Modulation of noradrenergic transmission in the guinea-pig mesenteric artery: an electrophysiological study

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.

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)

Spontaneous and noradrenaline-induced transient depolarizations in the smooth muscle of guinea-pig mesenteric vein

1. Recordings of membrane current were made in the smooth muscle of short segments of mesenteric vein before or during stimulation with noradrenaline (NA). 2. Small veins (diameter less than 150 microns) when cut into short segments (of length less than 250 microns) had the passive electrical characteristics of short cables both before and during activation with NA. 3. Spontaneous transient depolarizations (STDs) or the underlying inward currents (STICs) were recorded in these preparations. STDs were of myogenic origin as they were not blocked by tetrodotoxin or antagonists to the alpha-adrenoreceptor and persisted after either denervation or disruption of the endothelium. 4. STDs had time courses similar to the underlying currents and were generally slow compared to the membrane time constant of the short segments. 5. STDs and the underlying currents showed large variability in frequency and amplitude both within and between short segments. Currents were typically less than 0.3 nA, were characteristic in shape, had half-durations normally in the range 0.1-0.7 s and reversed at about -25 mV. 6. STDs persisted, but at markedly reduced frequencies, after exposure (3-10 min) to a solution in which cobalt ions had been used to substitute for Ca2+. STDs were also substantially suppressed by exposure to low-chloride solution. 7. Caffeine induced excitatory and inhibitory conductances. An initial component of the caffeine-induced responses showed similar voltage dependence to STDs and was also suppressed by exposure to low-chloride solution. 8. NA, through activation of alpha-adrenoreceptors, caused a sustained depolarization or inward current (under voltage clamp) with considerable membrane potential or current noise often in the form of agonist-induced spontaneous transient depolarizations (ASTDs) or currents (ASTICs). There were marked increases in amplitude and frequency of ASTDs with increase in NA concentrations. 9. ASTDs appeared to be generated within the smooth muscle as they were activated in preparations which had been denervated or in which the endothelium had been disrupted. 10. Except for the pathway of activation, ASTDs were indistinguishable from STDs having half-durations in the same range (0.1-2 s with the majority less than 0.7 s). The underlying currents again showed large variation in amplitude (typically less than 0.3 nA; maximum recorded 0.9 nA). They reversed at about -25 mV, could still be elicited in cobalt solution (but at reduced intensity for long exposures to this low-Ca2+ solution) and were reduced by long term exposure to low-chloride solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Pinacidil inhibits neuromuscular transmission indirectly in the guinea-pig and rabbit mesenteric arteries

1. Effects of pinacidil were investigated on neuromuscular transmission in smooth muscle tissues of the rabbit and guinea-pig mesenteric arteries by both electrophysiological procedures and a bioassay of noradrenaline (NA) outflows. 2. Pinacidil (over 1 microM) hyperpolarized smooth muscle cell membranes in both tissues, in a concentration dependent manner. Pinacidil hyperpolarized and increased the ionic conductance of smooth muscle membrane more markedly in the rabbit mesenteric artery than in the guinea-pig. The hyperpolarization induced by pinacidil occurred in the presence or absence of endothelial cells and was blocked by glibenclamide. 3. Perivascular adrenergic nerve stimulation produced excitatory junction potentials (e.j.ps) and repetitive stimulation produced a facilitation of e.j.ps in both tissues. Pinacidil (over 1 microM) reduced the amplitude and the decay time of e.j.ps to a consistently greater extent in the rabbit mesenteric artery than in the guinea-pig. However, the facilitation process of e.j.ps was not modified following application of pinacidil (1 microM). The pinacidil-induced inhibition of e.j.ps was prevented by pretreatment with glibenclamide. 4. Pinacidil (30 microM) marginally increased the overflows of NA and its metabolite, 3,4-dihydroxyphenylglycol (DOPEG) released following repetitive perivascular nerve stimulations. 5. Pinacidil (10 microM) partly inhibited the voltage-dependent Ca channel, as estimated from the recovery process following removal of pinacidil, of action potentials evoked on e.j.ps. 6. It is concluded that pinacidil increases ionic conductance and hyperpolarizes smooth muscle cell membranes of the guinea-pig and rabbit mesenteric arteries and as a consequence, inhibits the neuromuscular transmission process occurring on adrenergic nerve stimulation with no reduction in the amount of released transmitter.

Presynaptic dopamine DA2-receptors in rabbit jejunal arteries. An electrophysiological study

Excitatory junction potentials (e.j.ps) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of rabbit isolated jejunal arteries. LY 171555 1 mumol/l, SKF 38393 10 mumol/l, dopamine 10 mumol/l and clonidine 0.1 mumol/l depressed all e.j.ps in the train. The percentage inhibition was inversely related to the number of pulses. S- and R-sulpiride, 10 mumol/l, domperidone 1 mumol/l, SCH 23390 1 mumol/l and rauwolscine 1 mumol/l did not change, or even depressed the first e.j.ps. Of these compounds only S- and R-sulpiride, 10 mumol/l and rauwolscine 1 mumol/l facilitated the late e.j.ps. The percentage facilitation increased with the number of pulses until a maximum was reached; rauwolscine 1 mumol/l had the largest effect. S- and R-sulpiride, 10 mumol/l, as well as domperidone 1 mumol/l antagonized the action of LY 171555 1 mumol/l. S-Sulpiride was more potent than its R-isomer. SCH 23390 1 mumol/l and rauwolscine 1 mumol/l blunted the effect of SKF 38393 10 mumol/l. Rauwolscine 1 mumol/l slightly reduced the inhibition by dopamine 10 mumol/l; S-sulpiride 10 mumol/l was antagonistic only in the presence of rauwolscine 1 mumol/l. When rauwolscine 1 mumol/l, prazosin 0.1 mumol/l, propranolol 1 mumol/l and cocaine 10 mumol/l was added to the medium, dopamine 10 mumol/l continued to produce the same depression of e.j.ps, as in the absence of these compounds. Under such conditions S-sulpiride 10 mumol/l also counteracted dopamine 10 mumol/l. Rauwolscine 1 mumol/l prevented the effect of clonidine 0.1 mumol/l. The antagonists were not absolutely selective against only one type of agonist. We suggest that both presynaptic DA2- and postsynaptic DA1-receptors are present in rabbit jejunal arteries. The activation of either receptor-type may depress the e.j.ps. Dopamine interferes with neuroeffector transmission due to alpha 2-adrenoceptor agonist properties; its DA2-effect is unmasked only after alpha 2-adrenoceptor blockade. There was no evidence for a co-transmitter function of dopamine.

Sympathetic transmission in small mesenteric arteries from the rat: highly calcium-dependent at low stimulation rates

In the present study we examine the calcium requirements of the neurogenic response in vitro of small arteries (150-200 microns diameter) from the mesentery of Wistar rats. Intramural nerves were activated with electrical field stimulation. Responses to single impulses and to low-frequency repeated stimulation were reduced or abolished by reducing the Ca2+ concentration in the bathing solution from 2.5 to 1.0 mM. Responses to higher frequencies (16 Hz) were only slightly affected. Since calcium reduction had markedly less effect on responses to direct activation of the smooth muscle and on responses to any dose of exogenous noradrenaline, the calcium reduction had mainly pre-junctional effects. The data show that part of the neurogenic response is highly calcium-sensitive, perhaps more so than would be expected of a purely noradrenergic transmission.

Evidence for adenosine triphosphate as an excitatory transmitter in guinea-pig, rabbit and pig urinary bladder

1. The effects of alpha,beta-methylene ATP (alpha,beta-MeATP) on membrane properties and excitatory junction potentials (EJPs) were examined in smooth muscle cells of the guinea-pig, rabbit and pig bladder. 2. Intracellular recording with microelectrodes was used to record membrane electrical activity from the guinea-pig bladder. ATP (10(-3) M) produced a rapid, large depolarization with a marked increase in spike frequency, while carbachol (10(-4) M) or acetylcholine (ACh; 10(-4) M) produced only a small or no depolarization with a smaller increase in spike frequency. alpha,beta-MeATP produced a similar response to that of ATP but at a much lower concentration (5 x 10(-6) M), and the response was transient even in the continuous presence of this agent. 3. Changes in the membrane potential and conductance elicited by alpha,beta-MeATP were also measured with the double sucrose-gap method. alpha,beta-MeATP (5 x 10(-6) M) depolarized the membrane and increased the membrane conductance in all three species, but both parameters returned to control values during continuous exposure to this agent. 4. Intracellular recording with microelectrodes showed that in the guinea-pig bladder treatment with alpha,beta-MeATP abolished the response to ATP, while the response to ACh was unchanged. 5. With the double sucrose-gap method, EJPs were elicited by transmural nerve stimulation of strips of the guinea-pig, rabbit and pig bladder and had spikes superimposed, leading to contractions. Desensitization of P2-purinoceptors by alpha,beta-MeATP (3-5 x 10(-6) M) abolished the EJPs and spikes, and reduced the contraction. Atropine (10(-6) M) alone did not alter the EJPs but reduced the contraction. Combined application of both agents abolished the contraction. 6. It is concluded that in the guinea-pig, rabbit and pig bladder ATP is an excitatory transmitter with ACh and EJPs are mediated by ATP.

Electrophysiology of neuromuscular transmission in guinea-pig mesenteric veins

1. Neuromuscular transmission in the smooth muscle of mesenteric veins has been investigated by recording intracellular potential changes resulting from stimulation of the sympathetic nerves and comparing these potential changes with responses obtained by ionophoresis of noradrenaline. 2. Neural stimulation or exogenous noradrenaline acted similarly to cause two excitatory depolarizations, a slow response reported previously (Suzuki, 1981) and a separate fast depolarization. 3. The fast depolarization was distinct from the slow depolarizing response in that it had a different dependence on the level of stimulation, was readily desensitized and was more suppressed in low-chloride solution. 4. The fast but not the slow depolarization shared certain characteristics with constriction. The fast depolarization and constriction both increased with the intensity of stimulation; inactivation in both was dependent on the recovery interval between trains of stimuli and both were suppressed to a similar degree by antagonists to alpha-adrenoceptors. The fast depolarization was, however, not a prerequisite for constriction to occur. 5. The fast and slow depolarizations were activated after a long latency which had a high temperature coefficient consistent with the postulate that these responses are rate limited by intracellular biochemical reactions. 6. The fast depolarization was preferentially suppressed by prazosin, an antagonist to the alpha 1-adrenoceptor subtype. Suppression of the slow depolarization required relatively higher concentrations of antagonist, indicating that these responses were mediated by receptor interactions involving a different alpha-adrenoceptor subtype. 7. It is concluded that neuromuscular transmission in mesenteric veins occurs through activation of alpha-adrenoceptors. A number of responses result, including voltage-independent constriction and two distinct excitatory depolarizations which can lead to voltage-dependent constriction.

Vascular responsiveness to periarterial electrical nerve stimulation on canine ophthalmic arteries

Both periarterial electrical stimulation (ES) and intraluminal application of norepinephrine (NE) produced a vasoconstriction in the isolated canine ophthalmic artery (OA). The ES-induced vasoconstrictor response was markedly inhibited by treatment with tetrodotoxin and slightly but significantly depressed by xylazine, a selective alpha-2 adrenoceptor agonist. Moreover, the ES-induced response was enhanced by either DG-5128, a selective alpha-2 adrenoceptor antagonist, or angiotensin II. The inhibitory effect of bunazosin on the ES-induced response was significant, but it was rather small compared with that on the NE-induced response. The NE-induced constriction was not affected by xylazine and DG-5128, but significantly enhanced by angiotensin II. These results indicate that (1) the ES-induced response is mediated via sympathetic nerve activation, (2) there are inhibitory prejunctional alpha-2 adrenoceptors, (3) there are more postjunctional alpha-1 than postjunctional alpha-2 adrenoceptors in canine OAs, and (4) prejunctional angiotensin II receptors may exist in isolated OA preparations.

Electrical responses of guinea pig coronary artery to transmural stimulation

Isolated segments of the guinea pig circumflex coronary artery were stimulated transmurally with brief duration (0.05 msec) pulses while recording intracellularly. Resting membrane potential was -60.3 +/- 4.3 mV (n = 66). Transient depolarizations (TDs) graded in amplitude with stimulus intensity were elicited with single stimuli. An action potential of 47 +/- 5 mV was superimposed upon the TD above a threshold membrane potential of -48 +/- 4 mV. Following the spike, a period of slow repolarization requiring 77 +/- 33 seconds was observed. Repetitive stimulation led to summation of TDs and a slow depolarization (less than or equal to 21 mV) that persisted for up to 90 seconds. The TD was reduced in amplitude when the membrane was depolarized with increased [K]o and enhanced when the membrane was hyperpolarized by adding potassium back to a potassium-depleted tissue. Phentolamine (10(-5) M), prazosin (10(-6) M), guanethidine (2 X 10(-5) M), alpha,beta-methylene ATP (2 X 10(-5) M), tetrodotoxin (10(-6) M), and 10-20 mM norepinephrine did not block TDs, action potentials, or slow depolarization. TDs were abolished following cold storage (4 degrees C) for 4 days and significantly reduced in a 2.25-mM CdCl2 solution with 0.25 mM CaCl2. Acetylcholine (10(-8) M to 10(-5) M) produced membrane hyperpolarization and reduced the amplitude of TDs and altered their time course. Atropine (10(-5) M) blocked the effects of acetylcholine but had no effect on the response to nerve stimulation. These observations indicate that TDs exhibit some, but not all, of the properties characteristic of excitatory junction potentials obtained with release of neurotransmitter substances in other vessels. If these TDs are neural events, they must involve a transmitter other than norepinephrine, acetylcholine, or ATP.

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.

Electrophysiological evidence for an alpha 2-adrenergic inhibitory control of transmitter release in the rabbit mesenteric artery

Excitatory junction potentials (e.j.p.s) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of the rabbit isolated mesenteric artery. Clonidine and B-HT 933 depressed all e.j.p.s in the train. The percentage inhibition was inversely related to the number of pulses. Yohimbine, rauwolscine and tolazoline reduced the early e.j.p. amplitudes but enhanced the later ones. The percentage facilitation of e.j.p.s increased with the number of pulses until a maximum was reached. Prazosin and corynanthine did not influence the first few e.j.p.s but potentiated the subsequent ones; their effects were less pronounced than those of yohimbine and rauwolscine. All the drugs antagonized the inhibition by clonidine but the effects of yohimbine and rauwolscine were more marked than those of prazosin and corynanthine. Phenylephrine, St 587 and noradrenaline depressed the e.j.p.s. Yohimbine diminished the effects of these substances and was a stronger antagonist of phenylephrine than prazosin. We suggest that, in the rabbit mesenteric artery, noradrenaline and the neuroeffector transmitter (probably ATP) are co-released from the terminals of postganglionic sympathetic nerves. Noradrenaline activates presynaptic alpha 2-adrenoceptors and thereby depresses transmitter release. The degree of presynaptic inhibition depends on the number of pulses applied, i.e. on the biophase concentration of noradrenaline.

Identification of the neuroeffector transmitter in jejunal branches of the rabbit mesenteric artery

Vasoconstriction or excitatory junction potentials (e.j.ps) evoked by nerve stimulation (15 field pulses at 2 Hz every 3 min) were recorded in rabbit isolated jejunal arteries. The resting diameter of the arteries and its decrease in response to stimulation was measured by a photoelectric method. Vasoconstriction was insensitive to prazosin 0.1 or 1 mumol/l. Yohimbine 1 mumol/l considerably enhanced, whereas alpha,beta-methylene ATP (alpha,beta-meATP) 1 mumol/l abolished the contractile response. In order to test the effect of exogenously applied transmitter candidates, noradrenaline (0.1-1 mumol/l) and ATP (10-30 mumol/l) were added in concentrations which evoked a vasoconstriction comparable to that induced by electrical stimulation. The action of noradrenaline was prevented by prazosin 0.1 mumol/l, but was unaffected by both yohimbine 1 mumol/l and alpha,beta-meATP 1 mumol/l. Alpha,beta-meATP 1 mumol/l depressed the effect of ATP. The e.j.ps evoked by a train of 15 pulses showed facilitation up to the third response and thereafter depression; a partial summation was also observed. Prazosin 0.1 mumol/l did not change the e.j.p. amplitudes. By contrast, when yohimbine 0.1 or 1 mumol/l was added to the prazosin-containing medium, both the late e.j.ps in the train and the summation were enhanced in a concentration-dependent manner. Alpha,beta-meATP 1 mumol/l almost abolished the e.j.ps. In conclusion, in rabbit jejunal arteries, stimulation of postganglionic sympathetic nerves may release noradrenaline together with ATP which is probably the sole neuroeffector transmitter under our conditions. Transmitter release seems to be modulated by the activation of presynaptic alpha 2-adrenoceptors. Under the stimulation conditions of the present experiments the released transmitter does not activate postsynaptic alpha 1-adrenoceptors.

The effect of reserpine on sympathetic, purinergic neurotransmission in the isolated mesenteric artery of the dog: a pharmacological study

Electrical transmural stimulation evoked a transient contraction in the isolated mesenteric artery of the dog. This contraction was abolished by guanethidine or tetrodotoxin and was partially inhibited by prazosin. Noradrenaline was competitively antagonized by prazosin. Similarly, in the reserpine-treated artery, electrical transmural stimulation produced a transient contraction which was abolished by guanethidine or tetrodotoxin. However, prazosin failed to inhibit this contraction. The contraction to noradrenaline was not significantly different from the response it produced in control vessels. Tyramine (10(-5) M), which acts on sympathetic nerves to release noradrenaline, evoked a tonic contraction in the untreated artery. This contraction was abolished or markedly attenuated by prazosin or guanethidine. The response was not observed in the reserpine-treated artery, indicating that reserpine had depleted the nerves of noradrenaline. In the control vessel alpha,beta-methylene-ATP produced a transient contraction which was followed by a complete relaxation to the basal level. This contractile response was not significantly different in the presence of guanethidine or prazosin or in the reserpine-treated artery. After desensitization of the vessel to alpha,beta-methylene ATP (5 X 10(-6) M) the prazosin-resistant contractions induced by electrical transmural stimulation were abolished both in reserpine-treated and untreated arteries. Also the contractile responses to ATP and alpha-beta-methylene-ATP were abolished but the responses to tyramine (control vessels), noradrenaline and KCl were not affected. 8-Phenyltheophylline (10(-5) M) showed no inhibitory effect on the contractile responses to electrical transmural stimulation, tyramine, ATP or alpha,beta-methylene-ATP. 7. Neuropeptide Y, peptide YY, vasoactive intestinal polypeptide, bombesin and substance P (10-7 and 10-6 M for each peptide) caused no contractile response in the dog mesenteric artery. 8. These experiments provide further evidence that the sympathetic contraction of the isolated mesenteric artery of the dog induced by electrical transmural stimulation consists ofan adrenergic and a purinergic component and that the latter component is mediated through postsynaptic P2- purinoceptors.

The action of noradrenaline on single smooth muscle cells freshly dispersed from the guinea-pig pulmonary artery

Responses to ionophoretically-applied noradrenaline were investigated with micro-electrodes in whole tissue preparations and with patch pipettes in isolated cells dispersed from the guinea-pig pulmonary artery. In whole tissue and dispersed cells noradrenaline evoked monophasic depolarizations which had a similar time course. In dispersed cells the amplitude of electronic potentials was reduced during the noradrenaline-evoked depolarization. Under voltage clamp noradrenaline elicited an inward current, which persisted in 18 mM external potassium with the membrane potential set at the potassium equilibrium potential. In voltage clamp experiments the amplitude of current steps to hyperpolarizing voltage jumps was increased during the noradrenaline-induced inward current. These data suggest that the depolarization to noradrenaline in the guinea-pig pulmonary artery is mediated by an increase in membrane conductance.

Distribution of excitatory junction potential amplitude in cat cerebral arteries: examination of its quantal nature and modulation by opiates

We used scorpion venom to release small amounts of an excitatory neurotransmitter from adventitial nerves in cat left anterior descending cerebral artery. We used glass microelectrodes to measure and record postsynaptic electrical events of minimal amplitude. These events were similar to postsynaptic spontaneous and electrically evoked excitatory junction potentials (ejp's) seen in skeletal muscle. We performed a frequency analysis of the ejp amplitudes to determine if they fit a unimodal or multimodal distribution. We also investigated the effects of phentolamine, norepinephrine, hydromorphone, and morphine on ejp amplitude and frequency in the artery. Statistical analysis of the ejp frequency and amplitude revealed a multipeaked distribution with decreasing peaks. These results were similar to the distribution reported for acetylcholine release in skeletal muscle. The ejps were inhibited by phentolamine, which suggested that these events were adrenergically mediated. Norepinephrine and the opiates, hydromorphone and morphine, reduced the frequency and amplitude of the ejp's. The vessels also constricted to increasing doses of norepinephrine both under control conditions and in the presence of opiate. These results suggest that norepinephrine blocks the ejp's by a feedback mechanism at the presynaptic membrane and that endorphins and/or enkephalins, also acting at this presynaptic site, may modulate neurotransmission in the cerebral circulation.

Comparison of the frequency dependence of venous and arterial responses to sympathetic nerve stimulation in guinea-pigs

1. Intracellular potentials and measurements of contractions were recorded in adjacent veins and arteries in the colonic mesentery of the guinea-pig in vitro during stimulation of post-ganglionic nerve trunks. 2. Repetitive stimulation (0.5-5 Hz) of lumbar colonic nerve trunks produced frequency-dependent slow depolarizations in all venous and in 92% of arterial smooth muscle cells. Excitatory junction potentials were observed for each nerve shock in arteries, but not in veins. 3. Low-frequency stimulations produced slow depolarizations of greater amplitude and longer duration in veins than in arteries. The frequencies at which half-maximal depolarizations and contractions occurred were always lower for veins than for arteries. 4. The alpha 1-adrenergic antagonist prazosin (5 X 10(-7) M) reduced the mean arterial slow depolarizations by 82% and reduced mean venous slow depolarizations by 58% for 5 Hz stimulations. Arterial contractions were completely inhibited by prazosin but venous contractions were incompletely reduced in a frequency-dependent manner. 5. These findings suggest that functional differences in activation between mesenteric veins and arteries during sympathetic stimulation are a result of differences in neuromuscular transmission.

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.

Interactions between the effects of yohimbine, clonidine and [Ca]o on the electrical response of the mouse vas deferens

Excitatory junction potentials (e.j.ps) were recorded from mouse vas deferens and resolved into families of 'discrete events' (d.es) reflecting intermittent release of packets of transmitter from one or a few sites. Within families d.es vary in amplitude between a few preferred values unaffected by any treatments used in these experiments. As [Ca]o is raised from 1.1 to 4.0 mM there is a rise in d.e. amplitude due to an increase in the frequency of large events and a decrease in that of small. At all [Ca]o clonidine reduces d.e. amplitude by increasing failures and small events and decreasing large events. Yohimbine has opposite effects. Both drug effects are concentration-dependent in the range 5 X 10(-9) - 10(-6)M. As [Ca]o is raised from 1.1 to 4.0 mM, and therefore more natural agonist is released, clonidine becomes more effective at altering d.e. amplitude whereas yohimbine becomes less so. With very low frequency stimulation yohimbine elevates e.j.p. amplitude only if [Ca]o is below 1.6 mM. These results are not easily compatible with the notion that yohimbine breaks a 'negative feedback' control of transmitter release.

The different effects of exogenous and neuronally released norepinephrine on renin release in rat kidney cortical slices

The effects of veratrine on renin release from rat kidney cortical slices were compared with those of norepinephrine (NE). Veratrine (10-100 micrograms/ml) produced a concentration-dependent increase in renin release. This action was markedly attenuated by propranolol but not influenced by prazosin. On the other hand, relatively higher concentrations of NE produced an inhibition of renin release and the inhibitory action was reversed by prazosin. These results suggest that exogenous NE inhibits renin release by preferential activation of alpha 1-adrenoceptors while neuronally released NE stimulates renin release by exclusive activation of beta-adrenoceptors.

Evidence for sympathetic, purinergic transmission in the mesenteric artery of the dog

Electrical transmural stimulation of isolated mesenteric artery of the dog produced a transient contraction which consisted of adrenergic and non-adrenergic components. In contrast to the adrenergic component, the nonadrenergic component was resistant to prazosin and other adrenoceptor-blocking agents. However, the nonadrenergic component was completely blocked by guanethidine and by desensitization with alpha,beteta-methylene-ATP (alpha,beta-MeATP). Desensitization induced by alpha,beta-MeATP also inhibited the contractile response to ATP but not the adrenergic responses induced by electrical transmural stimulation and exogenous noradrenaline. These results suggest that the nonadrenergic contraction induced by electrical transmural stimulation is a sympathetic, purinergic response.

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.

Calcium-activated potassium channels in single smooth muscle cells of rabbit jejunum and guinea-pig mesenteric artery

Single-channel studies were made using the patch-clamp technique of K channels in dispersed single smooth muscle cells from rabbit longitudinal jejunal muscle and guinea-pig small (less than 0.2 mm o.d.) mesenteric arteries. In isolated inside-out patches from these two types of smooth muscle cell there was a population of K channels which had single-channel conductances of about 100 pS in near physiological K gradients and about 200 pS with symmetrical 126 mM-K solutions. Their conductance and other properties distinguish them from a K channel of smaller conductance which we have previously described in these cells. The relative permeability of the channel with respect to K was 1.4 Tl:1.0 K:0.7 Rb: less than 0.05 Na: less than 0.05 Cs. Cs (1 mM applied to the outside of the membrane) interfered with inward K movement when the membrane was hyperpolarized. Rb conductance of the channel when both sides of the membrane were exposed to 126 mM-Rb was 30 pS. When the Ca concentration on the inside of the membrane ([Ca]i) was about 10(-9) M, K channel opening was rarely observed and then only at strongly positive potentials. At [Ca]i between 10(-9) M and 10(-7) M mean channel open time increased and the probability of channel opening increased steeply; both were further increased by increasing membrane positivity. At [Ca]i between 10(-6) M and 2.5 mM the channel was mainly in the open state and the probability of channel conducting state often declined with increasing membrane positivity. The effects of varying [Ca]i from 10(-7) M to 2.5 mM on the kinetic activity of a single channel was studied largely in mesenteric artery patches containing one active channel. The distribution of open times could be fitted by a single exponential at low (less than 10(-6) M) [Ca]i but a component of fast openings (to less than 1.0 ms) was observed at all potentials at [Ca]i 2.5 mM. Closed time distribution required the sum of three exponentials to fit it all [Ca]i greater than 10(-7) M; at [Ca]i 10(-6) M or greater evidence of a fourth component, probably caused by Ca block of open channels, was obtained. Raising [Ca]i increased the mean duration of the (long) open state and decreased or had no effect on the duration of short, intermediate, and long mean closed states.

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.

Comparison of the antagonistic effects of phentolamine on vasoconstrictor responses to exogenous and neurally released noradrenaline in vivo

The antagonistic effects of the alpha-adrenoceptor blocking agent phentolamine on vasoconstrictor responses to intraluminal noradrenaline and lumbar sympathetic nerve stimulation were compared in the hindlimb of the anaesthetized dog. Sympathetic stimulation with 1 pulse or trains of 4-10 pulses at 0.4-40 Hz produced graded vasoconstrictor responses that were matched in amplitude by intra-arterial injections of 10(-8) - 10(-6) g noradrenaline. Phentolamine (0.5 mg kg-1 i.v.) attenuated amplitude-matched responses to both types of stimuli to quite similar extents. The extent of the effect of phentolamine on neurogenic responses was greater with 1 pulse stimulation than with trains, and greater with 4 pulse than with 10 pulse trains. The effect was maximal within 2 min of phentolamine administration and wore off in parallel with that on responses to injected noradrenaline. The results are consistent with the view that transmitter released from noradrenergic vasoconstrictor nerves acts primarily on subjunctional alpha-adrenoceptors.

Different sensitivity of blocking effects of alpha-adrenoceptor blocking agents on vascular responses to intraluminal norepinephrine and periarterial stimulation in isolated dog arteries

The stainless steel cannula inserting method was used to examine the effects of periarterial electrical nerve stimulation and intraluminal norepinephrine in the isolated and perfused mesenteric artery of the dog. The optimum conditions for inducing an increase in perfusion pressure over 50 mmHg to periarterial electrical stimulation were 3-5 msec duration, 40-50 volts and 10-20 Hz. After the treatment with phentolamine or prazosin, the vasoconstrictor response to norepinephrine was readily inhibited completely at a relative small dose of 0.3-3 micrograms. Periarterial stimulation-induced vasoconstriction was also significantly suppressed by a relatively large dose of 100 micrograms of phentolamine or prazosin in concentrations 30 to 100 times larger than that required for blocking the norepinephrine-induced constriction. Yohimbine in relatively small doses potentiated the stimulation-induced vasoconstriction, but rather suppressed it in a large dose. The constrictor response to periarterial stimulation was significantly suppressed by 1 and 10 micrograms of intraluminal tetrodotoxin. It is concluded that periarterial electrical stimulation in the cannula inserting method is useful for studying autonomic pharmacology and physiology in vasculature with due regard to the characteristics.

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.

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.

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.

Actions of nipradilol (K-351), a new alpha- and beta-adrenoceptor blocker, on the rabbit portal vein

Nipradilol but not desnitro nipradilol [N-) nipradilol) inhibited the norepinephrine (NE)-induced depolarization and contraction of the rabbit portal vein. The NE-induced contraction and depolarization were also blocked by prazosin, but not blocked by yohimbine. Therefore, nipradilol possesses an alpha 1-blocking action. The order of potency was prazosin greater than nipradilol greater than yohimbine greater than (N-)-nipradilol = 0. With applications of field stimulations to muscle tissues, the smooth muscle membrane was depolarized with a latency of several seconds, and the action potential was generated. These phenomena were blocked by tetrodotoxin (TTX), prazosin or nipradilol, but not by yohimbine. Isoproterenol (Isop) inhibited the 30 mM K-induced contraction, and this inhibitory action was blocked by (N-), nipradilol, nipradilol or propranolol, dose-dependently. The potency of beta-blocking actions of nipradilol was much the same as that observed by propranolol and (N-) nipradilol. When nipradilol (10(-5) M) was applied to the tissue, the amplitude of the 30 mM K contraction was slightly reduced. Such inhibitory action was not observed by application of (N-) nipradilol. The Ki values of nipradilol for blocking actions on the NE-induced contraction and Isop-induced relaxation were of the same order of 10(-7) M. Therefore, the potencies of alpha 1-blocking and beta-blocking actions of nipradilol may be the same in the rabbit portal vein. These findings suggest that the vasodilating action of nipradilol on the rabbit portal vein is mainly due to the alpha 1-blocking action and that the nitrate action of this agent may be weak.

Mechanisms of the ergonovine-induced vasoconstriction in the rabbit main coronary artery

The mean membrane potential of smooth muscle cells of the rabbit main coronary artery was -60.3 mV and an evoked action potential could be recorded in response to acetylcholine (ACh). Ergonovine or 5-hydroxytryptamine (5-HT) slightly depolarized the membrane and methysergide, a relatively selective antagonist for the 5-HT receptor, had a slight inhibitory action on these depolarizations. 5-HT produced larger contractions than ergonovine, and the concentration-effect relationships obtained for both agents shifted to higher concentrations following pre-equilibration with methysergide. ACh (10(-11)M) slightly hyperpolarized the membrane and relaxed the tissue, and high concentrations of ACh (greater than 10(-8)M) depolarized the membrane, increased the membrane resistance and produced a contraction. ACh but not ergonovine or 5-HT, produced a contraction in Ca-free EGTA-containing solution. Following a 60 min pre-equilibration with indomethacin, the ergonovine-induced contraction was markedly enhanced but the 5-HT- or ACh-induced contractions were not. Removal of the endothelium by rubbing the vascular lumen enhanced the ergonovine- or ACh-induced contractions, but not those to 5-HT. The results obtained can be summarized as follows: ergonovine probably accelerates Ca influx and thereby produces contraction in the rabbit main coronary artery. This contraction is due to activation of the 5-HT receptor as an agonist, but the ergonovine-induced contraction is attenuated due to activation of the endothelium from which inhibitory prostanoid substances may be released. Ergonovine, therefore, may produce greater contractions in coronary arteries with damaged endothelium than in intact tissues.

Mechanisms of action of noradrenaline and carbachol on smooth muscle of guinea-pig anterior mesenteric artery

Membrane potential was recorded by micro-electrode in segments of small (200-500 microns o.d.) mesenteric arteries of guinea-pig. Isotonic shortening was recorded in helical strips cut from these arteries. Raising the external potassium concentration, [K+]o, caused shortening and substantial depolarization. The threshold for contraction was about 30 mM which corresponded to a membrane potential of about -45 mV. Since high-potassium contractions were abolished in calcium-free solution it was suggested that they occur due to potential-sensitive calcium channels opening positive to about -45 mV. Noradrenaline weakly depolarized the muscle and produced contractions resistant to calcium-free conditions. It was suggested that noradrenaline contractions are mainly caused by mechanisms other than the opening of potential-sensitive calcium channels, namely entry of calcium via other channels and release of stored calcium. Carbachol had no effect on basal tension but inhibited shortening by noradrenaline or by raising [K+]o. The inhibitory effect of carbachol on tension under various conditions was associated with hyperpolarization or depolarization in a range negative to -45 mV, or no effect on potential, so that modulation of the number of open potential-sensitive calcium channels could not be evoked to explain its relaxant action. Removal or destruction of the endothelium by rubbing or by distilled water perfusion left tension responses to noradrenaline or raised [K+]o essentially unchanged. However, the inhibitory effect of carbachol on tension was attenuated and hyperpolarization of the resting artery was converted to a depolarization. It was concluded that carbachol has both a strong inhibitory and a weak excitatory effect on these vascular smooth muscle cells. Membrane potential changes are not essential to its inhibitory action but may, by closing potential-sensitive calcium channels, sometimes reinforce it. Hyperpolarization by carbachol may be caused by a factor released by the action of carbachol on endothelial cells: in its absence carbachol may weakly depolarize but this alone is normally insufficient to generate tension.

The presynaptic regulation of noradrenaline release differs in mesenteric arteries of the rabbit and guinea-pig

Prejunctional control mechanisms of neuromuscular transmission in mesenteric arteries of the guinea-pig and rabbit were compared by examining excitatory junction potentials (e.j.p.s). The various agents used did not modify the membrane potential or resistance of smooth muscle cells of either tissue at the concentrations used in the present experiments. In the rabbit mesenteric artery, 10(-7) M-phentolamine and yohimbine had almost no effect on the amplitude of e.j.p.s or on facilitation. In the guinea-pig mesenteric artery, phentolamine enlarged and yohimbine reduced the amplitude of the first e.j.p. but both agents markedly enlarged the amplitude of e.j.p.s evoked by repetitive perivascular nerve stimulation at frequencies over 0.1 Hz. In the rabbit mesenteric artery, isoprenaline and dibutyryl cyclic AMP inhibited the e.j.p.s and the facilitation evoked by repetitive stimulation, while in the guniea-pig mesenteric artery, isoprenaline enhanced the amplitude of e.j.p.s and the facilitation process. In the rabbit mesenteric artery, ATP inhibited the amplitude of e.j.p.s with no change in facilitation and adenosine inhibited mainly the facilitation process evoked by repetitive perivascular nerve stimulation. In the guinea-pig mesenteric artery, these agents had no effect on e.j.p.s or facilitation. In both the rabbit and guinea-pig, indomethacin enlarged the amplitude and prolonged the duration of e.j.p.s. Prostaglandin E2 and F2 alpha inhibited the amplitude of e.j.p.s. Enlarged amplitudes of the e.j.p.s following pre-treatment with indomethacin were inhibited by treatment with PGE2 or ATP. In the rabbit mesenteric artery, 8-phenyltheophylline (an antagonist of P1 subtype of purinergic receptor) had no effect on the e.j.p.s, but these agents did reverse the inhibitory action of 2-chloroadenosine (an agonist of P1 subtype of purinergic receptor) or adenosine on facilitation of e.j.p.s. Theophylline did not have any direct effect on e.j.p.s nor did it reverse the effects of adenosine or 2-chloroadenosine. In the rabbit mesenteric artery, ATP and adenyl-imidophosphate showed much the same potency in inhibiting the amplitude of e.j.p.s. Therefore, the ATP action on nerve terminals is not due to a high energy requiring mechanism. From these results we conclude that the control mechanisms related to noradrenaline (NA) release in nerve terminals differ in the guinea-pig and rabbit mesenteric arteries.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of a new alpha 1-adrenoceptor blocking agent, 3-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-(1H,3H)-quinazoline-2, 4-dione monohydrochloride (SGB1534), on electrical and mechanical properties of guinea-pig mesenteric artery

Effects of a newly synthesized alpha-adrenoceptor blocking agent, 3-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-1H, 3H)-quinazoline-2, 4-dione monochloride (SGB1534), on the electrical and mechanical properties of smooth muscles of guinea-pig mesenteric artery and on the electrical properties of smooth muscles of rat tail artery were investigated. SGB1534 (10(-10) M-10(-5) M) did not modify the membrane potential and ionic conductance of the membrane in guinea-pig mesenteric artery, but this agent did inhibit depolarizations induced by noradrenaline, phenylephrine, or histamine with similar potencies. This agent inhibited serotonin-induced depolarization, but with a weak potency. In rat tail artery, the noradrenaline-induced depolarization (10(-5) M) was inhibited by yohimbine (5 X 10(-7) M), but not by SGB1534 (10(-6) M). SGB1534 (10(-6) M) did not modify the amplitude of excitatory junction potentials (e.j.ps), the facilitation process or spike potential evoked by perivascular nerve stimulation in the mesenteric artery. Noradrenaline, phenylephrine or histamine evoked the contraction in guinea-pig mesenteric artery, and this contraction was inhibited by SGB1534 (over 10(-10) M). The serotonin-induced contraction was inhibited by higher concentrations of SGB1534 (over 10(-6) M). The concentration of SGB1534 required to inhibit the contractions evoked by these amines was much higher than that required to inhibit the depolarizations. SGB1534 (less than 10(-6) M) had no effect on the excess K-induced depolarization and contraction. These results indicate that SGB1534 possesses the property of an alpha 1-but not alpha 2-adrenoceptor blocking agent.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of alpha-adrenoceptor antagonists on electrical and mechanical responses of the isolated dog mesenteric vein to perivascular nerve stimulation and exogenous noradrenaline

The effects of four different alpha-adrenoceptor antagonists (prazosin, phentolamine, yohimbine, and nipradilol) on the electrical and mechanical responses of smooth muscle cells of the dog isolated mesenteric vein to perivascular nerve stimulation and exogenous noradrenaline were investigated. Perivascular nerve stimulation generated an excitatory junction potential (e.j.p.), a spike potential and a slow depolarization. The latter component was blocked by yohimbine or phentolamine at doses over 10(-7) M, while the former two components were suppressed by 10(-6)-10(-5) M yohimbine, but not by prazosin, nipradilol or phentolamine (up to 10(-5) M). Nerve-mediated muscle contractions were suppressed by these alpha-adrenoceptor antagonists in a concentration-dependent manner, at doses over 10(-7) M. The order of potency was yohimbine greater than nipradilol = phentolamine greater than prazosin. Exogenously applied noradrenaline (10(-6) M) depolarized the smooth muscle membrane and generated slow waves. The slow waves were blocked by all of these alpha-adrenoceptor antagonists (10(-5) M), while the depolarizations were inhibited by yohimbine (greater than 10(-7) M) or phentolamine (10(-5) M), but not by nipradilol or prazosin (up to 10(-5) M). Contractions produced by exogenously applied noradrenaline (10(-6) M) were inhibited by the alpha-adrenoceptor antagonists; yohimbine or phentolamine (10(-6)-10(-5) M) showed complete inhibition and prazosin or nipradilol (up to 10(-5) M) partial inhibition. Contractions produced by high-potassium or current-stimulation were suppressed by high-concentrations (10(-6)-10(-5) M) of these alpha-adrenoceptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-adrenoceptors and facilitation at a sympathetic neuroeffector junction

Excitatory junction potentials were recorded from the mouse vas deferens following 5 stimuli at 1Hz and a single stimulus at times from 100 msec to 9 sec later. E.j.ps facilitate and facilitation decayed over a biexponential time course, with time constants of 120 msec and 4.3 sec. Clonidine (10(-8)-10(-7) M) depressed e.j.p. amplitude, and also accelerated both phases of the decay of facilitation. Yohimbine (10(-7) M) and piperoxan (10(-7)-10(-6) M) increased the amplitude of all but the first e.j.p. in a train, and slowed both phases of the decay of facilitation. One way in which prejunctional modulation by alpha-adrenoceptors may occur is therefore by modifying the carry-over of facilitation from one stimulus to the next.

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

Adrenergic transmission was investigated in the dog mesenteric vein by recording electrical responses of single smooth muscle cells to perivascular nerve stimulation. Perivascular nerve stimulation generated an excitatory junction potential (e.j.p.) and a slow depolarization of the membrane. The amplitude of the e.j.p. was increased by increasing the stimulus intensity, and at high intensity, a spike potential was generated. Repetitive stimulation of the nerves showed facilitation of e.j.ps and enhanced the amplitude of slow depolarization. A linear relationship was observed between the amplitude of the e.j.p. and of slow depolarization. The slow depolarization was inhibited by application of yohimbine or phentolamine, but not by prazosin. The amplitude of e.j.p. was increased by prazosin and was decreased by yohimbine. Both e.j.p. and slow depolarization were inhibited by guanethidine or tetrodotoxin. Exogenously applied noradrenaline depolarized the muscle membrane and, in high concentrations (greater than 10(-7)M), generated slow waves. These effects of noradrenaline were blocked by yohimbine. High concentrations of prazosin (greater than 10(-6)M) showed weak inhibitory effects on the noradrenaline actions. The amplitude of e.j.p. was decreased by exogenously applied noradrenaline in a dose-dependent manner. The inhibitory effect of noradrenaline on the e.j.p. was suppressed by yohimbine, but not by prazosin or phentolamine. Phentolamine, but not prazosin, enhanced the facilitation process of e.j.ps. This effect was not suppressed by exogenously applied noradrenaline. Application of neostigmine but not of atropine, reduced the e.j.p. amplitude without affecting the slow depolarization. It was concluded that, in the dog mesenteric vein, perivascular nerve stimulation produced three types of electrical responses of the smooth muscle membrane, i.e., e.j.p., slow depolarization and spike potential. The slow depolarization was generated by activation of alpha 2-adrenoceptors. Exogenously applied noradrenaline reduced the e.j.p. amplitude through activation of prejunctional alpha 2-adrenoceptors, but the reduction may not involve alpha-autoinhibitory mechanisms.

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.

Effects of adrenoceptor agonists and antagonists on smooth muscle cells and neuromuscular transmission in the guinea-pig renal artery and vein

In the guinea-pig renal artery and vein, the membrane potential was -66.8 mV and -46.8 mV, the length constant 0.54 mm and 0.43 mm, and the time constant 240 ms and 98 ms, respectively. The maximum slope of the depolarization produced by a 10 fold increase [K]o was 46 mV in the renal artery and 39 mV in the renal vein. Noradrenaline (NA over 5 X 10(-7)M in the artery and over 10(-7)M in the vein) depolarized the membrane and slightly reduced the membrane resistance, assessed from relative changes in the amplitude of electrotonic potential. The action of NA was suppressed by prazosin in the artery but by yohimbine in the vein, i.e. the alpha 1-adrenoceptor is present in the extrajunctional muscle membrane in the renal artery while the alpha 2-adrenoceptor is present in the renal vein. Dopamine and isoprenaline did not modify the membrane properties. In the renal artery, repetitive perivascular nerve stimulation (0.1 ms, 50 Hz, 5 shocks) evoked excitatory junction potential (e.j.p.). Applications of guanethidine (10(-6) M) or tetrodotoxin (3 X 10(-7) M) abolished the generation of the e.j.p.. Low concentrations of phentolamine (5 X 10(-7) M), prazosin (10(-7) M) and yohimbine (5 X 10(-7) M) enhanced the e.j.p. amplitude, while high concentrations of phentolamine (10(-5) M) and prazosin (greater than 10(-5) M) reduced the amplitude of e.j.p.s. NA, dopamine and clonidine consistently suppressed the amplitude of e.j.ps, at any given concentration over 10(-7) M. Spontaneous generated miniature e.j.ps (m.e.j.ps) were recorded on rare occasions. Phentolamine and yohimbine both at 5 x 10(-7) M and prazosin 10(-7) M increased the appearance of m.e.j.ps. 5 In the renal vein, repetitive nerve stimulation failed to generate the e.j.p. Sympathetic innervation to this tissue seems to be sparse. 6 Specificity of innervation and adrenoceptors present on smooth muscle cells in both the renal artery and vein are discussed, and the presynaptic regulation ofNA release is compared with findings in other vascular tissues.

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.

The effects of 2-nicotinamidoethyl nitrate on smooth muscle cells of the dog mesenteric artery and trachea

The effects of 2-nicotinamidoethyl nitrate (nicorandil; 2-NN), a synthesized coronary vasodilator, on smooth muscle cells of dog mesenteric artery or trachea were investigated using microelectrode, double sucrose gap and isometric tension recording methods. Nicorandil hyperpolarized the smooth muscle membrane of the mesenteric artery (5 X 10(-6)M) and the trachea (5 X 10(-5)M). In both these smooth muscle cells, the nicorandil-induced hyperpolarization remained constant in various concentrations of [Cl-]o, but changed with various concentrations of [K+]o. This hyperpolarization was partly inhibited following pretreatment with tetraethylammonium (TEA greater than 1 mM) and was completely inhibited following pretreatment with procaine (1 mM or 5 mM), indicating that the nicorandyl-induced hyperpolarization is due to increase in the K-conductance, in both these membranes. Following pretreatment with TEA (5 mM), outward current pulses evoked an action potential in tracheal smooth muscle cells. Nicorandil inhibited the generation of action potential due to the hyperpolarization of the membrane but not due to inhibition of the spike generating mechanism. Nicorandil (10(-6)M) inhibited the contracture evoked by excess [K]o, in both tissues. The contracture evoked by noradrenaline or repetitive field stimulation with short duration (50 microseconds) pulses was also inhibited in the mesenteric artery, while a higher dose of nicorandil (10(-5)M) was required to inhibit the contracture evoked by acetylcholine or repetitive field stimulation in the trachea. Excitatory junction potentials (e.j.ps) evoked by field stimulation in the mesenteric artery due to the release of noradrenaline, or in the trachea due to release of acetylcholine, were suppressed by 10(-5)M or 5 X 10(-5)M nicorandil, respectively. The reduction in the amplitude of e.j.p. was mainly due to the hyperpolarization of the membrane with increase in the membrane conductance. In the mesenteric artery, following pretreatment with TEA (1 mM) an action potential was generated on the e.j.p.. Nicorandil suppressed the generation of the action potential by reduction in the amplitude of the e.j.p., below the threshold required for generation of the action potential. These results indicate that nicorandil hyperpolarizes the membrane by increasing K-conductance and inhibits the generation of contraction, in both tissues. Higher concentrations of nicorandil are required to suppress the mechanical response in the trachea than in the mesenteric artery. Depression of the mechanical responses in both tissues is partly due to suppression of Ca-mobilization inside the muscle cells and partly to hyperpolarization of the membrane.

Effects of a new alpha-adrenoceptor blocking agent, ethyl-7-[4-(2-methoxyphenyl)-1-piperazinyl] heptanoate dihydrochloride (SGB-483), on smooth muscle and neuromuscular transmission in guinea-pig mesenteric artery

The effects of ethyl-7-[4-(2-methoxyphenyl)-1-piperazinyl] heptanoate dihydrochloride (SGB-483) on the smooth muscle of the guinea-pig mesenteric artery were investigated using microelectrodes. The resting membrane potential was -70.3 +/- 2.1 mV.SGB-483 (10(-8)M - 10(-4)M) did not modify the membrane potential or membrane resistance, as estimated from measurement of current-voltage relationships. Noradrenaline (NA; above 10(-5)M) depolarized the membrane. After pretreatment with SBG-483 10(-5)M or prazosin 10(-6)M, the NA-induced depolarization of the membrane was inhibited; yohimbine (10(-5)M) was ineffective. Phenylephrine and NA (greater than 3 X 10(-7)M) but not clonidine (10(-6)M) contracted the artery. These contractions were inhibited by SGB-483. Following repetitive perivascular nerve stimulation, the amplitude of excitatory junction potentials (e.j.ps) increased to a certain steady state value (e.j.p.(s]. The amplitude of e.j.p.(s) was frequency-dependent. Application of SGB-483 (over 10(-8)M) enhanced the amplitude of e.j.p.(s), dose-dependently with no change in the amplitude of the first e.j.p. (e.j.p.(f] evoked by the first stimulus. After pretreatment with NA, the amplitudes of both e.j.p.(f) and e.j.p.(s) were inhibited, dose-dependently. Following pretreatment with SGB-483 (10(-6) - 10(-5)M), the NA-induced reduction in the amplitude of both e.j.p.(f) and e.j.p.(s) were reversed and the control values restored. Clonidine (10(-7)M) inhibited the amplitude of e.j.p.(f) and e.j.p.(s), and SGB-483 (10(-7)M) partially restored the amplitude of both. Yohimbine (10(-7)M) and phentolamine (10(-7)M) enlarged the amplitude of e.j.p.(s); the amplitude of e.j.p.(f) was inhibited by yohimbine and enlarged by phentolamine. Prazosin (10(-6)M) had no effect on the amplitude of either e.j.p.(f) or e.j.p.(s), at any given stimulus frequency. SBG-483 (10(-7)M) did not enhance the amplitudes of either e.j.p.(f) or e.j.p.(s) following pretreatment with phentolamine (10(-7)M) or yohimbine (10(-7)M) but did enhance the amplitude following pretreatment with prazosin (10(-7)M). SGB-483 possesses the property of an alpha 1- and alpha 2-adrenoceptor antagonist. The alpha-antagonistic action was apparent on post-junctional smooth muscle cells. The alpha 2-antagonistic action on neuromuscular transmission was mediated at pre-junctional nerve terminals to enhance the release of NA. The prejunctional actions of SGB-483 were more selective than those of yohimbine or phentolamine.

Effects of dopamine on neuromuscular transmission in the guinea-pig mesenteric artery

The effects of dopamine on neuromuscular transmission in the guinea-pig mesenteric artery were investigated using a microelectrode method. Dopamine did not modify the membrane potential or the membrane resistance of the smooth muscle, but did reduce the amplitude of excitatory junction potentials (e.j.p.) and enhance the facilitation of e.j.p. produced by repetitive stimulation. Phentolamine (10(-7) M) enhanced the amplitude and the facilitation of the e.j.p., and with the addition of dopamine (10(-6) M) there was a reduction in the amplitude of e.j.p. but not in the facilitation. Haloperidol and sulpiride (greater than 10(-6) M) increased the amplitude of e.j.p. without altering the postjunctional membrane properties. Haloperidol and sulpiride did not increase the facilitation of e.j.p. produced by repetitive stimulation. In the presence of haloperidol or sulpiride (10(-5) M), dopamine (10(-6) M) did not suppress the amplitude of the e.j.p. These results indicate that in the guinea-pig mesenteric artery, dopamine inhibits the release of transmitter at the presynaptic membrane.

The effects of 3,4-dihydro-8-(2-hydroxy-3-isopropylaminopropoxy)-3-nitroxy-2H-1-benzopyran (K-351) and its denitrated derivative on smooth muscle cells of the dog coronary artery

1 Effects of 3,4-dihydro-8-(2-hydroxy-3-isopropylaminopropoxy)-3-nitroxy-2H-1-benzopyran (K-351) and its derivative, 3,4-dihydro-8-(2-hydroxy-3-isopropylaminopropoxy)-3-hydroxy-2H-1-benzopyran (K-351 (N-)) on the electrical and mechanical properties of smooth muscles of the dog coronary and mesenteric arteries were investigated, and the findings were compared with data obtained with nitroglycerin. 2 In both proximal and distal regions of the coronary arteries, K-351 and nitroglycerin reduced the resting tone and suppressed the contractions produced by high-potassium solution or by current passage to the same extent, with no remarkable change in the electrical properties of the smooth muscle membrane. 3 In the proximal regions of the descending coronary artery, low and high concentrations of noradrenaline (NA) produced relaxation and contraction of the muscle, respectively. In the distal region, NA consistently relaxed the muscle with concentrations up to 10(-5) M. In both regions, the contraction or relaxation was suppressed by phentolamine or propranolol, respectively. 4 K-351 suppressed the NA-induced contraction. K-351(N-) potentiated the NA-induced contraction and suppressed the relaxation, but these actions were weaker than those of propranolol. Nitroglycerin suppressed the NA-induced contraction and the potency was weaker than that of K-351. 5 In the mesenteric artery, K-351 depressed excitatory junction potentials, spikes and contractions evoked by perivascular nerve stimulation, while K-351(N-) potentiated or depressed mechanical responses, with no change in the electrical responses. Nitroglycerin also depressed the mechanical responses evoked by perivascular nerve stimulation with no change in the electrical responses. 6 These results suggest that K-351 has a blocking action on postjunctional adrenoceptors, and also dilator actions similar to the actions of nitroglycerin on the dog coronary artery, while K-351 (N-) possesses a weak beta-adrenoceptor blocking action.

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

1. The effects of cocaine on the neuromuscular transmission and smooth muscle cells of the guinea-pig mesenteric artery were observed using various experimental procedures.2. Cocaine (10(-7) M) depolarized the membrane and increased the membrane resistance of single smooth muscle cells. Outward current pulses produced neither spikes nor graded responses in Krebs solution, but in the presence of 10(-5) M-cocaine, outward current did produce spikes.3. Perivascular nerve stimulation evoked excitatory junction potentials (e.j.p.s). Repetitive stimulation (0.25-1.0 Hz) produced a frequency-dependent facilitation. Application of cocaine (10(-7) M) reduced the amplitude of the first e.j.p. (e.j.p.(f)) and also after completion of facilitation (e.j.p.(s)). However, the facilitation process was not affected by cocaine (10(-5) M).4. On pre-treatment with phentolamine (3 x 10(-7) M), both e.j.p.(f) and e.j.p.(s) were enhanced, but on pre-treatment with yohimbine (3 x 10(-7) M), e.j.p.(f) was inhibited and e.j.p.(s) was enhanced. Both phentolamine and yohimbine accelerated the facilitation of e.j.p.s in the absence and presence of cocaine (10(-7) M).5. The conduction velocity of nerve excitation measured from the latency of generation of e.j.p.s was slightly lowered by cocaine. The number of nerve fibres or varicosities contributing to the generation of an e.j.p. was not reduced in the presence of cocaine (10(-5) M).6. Mechanical responses could be recorded on perivascular nerve stimulation, and direct muscle stimulation on treatment with tetrodotoxin. Cocaine (10(-7) to 10(-4) M) enhanced the contraction evoked by direct muscle stimulation and inhibited the contraction evoked by perivascular nerve stimulation.7. Cocaine (10(-5) M) enhanced the contraction evoked by 5 x 10(-6) M-noradrenaline (NA) and direct muscle stimulation (5 sec pulse) but no effect was observed on the K-induced contraction (39.2 mM-K). On pre-treatment with guanethidine (10(-6) M) these effects of cocaine were not affected.8. In the presence of cocaine (10(-5) M), the depolarization of the membrane induced by NA was additively increased, and the dose response curve for NA was shifted to the left with no change in the maximum amplitude of contraction.9. When 10(-5) M-cocaine was applied during contractions evoked by alternate perivascular nerve stimulation and exogenously applied NA, the contraction evoked by perivascular nerve stimulation was reduced, while that evoked by NA was enhanced.10. In saponin-treated skinned muscles, the pCa-tension relationship was not affected by application of 10(-4) M-cocaine. The effects of cocaine on the Ca accumulation and release from the store site were estimated. It was found that cocaine (10(-4) M) slightly inhibited the Ca accumulation (0.89 times the control) but did not modify the Ca-release mechanism.11. The overflow of NA, 3,4-dihydroxyphenylglycol (DOPEG) and 3-methoxy-4-hydroxyphenylglycol (MOPEG) were measured in the same tissue before and after application of perivascular nerve stimulation in the presence or absence of cocaine (10(-7) to 10(-5) M). Cocaine induced a concentration-dependent increase in the overflow of NA and a reduction in the amounts of DOPEG and MOPEG.12. We conclude from these studies that cocaine mainly inhibits the sensitivity of the intra-junctional adrenoceptor, but increases the sensitivity of the extrajunctional adrenoceptor distributed on the post-junctional muscle membrane, with increase in the overflow of NA. The enhancement of mechanical response in the presence of cocaine is probably due to an increased sensitivity of the extra-junctional adrenoceptor and changes in the post-junctional muscle membrane, without any marked effect on the prejunctional mechanism.

Differences and similarities in the noradrenaline- and caffeine-induced mechanical responses in the rabbit mesenteric artery

1. The properties of noradrenaline (NA)-induced and caffeine-induced contractions in the rabbit mesenteric artery were investigated and compared, using intact and skinned muscles.2. NA (> 10(-7) M) depolarized the membrane and reduced the membrane resistance. Increased concentrations of NA (> 10(-5) M) rarely produced oscillatory potential changes yet consistently produced oscillatory contractions which were abolished by pre-treatment with prazosin or Ca-free solution. Application of low concentrations (0.5 mM) of caffeine hyperpolarized the membrane and high concentrations (5 mM) depolarized the membrane.3. NA- and caffeine-induced contractions, but not a K-induced contraction, could be recorded in Ca-free, EGTA (2 mM) containing solution.4. In Ca-free, EGTA containing solution, repetitive applications of NA or caffeine led to a cessation of contractions. Responses persisted longer with applications of NA than with caffeine. In Na-free, Ca-free solution, the contractions evoked by both agents applied repetitively persisted much longer than those evoked in the presence of [Na](o).5. When 50 mug/ml. saponin was applied for 8 min, NA produced only one contraction and with application of 10(-6) M-Ca, only a small contraction was evoked. When saponin was applied for 20 min, NA produced no contraction while 10(-6) M-Ca produced a large contraction, indicating that NA produced a contraction under conditions of incomplete skinning, while caffeine consistently released Ca from incompletely and completely skinned muscles.6. After 20 min of saponin treatment (50 mug/ml.), the pCa-tension relationship could be measured. The minimum concentration of Ca required to produce the contraction was just over 10(-7) M, and the maximum contraction occurred with 3 x 10(-6) M-Ca and was greater than that produced by 128 mM-K in intact muscles. The sigmoidal relationship with this rabbit tissue was sharper than that observed in the guinea-pig mesenteric artery. Application of 10 mM-procaine and 10 mM-caffeine slightly inhibited the pCa-tension relationship.7. In skinned muscles, the amount of stored Ca was estimated from the amplitude of the caffeine-induced contraction. Addition of NA (10(-5) M) with 10(-6) M-Ca had no effect on the caffeine-induced contraction. A reduction of Na from 10 mM to zero in the relaxing solution slightly suppressed the caffeine - induced contraction, i.e. there was a slight inhibition of Ca accumulation.8. Thus, different mechanisms are involved in the NA- and caffeine-induced activations, but both agents do release Ca stored in the same site. Release of Ca from the store site by NA requires the presence of an intact muscle membrane with adrenoceptors but this is not the case with caffeine. The different mechanical responses to NA or caffeine may also relate to the behaviour of Ca; i.e. Ca may be extruded into the extracellular space in the presence of caffeine, while this ion may be extruded and to some extent, re-accumulate into the store site in the presence of NA. The oscillatory contraction evoked by treatment with NA is discussed in relation to the activation of a Ca-induced Ca-release mechanism in the store site.