the excitation spreads from these cells throughout the syncytium. The contractile response to exogenous noradrenaline may also be mediated via receptors on the innervated key cells. Reuptake of noradrenaline into the releasing varicosities, i.e. in "active junctions", is highly efficient for single quanta but rapidly saturated by repeated release, while reuptake of noradrenaline in the "surround" of active junctions is probably rarely saturated and more independent of nerve activity. Saturation of the transporter by repeated release of quanta from the same varicosity and the consequent accumulation of "residual" noradrenaline and increased diffusion out of the junction and recruitment of noradrenaline receptors in the surround may be the cause of the rapid growth of the contraction during a high frequency train. Diffusion of released noradrenaline away from the postjunctional receptors is restricted by a local nerve activity-dependent buffering mechanism which, in spite of fading of the per pulse release, helps maintain the noradrenaline concentration at the receptors and the contraction during long high-frequency trains. Reactivation of the clearance mechanisms upon cessation of nerve activity accelerates the relaxation.(ABSTRACT TRUNCATED AT 400 WORDS)

Extracellular Ca(2+)-dependent potentiation by cocaine of serotonin- and norepinephrine-induced contractions in rat vascular smooth muscle

Using front-surface fluorometry, we determined the effects of cocaine on force and cytosolic Ca2+ concentration ([Ca2+]i) in the rat aorta. We also examined the effects of cocaine on 45Ca2+ influx. Cocaine (10(-7) to 10(-4) M) alone did not alter the resting level of [Ca2+]i and force. Cocaine (< 10(-4) M), in a concentration-dependent manner, potentiated the 10(-6) M serotonin (5-HT)-induced or 10(-8) M norepinephrine (NE)-induced sustained increase in [Ca2+]i and force in the presence of extracellular Ca2+, whereas it had no potentiating effects in Ca(2+)-free solution. Similar potentiating effects of cocaine were observed in pharmacologically denervated strips. Cocaine (10(-5) M) produced a leftward shift of concentration-response curves for both 5-HT- and NE-induced increases in [Ca2+]i and force with no effect on the maximal response or the relations between [Ca2+]i and force. Cocaine (10(-5) M also accelerated the 45Ca2+ influx during activation by 10(-6) M 5-HT or by 10(-8) M NE. Cocaine (> 10(-3) M) inhibited 5-HT-, NE-, and high-K+ depolarization-induced contractions accompanied by decreases in [Ca2+]i in normal physiological salt solution and 5-HT- or NE-induced transient increase in [Ca2+]i and force in Ca(2+)-free physiological salt solution. Thus, low concentrations of cocaine potentiate NE- or 5-HT-induced contraction by augmenting the increase in [Ca2+]i. These potentiating effects may derive from either an increase in the affinity of the receptors to agonists or an increase in the Ca2+ influx. On the other hand, high concentrations of cocaine (> 10(-3) M) have a relaxant effect on vascular smooth muscle, as a result of a decrease in [Ca2+]i.

Electrophysiological and electrochemical analysis of the secretion of ATP and noradrenaline from the sympathetic nerves in rat tail artery: effects of ?2-adrenoceptor agonists and antagonists and noradrenaline reuptake blockers

The aim of this study was to investigate whether or not nerve impulses release ATP and noradrenaline in parallel from the sympathetic nerve terminals of the rat tail artery. The extracellularly recorded excitatory junction current (EJC) was used to study, pulse by pulse, the release of ATP. An electrochemical method was used to study online the nerve stimulation-induced rise in the extracellular concentration of endogenous noradrenaline at the probe, a carbon fibre electrode (CF). This parameter, which does not directly represent noradrenaline release, but reflects release minus clearance, has been termed delta[NA]CF. The effects of a number of pharmacological agents on the EJCs were examined both at 0.1 and 2 Hz, and the effects on the EJC response to 100 pulses at 2 Hz compared with that on the delta[NA]CF response. Clonidine and xylazine were used as alpha 2-agonists, yohimbine and idazoxan as alpha 2-antagonists and desipramine and cocaine as blockers of noradrenaline reuptake. Most of these agents had unwanted side effects, especially at higher concentrations. However, clonidine and xylazine depressed at lower concentrations the EJC and delta[NA]CF responses to about the same extent; these effects were partially or completely reversed by yohimbine. Yohimbine or idazoxan did not affect the EJCs at 0.1 Hz but enhanced the EJC and delta[NA]CF responses to 100 pulses at 2 Hz to the same extent. All effects of desipramine (1 microM) seemed explainable as a result of block of noradrenaline reuptake, while cocaine (10 microM) in addition exerted an 'unspecific' depressant (probably local anesthetic) effect. Under control conditions, both agents depressed the EJC but dramatically enhanced the delta[NA]CF response to 100 pulses at 2 Hz. Addition of yohimbine prevented the depressant effect of desipramine on the EJCs completely and reduced that of cocaine, but increased their effects on the delta[NA]CF response. These results are compatible with the view that ATP and noradrenaline are released in parallel from the sympathetic nerve terminals of this tissue. The different, and under some conditions even opposite, effects of desipramine or cocaine on the EJC and delta[NA]CF responses are explainable in terms of the known post-secretory effects of these agents.

Influence of neuronal uptake on pre- and postjunctional effects of alpha-adrenoceptor agonists in tissues with noradrenaline--ATP cotransmission

This study was undertaken in an attempt to explain why in some of the tissues in which noradrenaline and ATP act as co-transmitters the noradrenergic component predominates, while in others the predominant component is purinergic. Four different tissues were used: the epididymal portion of the rat vas deferens and the rabbit ear artery, tissues where the noradrenergic component predominates, and the prostatic portion of the rat vas deferens and the rabbit jejunal artery, where the purinergic component predominates. The noradrenaline content as well as the electrically-evoked release of noradrenaline were determined in all tissues. To determine the evoked release, the tissues were pretreated with pargyline (1 mmol.l-1) and then exposed to 3H-noradrenaline, washed out and transmurally stimulated (1 Hz). In addition, the influence of inhibition of neuronal uptake by desipramine (40 nmol.l-1) on pre- and postjunctional effects of adrenaline and alpha-methylnoradrenaline (and/or noradrenaline) was compared. The noradrenaline content of the tissues averaged: 17.4, 23.2, 3.1, and 4.8 micrograms.g-1 for the epididymal and the prostatic portions of the rat vas deferens and for the ear and the jejunal arteries of the rabbit, respectively. The fractional electrically-evoked release of 3H-noradrenaline was 2.02 and 2.04 x 10(-5) for the epididymal and the prostatic portions of the rat vas deferens, respectively, and 3.33 and 3.26 x 10(-5) for the ear and the jejunal arteries of the rabbit, respectively. Desipramine enhanced much more the postjunctional effect of noradrenaline, adrenaline, and alpha-methylnoradrenaline in the epididymal than in the prostatic portion of the rat vas deferens.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cocaine on excitation-contraction coupling of aortic smooth muscle from the ferret

The mechanism by which cocaine alters vascular tone is not fully understood. We determined the effects of cocaine on excitation-contraction coupling of isolated ferret aorta. Cocaine in concentrations less than or equal to 10(-4) M caused a contractile response in a dose-dependent manner. The response of control muscle was significantly larger than that in muscle from ferrets pretreated with reserpine. Cocaine-induced contraction was not affected by endothelial factors, but was significantly inhibited by prazosin 10(-7) M pretreatment. The intracellular calcium [( Ca++]i), as measured with aequorin, rose in conjunction with cocaine-induced contraction. The degree of contraction generated by 10(-4) M cocaine decreased after higher concentrations of cocaine greater than or equal to 10(-3) M, while aequorin luminescence remained elevated above the levels before 10(-6) M cocaine. The dose-response relationships of norepinephrine and sympathetic nerve stimulation were enhanced by 10(-6) M cocaine in control muscles; this did not occur in muscles from reserpine pretreated ferrets. In conclusion, (a) cocaine in concentrations less than or equal to 10(-4) M caused vascular contraction presumably by its presynaptic action with consequent alpha-1 adrenoceptor activation and consequent [Ca++]i rise; (b) high concentrations of cocaine greater than or equal to 10(-3) M reduced muscle tone by decreasing the Ca++ sensitivity of the contractile proteins; and (c) supersensitivity to norepinephrine was mediated by cocaine's action on adrenergic nerve endings.

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.

Effects of cocaine on pial arterioles in cats

We used the closed cranial window technique to observe the responses of pial arterioles to topical application of cocaine in 29 anesthetized cats. Alterations in arteriolar diameter were dependent on the concentration of cocaine applied. Cocaine dissolved in artificial cerebrospinal fluid at concentrations of 10(-8) or 10(-7) M was without effect. Concentrations of 10(-6) and 10(-5) M produced dilation (4.9 +/- 1.5% [mean +/- SEM] and 5.9 +/- 2.0%, respectively) in large arterioles (greater than 100 microns) but no significant change in the diameter of small arterioles (less than 100 microns). A concentration of 10(-4) M dilated both large and small arterioles (20.3 +/- 3.1% and 12.0 +/- 7.1%, respectively). Pretreatment with 1 mg/kg i.v. propranolol blocked the increase in pial arteriolar diameter after application of 10(-4) M cocaine and produced significant vasoconstriction in small arterioles (-8.3 +/- 3.1%). Cocaine produces vasodilation of cat cerebral arterioles. This effect appears to be mediated, at least in part, by mechanisms that depend on stimulation of beta-adrenergic receptors.

Characterization of a neurogenic and a direct smooth muscle component in the contractile response to electrical field stimulation in rat tail artery

1. The extent to which neuronal transmitter release contributes to the contractions induced by transmural nerve stimulation of the rat tail artery at various stimulus intensities was characterized. 2. Using tetrodotoxin, which blocks conduction of the action potential along the nerves, and omega-conotoxin GVIA, a blocker of transmitter release from the nerve terminals, as well as chemical and surgical denervations of the perivascular sympathetic nerves, a neurogenic and a direct smooth muscle component could be clearly separated. 3. The neurogenic component was fast in onset, rise and decline (after the end of stimulus), and showed a voltage dependency only at lower stimulus intensities. The non-neurogenic component was slower in onset, rise and decline, and showed a strict voltage dependency throughout the whole stimulus range. This implies that the non-neurogenic component becomes increasingly prominent at high, non-physiological voltages. Mechanisms underlying the declining neurogenic contractile response at the stronger stimulus intensities are discussed. 4. We found no evidence supporting the existence of a possible tetrodotoxin- or omega-conotoxin GVIA-resistant contractile component originating from the perivascular nerves (sympathetic or non-sympathetic). Thus, in order to get a purely neurogenic response stimulus intensities should be minimized to give a contraction that is fully sensitive to these two agents. 5. Transmitter release from the perivascular sympathetic nerves was fully responsible for the purely neurogenic contractions. Activation of postjunctional alpha 1-adrenergic receptors was mainly involved, with a substantial contribution from alpha 2-receptors, and a minor contribution from neuropeptide Y receptors. There was no evidence for a contractile component linked to activation of so-called gamma-adrenergic receptors. 6. Beta-adrenergic receptors, serotonergic, cholinergic, prostanoic or purinergic mechanisms do not appear to contribute to the neurogenic (or the non-neurogenic) response. The neurogenic contraction does not utilize potential-sensitive calcium channels.

Dose-response inhibition of rat compound nerve action potential by dextropropoxyphene and codeine compared to morphine and cocaine in vitro

1. The effects of dextropropoxyphene hydrochloride (dextropropoxyphene) codeine phosphate (codeine), morphine hydrochloride (morphine), and cocaine hydrochloride (cocaine) on the compound action potentials (cAP) of the rat phrenic nerve were studied in vitro. 2. Dextropropoxyphene, cocaine, codeine and morphine depressed the cAP in a reversible, dose-dependent way. After 60 min exposure to the drugs, dextropropoxyphene, inhibited the cAP between 0.025 and 0.5 mM, cocaine between 0.01 and 5 mM, codeine between 0.1 and 10 mM and morphine between 0.5 and 70 mM. The inhibition caused by 5 mM morphine could not be reversed by naloxone. 3. Dextropropoxyphene demonstrated the most potent cAP inhibition compared to the other drugs tested. The observed dose range for the inhibition of the cAP of rat phrenic nerve includes that observed after intoxications with dextropropoxyphene (0.004-0.06 mM) in the clinic. 4. For all other drugs tested the present observed dose ranges are much greater than the plasma concentrations previously reported after therapeutic use and intoxication.

Acetylcholine is an indirect inhibitory transmitter in the canine ileocolonic junction

The effects of cholinergic agents, electrical stimulation and vasoactive intestinal polypeptide (VIP) were studied on transverse muscle strips of the canine ileum, ileocolonic junction and colon. Acetylcholine, methacholine and carbachol caused concentration-dependent contractions in the three gut tissues. Only acetylcholine (greater than 10(-5) M) evoked transient relaxations in the ileum and the ileocolonic junction before the onset of contractions. During contractions by noradrenaline, acetylcholine induced relaxations, which were enhanced by atropine; electrical stimulation also caused frequency-dependent relaxations. Propranolol or naloxone did not affect the relaxations. Hexamethonium, cocaine or lidocaine inhibited the relaxations induced by acetylcholine but not those evoked by electrical stimuli. Tetrodotoxin inhibited all relaxations, VIP did not evoke relaxation in the ileocolonic junction. These data indicate that acetylcholine stimulates nicotinic receptors on non-adrenergic non-cholinergic neurons, which do not release VIP or opioids. It is thus suggested that there is a nicotinic inhibitory innervation at the canine ileum and ileocolonic junction.

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.

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.

Actions of ATP and alpha, beta-methylene ATP on neuromuscular transmission and smooth muscle membrane of the rabbit and guinea-pig mesenteric arteries

In the rabbit mesenteric artery, adenosine triphosphate (ATP), showed two actions on the membrane potential of muscle cells: low concentrations (1-10 microM) hyperpolarized and high concentrations (greater than or equal to 50 microM) depolarized the membrane. Both changes in the potential were accompanied by increases in ionic conductance. In the rabbit mesenteric artery, alpha, beta-methylene ATP (MeATP), (greater than or equal to 30 nM) depolarized the muscle membrane at a lower concentration than ATP (greater than or equal to 50 microM), and increased the ionic conductance of the membrane. The depolarization induced by ATP was prevented by low concentrations of MeATP, but the hyperpolarization was retained. Furthermore, the hyperpolarization was not affected by theophylline (10 microM). In the guinea-pig mesenteric artery, ATP and MeATP depolarized and increased the ionic conductance of muscle membrane, but to depolarize the membrane, higher concentrations of both agents were required, compared to those in the rabbit mesenteric artery. In the mesenteric arteries from both species, perivascular nerve stimulation evoked excitatory junction potentials (e.j.ps). In both tissues, MeATP inhibited the amplitude of e.j.ps at lower concentrations than did ATP, and both agents had more potent inhibitory actions on rabbit than on guinea-pig. The inhibition of e.j.p. induced by low concentrations of these agents showed no relationship to depolarization, but the inhibition induced by high concentrations was paralleled by depolarization and increase in ionic conductance of the membrane. In the rabbit mesenteric artery, overflow of noradrenaline (NA) and its metabolite (3,4-dihydroxyphenylglycol; DOPEG) produced by perivascular nerve stimulation was examined. ATP (0.1 mM) but not MeATP (0.1 microM) reduced the overflow of NA, whereas both agents had no effect on the overflow of DOPEG. Exogenously applied high concentrations of NA (greater than or equal to 3 microM) depolarized the muscle membrane in both species. These NA-induced depolarizations were not affected by treatment with ATP or MeATP. It is concluded that, in the rabbit mesenteric artery, ATP is more likely to be involved in generation of e.j.ps than is NA. A similar interpretation in the guinea-pig mesenteric artery is complicated by the depolarization produced by high concentrations of ATP or MeATP.

Comparison of the effects of caffeine and procaine on noradrenergic transmission in the guinea-pig mesenteric artery

The effects of caffeine and procaine on noradrenergic transmission in the guinea-pig mesenteric artery were investigated by recording electrical responses of smooth muscle cells and by measuring the outflow of noradrenaline (NA) and 3,4-dihydroxyphenylglycol (DOPEG) induced by perivascular nerve stimulation. Caffeine possessed dual actions on the membrane, i.e., at low concentrations (2.5 X 10(-4)-5 X 10(-4)M), it hyperpolarized the membrane and decreased the membrane resistance and at high concentrations (over 2.5 X 10(-3)M) it depolarized the membrane and increased the membrane resistance. Procaine (over 10(-4)M) consistently depolarized the membrane and increased the membrane resistance. The amplitude of the excitatory junction potential (e.j.p.) produced by perivascular nerve stimulation was increased by low concentrations of procaine (2.5 X 10(-5)-10(-4)M) or high concentrations (10(-3)-5 X 10(-3)M) of caffeine and was decreased by low concentrations of caffeine (2.5 X 10(-5)-10(-4)M) or high concentrations of procaine (5 X 10(-4)-10(-3)M). Higher concentrations of caffeine (over 5 X 10(-3)M) induced a spike potential on the e.j.p., while higher concentrations of procaine (over 2.5 X 10(-3)M) inhibited the generation of e.j.ps. Facilitation of e.j.ps produced by repetitive stimulation of perivascular nerves remained unchanged by caffeine, while it was enhanced by procaine at any given concentration (caffeine 2.5 X 10(-4)-10(-3)M; procaine 10(-4)-10(-3)M). The membrane depolarization produced by exogenously applied NA (10(-5)M) was not blocked by pretreatment with procaine. Conduction velocity of perivascular nerve excitation remained unchanged by application of caffeine (up to 5 X 10(-3)M), and was reduced by application of procaine (over 2.5 X 10(-4)M). Outflow of NA during perivascular nerve stimulation remained unchanged by caffeine (10(-4)-3 X 10(-3)M), while it was enhanced by procaine (over 2.5 X 10(-4)M). The outflow of DOPEG was slightly reduced by caffeine (10(-3)-5 X 10(-3)M) and by lower concentrations of procaine (10(-4)-2.5 X 10(-4)M) but was not altered by higher concentrations of procaine (10(-3)-5 X 10(-3)M). It is concluded that in the guinea-pig mesenteric artery, high concentrations of caffeine (over 10(-3)M) increased the e.j.p. amplitude which might be due to an increase in membrane resistance of the smooth muscle cells. No marked effect of caffeine was observed on transmitter release from the nerve terminals. Procaine (over 2.5 X 10(-4)M) increased transmitter release from perivascular nerves and blocked the re-uptake mechanism of released NA. The mechanisms underlying the decrease in ej.p. amplitude by procaine remain to be determined.

Electrical responses of smooth muscle cells of the rabbit ear artery to adenosine triphosphate

Ionophoretic application of ATP to smooth muscle cells of the rabbit ear artery produced rapid depolarization of the membrane and, in the case of large doses of ATP, spike potentials or slow oscillatory potentials. The ATP response desensitized rapidly, and required over 70 s for recovery. When the intervals between repetitive application of ATP were shorter than 70 s, the amplitudes of the ATP responses successively decreased. Ejection of ATP with increasing intensities of current (10-15% of the first) was required to produce successively increasing amplitudes of ATP responses. Repetitive stimulation of perivascular nerves (at intervals of less than 10 s) evoked increasing amplitude of excitatory junction potentials (e.j.p.s). Quinidine (over 5 X 10(-5) M) inhibited and theophylline (over 5 X 10(-4) M) enhanced the ATP response, with associated depolarization or hyperpolarization of the membrane, respectively. Cocaine (over 10(-6) M) depolarized the membrane and enhanced the ATP response. Phentolamine reduced the amplitude of the ATP response with no change in the membrane potential, only when the concentration was extremely high (over 10(-4) M). These all therefore appear to represent non-specific interactions with the effects of ATP. Bath application of ATP depolarized the membrane dose dependently and, at concentrations over 5 X 10(-7) M, produced spike potentials. The amplitude of electrotonic potentials decreased during the ATP-induced depolarization, thereby suggesting an increase in ionic conductance of the membrane. ADP depolarized the membrane, the effect being weaker than that of ATP. Both AMP and adenosine hyperpolarized the membrane. The results provide evidence that in the rabbit ear artery, the e.j.p. could be mimicked by ATP. ATP can however only account for the fast e.j.p. if it is released in increasing amounts with successive nerve discharges. Reported blocking agents for ATP receptors did not block the response to ATP in this tissue.

Effects of nifedipine derivatives on smooth muscle cells and neuromuscular transmission in the rabbit mesenteric artery

The effects of nifedipine and its derivatives, nisoldipine, nimodipine and nitrendipine, on smooth muscle cells and neuromuscular transmission were investigated in the rabbit mesenteric artery. These agents in concentrations of up to 2 X 10(-7) M modified neither the membrane potential nor the membrane resistance, yet did inhibit the spike potential evoked by direct muscle stimulation in the presence of TTX or by perivascular nerve stimulation. The inhibitory action of nitrendipine was weaker than that of the other derivatives. These agents had no effect on the miniature excitatory junction potentials (m.e.j.ps) and e.j.ps evoked by the first stimuli and after completion of the facilitation in a train stimulation. Nifedipine and its derivatives had no effect on the K-induced depolarization but did have a marked effect on the K-induced contraction. Nisoldipine showed the highest inhibitory potency for the K-induced contraction [IC50 was 1.2 X 10(-9) M for the 128 mM (K)o-induced contraction]. Noradrenaline depolarized the membrane (greater than 5 X 10(-7 M) and produced contraction (greater than 3 X 10(-7) M). The contraction evoked by high concentrations of noradrenaline was inhibited by these agents to a greater extent than that evoked by low concentrations. The contraction evoked by perivascular nerve or direct muscle stimulation was partly inhibited by nifedipine and its derivatives. The contraction elicited by Na-free solution was inhibited by these agents but the noradrenaline- or caffeine-induced contraction in Ca-free solution was not. These results indicate that in smooth muscle cells of the rabbit mesenteric artery, nifedipine and its derivatives inhibit the voltage dependent Ca-influx which occurs during the spike potential and in response to K-, electrically- or noradrenaline-induced depolarization. These derivatives appear to have no effect on the adrenoceptor operated Ca increase in myoplasm which occurs in the absence of depolarization at low noradrenaline concentrations. The derivatives act as Ca antagonists with a quantitative difference in potency, i.e. the strongest action was observed with nisoldipine and the weakest with nitrendipine.

Roles of extrajunctional receptors in the response of guinea-pig mesenteric and rat tail arteries to adrenergic nerves

Studies of the electrical response of isolated guinea-pig mesenteric and rat tail arteries to perivascular nerve stimulation were made by micro-electrodes inserted from the outer surface of the vessels. Membrane potential of both arteries was -68 to -69 mV and usually stable, though with occasional miniature excitatory junction potentials (m.e.j.p.s). Perivascular nerve stimulation produced excitatory junction potentials (e.j.p.s) which usually increased in size with repetitive stimulation, particularly in the mesenteric artery, but rarely triggered a spike or other regenerative response. Phentolamine and yohimbine in low concentrations increased the size of the e.j.p.s in both arteries, and increased the mechanical response of the mesenteric artery, probably by blocking prejunctional alpha 2 receptors which depress release of noradrenaline by the nerves; they reduced the mechanical response of the tail artery, probably by blocking alpha 2 receptors of the smooth muscle. Prazosin in low concentration had no effect on the e.j.p.s but inhibited contraction in both arteries, probably by blocking alpha 1 receptors of the smooth muscle. In the tail artery, but not the mesenteric artery, e.j.p.s produced by repetitive perivascular nerve stimulation were followed by a slow depolarization reaching a maximum at about 20 s and then decaying over 1-3 min; it did not reach the threshold for contraction, assessed by K depolarization. Yohimbine reduced the size and duration of the slow depolarization. High concentrations of noradrenaline (10(-5) M) caused depolarization and contraction of the mesenteric artery, both of which were blocked by prazosin and little affected by yohimbine. In the tail artery, yohimbine did but prazosin did not block the depolarization produced by any concentration of noradrenaline, although yohimbine was almost as effective as prazosin in blocking the contraction produced by low concentrations of noradrenaline. Extrajunctional adrenoceptors in the mesenteric artery therefore included high sensitivity types of alpha 1 receptor, and in the tail artery high sensitivity types of alpha 1 and alpha 2 receptor. Some of the extrajunctional receptors, as well as the junctional receptors responsible for e.j.p.s in both arteries, produced depolarization. Most of the contraction induced by either nerves or exogenous noradrenaline was produced by the extrajunctional receptors, and was not dependent on the depolarization which some of these receptors induced.