Effects of suramin on the concentration--response relationship of alpha, beta-methylene ATP on the mouse vas deferens

1. The effect of suramin on the concentration-effect curve for the contractile response of the isolated mouse vas deferens to alpha, beta-methylene ATP (alpha, beta-meATP) was investigated. 2. The concentration-response curve to alpha, beta-meATP had a consistent discontinuity at about 3 x 10(-6) M, giving it a biphasic appearance. 3. Suramin in a dose-dependent, reversible manner both shifted the curve to the right and at the same time elevated the maximum response. 4. The P2y inhibitor Reactive Blue 2, on the other hand, both shifted the curve to the left and elevated the maximum response. 5. These results show that alpha, beta-meATP in this preparation is an agonist both at excitatory, presumably P2y receptors and inhibitory P2y receptors, and that suramin antagonizes both effects.

Properties of excitatory junction potentials and currents in smooth muscle cells of the mouse vas deferens

Intracellular recordings made from superficial smooth muscle cells of the mouse vas deferens confirmed the presence of two populations of cells, distinguishable by their membrane potential and measured input resistance. In this study, we have concentrated on cells with high input resistance (50-500 M omega) and membrane potentials of -45 to -65 mV. These cells fired action potentials when depolarized by intracellular current injection and appeared to be electrically isolated from adjacent cells. Stimulation of the intramural nerves evoked excitatory junction potentials (e.j.p.s.), which fluctuated in amplitude, with the largest firing action potentials. The e.j.p. amplitude was increased by hyperpolarizing and decreased by depolarizing the cell membrane. Under voltage-clamp, nerve stimulation evoked excitatory junction currents (e.j.c.s.) which reached a peak in 5 ms, and declined exponentially with a time-constant of 28 ms. The e.j.c. amplitude was linearly related to membrane potential with a reversal potential near -10 mV.

A study of the actions of P1-purinoceptor agonists and antagonists in the mouse vas deferens in vitro

1. We have examined the effects of purinoceptor agonists and antagonists on the mechanical 'twitch' response, excitatory junction potential (e.j.p.) amplitude and [3H]-noradrenaline overflow in the mouse vas deferens. 2. The agonist profile for inhibition of the mechanical response was N6-([R]-2-phenylisopropyl)adenosine (L-PIA) congruent to N6-cyclohexyladenosine (CHA) greater than 5' N-ethylcarboxamido-adenosine (NECA) greater than 2-chloroadenosine (2ClA) congruent N6-([S]-2-phenylisopropyl)adenosine (D-PIA). 3. The P1-purinoceptor agonists inhibited the e.j.p. with an agonist profile of CHA greater than L-PIA congruent to NECA greater than 2ClA. 4. 2ClA inhibited [3H]-noradrenaline overflow with an EC50 of 1.2 microM which was not significantly different from the values for inhibition of the e.j.p. and the mechanical response. 5. The inhibitory action of 2ClA on the mechanical response was antagonized by 5 microM 8-phenyltheophylline (8-PT). However, neither blockade of P1-purinoceptors by 8-PT nor increasing the rate of degradation of endogenous adenosine by addition of adenosine deaminase had any effect on the mechanical response per se. 8-PT (5 microM) also failed to alter the e.j.p. amplitude or [3H]-noradrenaline overflow. 6. These results indicate that there are P1-purinoceptors present on sympathetic nerve terminals of the mouse vas deferens which are more like A1- than A2-receptors, but may be better classified as being of the A3-subtype (Ribeiro & Sebastiao, 1986). These receptors are not normally involved in the feedback regulation of transmitter release in this tissue.

Suramin: a reversible P2-purinoceptor antagonist in the mouse vas deferens

The trypanocide Suramin was tested as a possible antagonist at the P2-purinoceptor of the mouse vas deferens. At a concentration of 100 microM, Suramin antagonized the response to alpha,beta-methylene ATP, while responses to carbachol and noradrenaline were unaffected. These results suggest that Suramin may provide a starting point for the development of specific antagonists for P2-purinoceptors.

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.

Facilitation at single release sites of a sympathetic neuroeffector junction in the mouse

Electrophysiological techniques were used to observe the release of transmitter from one or a few release sites of the sympathetic neuroeffector junction of the mouse vas deferens. Release produces transient accelerations of the depolarizing phase of the excitatory junction potential, known as 'discrete events'. Discrete events associate into families at a constant latency and peak time, but vary in amplitude between a few preferred values. As facilitation develops there is a decrease in the frequency of small members of families and an increase in the frequency of large members, a change in apparent 'quantal content'. A similar change in amplitude distribution occurs when the [Ca]o is raised. The alpha-adrenoceptor antagonist yohimbine increases quantal content when facilitation has developed, but has no significant effect on unfacilitated discrete event amplitude unless the [Ca]o is below 2.0 mM. The alpha-adrenoceptor agonist clonidine reduces facilitated and unfacilitated quantal content under all conditions examined.

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.

An electrophysiological analysis of the effects of cooling on autonomic neuromuscular transmission in the guinea-pig vas deferens

The effects of cooling upon the excitatory junction potential (e.j.p.) and spontaneous e.j.p.s. in the guinea-pig vas deferens have been studied. A series of strict criteria have been used to select 'successful' penetration of smooth muscle cells by the micro-electrodes. Cooling the vas from 35 to 22 degrees C has little effect on the membrane potential. The e.j.p. is increased in size by cooling from 5 . 1 +/- 0.2 mV, n = 120 at 35 degrees C to 15 . 9 +/- 0 . 5 mV, n = 29 at 22 degrees C. Facilitation is less marked at lower temperatures. The effects of cooling are not due to any effects under the electrodes used to stimulate the innervation of the vas. The frequency and size of spontaneous e.j.p.s. is reduced by cooling. The significance of these observations is discussed.

Local regulation of transmitter release from rodent sympathetic nerve terminals?

1. Electrophysiological techniques were used to observe the release of transmitter from single release sites in the sympathetic neuro-effector junction of the rodent vas deferens. Transmitter release produces transient peaks in the rate of depolarization of the smooth muscle cells, known as 'discrete events'. 2. The amplitude distributions of stimulus-evoked discrete events in both mouse and guinea-pig vas are multi-modal. In the mouse, the distribution fits a Poisson with a 'quantal content' of about two. There are too many zeros in the amplitude distributions of guinea-pig discrete events to fit a Poisson distribution, and it is likely that in this species there is a mechanism preceding the transmitter release process which may occasionally prevent it operating. 3. alpha-adrenoreceptor agonists and antagonists produce, respectively, left and right shifts in the amplitude distributions of discrete events at a single latency, with no change in the amplitudes at the modes. 4. There is, however, no evidence of any inhibitory relationship between either discrete events evoked by successive stimuli, or early and late discrete events following a given stimulus. 5. Transmitter release at this junction is therefore packeted, with few quanta released by each stimulus. Release from single sites is affected in ways compatible with the 'alpha-feed-back' hypothesis by alpha-adrenoreceptor agonists and antagonists, but in the absence of drugs we can find no evidence of any local feed-back inhibition of transmitter release.

An electropharmacological analyses of the effects of some drugs on neuromuscular transmission in the vas deferens of the guinea-pig

1 The effects of several drugs upon the excitatory junction potential (e.j.p.) in the guinea-pig vas deferens have been investigated. 2 Amiodarone, a noradrenergic neurone blocker which also blocks both alpha and beta-adrenoreceptors, did not reduce the e.j.p. 3 The alpha-adrenoreceptor antagonists, azapetine, piperoxan and prazosin, only enhanced the e.j.p. irrespective of their relative potencies at alpha 1 and alpha 2-adrenoreceptors. 4 Sotalol, a beta-adrenoreceptor antagonist, was without effect upon the e.j.p. 5 Clonidine and lysergic acid diethylamide, alpha-adrenoreceptor agonists, produced a dose dependent inhibition of the e.j.p. without apparently affecting the frequency or size of spontaneous junction potentials. 6 The effects of clonidine were antagonized by piperoxan in a competitive reversible manner. 7 It is argued that these results confirm the presence of alpha-adrenoreceptors prejunctionally upon the sympathetic excitatory innervation of the vas deferens but that although an endogenous alpha-adrenoreceptor agonist is released by nerve stimulation either the transmitter that is responsible for the e.j.p. is not noradrenaline or that the postjunctional receptors responsible for the generation of the e.j.p. are not adrenoreceptors.

The packeted release of transmitter from the sympathetic nerves of the guinea-pig vas deferens: an electrophysiological study

1. Excitatory junction potentials (e.j.p.s) were recorded intracellularly in the guinea-pig vas deferens following stimulation of the hypogastric nerve. Differentiation of the rising phase of the e.j.p. showed them to be made up of transient peaks in the rate of depolarisation, the 'discrete events'. 2. In any one cell discrete events occurred at one or several latencies, intermittently, the frequency of occurrence varying between 1 in 1.8 to 1 in 45 stimuli. 3. Intermittence was not an artifact due to the use of submaximal stimulation nor the result of a ganglionic relay between the hypogastric and vas deferens nerve. 4. Discrete events occurring with a single latency had amplitudes that were multimodally distributed. In some cells the preferred values of amplitude were simple whole number multiples of the smallest preferred value. 5. The time course of discrete events varied from cell to cell and at different latencies. The discrete event had a time to peak of 5.3 +/- 1.9 msec, n = 220 (mean +/- S.D.) and a time to half decay of 8.3 +/- 3.6 msec, n = 220. 6. Discrete events in a cell could be matched for amplitude and time course by spontaneous excitatory junction potentials in the same cell and both probably represent the release of a single packet of transmitter. 7. The e.j.p. is made up of (1) discrete events which represent the release of transmitter from a single varicosity, (2) a non intermittent slow component which represents the electronic spread of activity from smooth muscle excited from distant release sites. 8. It is concluded that transmitter release from individual varicosities is packeted and the number of packets liberated per stimulus from a single varicosity is small, varying between zero and 10.

The electrical responses of the rabbit rectococcygeus following extrinsic parasympathetic nerve stimulation

1. The responses of the rabbit rectococcygeus muscle to stimulation of the extrinsic pelvic nerves have been investigated using intracellular micro-electrode recording techniques. 2. Submaximal pelvic nerve stimulation evoked a depolarization (e.j.p.) which was graded with stimulus strength and abolished by atropine (10(-6) g/ml.) and tetrodotoxin (TTX). 3. Single supramaximal stimuli evoked action potentials associated with muscle contraction. 4. In the presence of atropine, to abolish the e.j.p.s, supramaximal stimulation of the pelvic nerves evoked hyperpolarizations (i.j.p.s) which were graded with stimulus strength and abolished by TTX. 5. The transmitter responsible for the i.j.p.s is unknown. Phentolamine (10(-5) g/ml.) and propranolol (3 x 10(-5) g/ml.) in concentrations which block respectively alpha and beta adrenoceptors and the adrenergic neurone blocking agent guanethidine (10(-6) g/ml.) were ineffective in blocking the inhibitory response. 6. It is concluded that stimulation of the pelvic nerves to the rectococcygeus releases two transmitters, acetylcholine, responsible for the e.j.p. (and muscle contraction) and a non-adrenergic non-cholinergic transmitter which is the basis for the mechanical relaxation.

The effects of piperoxan on uptake of noradrenaline and overflow of transmitter in the isolated blood perfused spleen of the cat

1 The competitive alpha-adrenoceptor blocking agent, piperoxan, in concentrations up to 2 x 10(-4) M, produced large dose-dependent increases in transmitter overflow from the isolated blood perfused spleen of the cat following nerve stimulation at 10 hertz. 2 At concentrations greater than 2 x 10(-4) M, piperoxan produced a rise in perfusion pressure, a contraction of the splenic capsule, and a marked dose-dependent decrease in transmitter overflow. 3 Phenoxybenzamine (10(-4) M) and desmethylimipramine (3 x 10(-5) M) produced further increases in transmitter overflow when added after piperoxan. 4 Piperoxan (5.8 to 6.6 x 10(-6) M) had no effect on the recovery of 3H in the venous blood following the close arterial infusion or injection of (3H)-(--)-noradrenaline, indicating that the drug does not inhibit uptake of the amine. 5 Piperoxan produced dose-dependent inhibition of responses of the splenic vasculature to close arterial injection of 1 microgram of (--)-noradrenaline but was much less effective at inhibiting responses to nerve stimulation. At 2 x 10(-6) M piperoxan produced a considerable reduction of the response to injected noradrenaline but potentiated the response to nerve stimulation. 6 In isolated strips of cat splenic capsule, piperoxan produced a shift to the right of the dose-response curve to noradrenaline with no change of the maximum response. There was no evidence of a postsynaptic sensitizing effect of the type observed in the rat vas deferens.

Accumulation of amines by rabbit erythrocytes in vitro

1. Accumulation of noradrenaline (NA), 5-hydroxytryptamine (5HT) and tyramine by rabbit erythrocytes was measured at 37 degrees C in vitro. 2. Of the amines used only NA was broken down during incubation. This was a result of intracellular catechol-O-methyl transferase activity. 3. NA and 5HT entered the red cells by similar processes which were temperature-sensitive (cooling to 0 degrees C inhibited accumulation) and had saturation kinetics. The entry of NA was partially stereospecific; the (-)-isomer accumulated twice as fast as did (+)-NA. 5HT and NA competed for entry. Tyramine entry was unaffected by cooling, was not saturable and did not affect the entry of either NA or 5HT. NA and 5HT entered the erythrocytes at rates which were proportional to their lipid solubilities. 4. Metabolic inhibitors had no effect on amine transport. Inhibitors of amine transport in other tissues produced only small non-specific reductions of NA accumulation in the red cells. 5. Amine accumulation was a symmetrical process (no amine was retained by the red cells if the concentration gradient was reversed). It is concluded that NA and 5HT enter the cells by facilitated diffusion. The entry of NA and 5HT displayed countertransport, an additional feature of facilitated diffusion. 6. The relationship between the physical properties of the amines and the routes by which they entered the erythrocytes is discussed.

The effects of labetalol (AH 5158) on adrenergic transmission in the cat spleen

1. The competitive alpha- and beta-adrenoceptor blocking agent labetalol, in concentrations up to 10(-4) M, produced dose-dependent increases in transmitter overflow from the isolated blood perfused spleen of the cat following nerve stimulation at 10 and 30 Hz. 2. At concentrations above 10(-4) M labetol produced a pronounced decrease in transmitter overflow. 3. Labetalol (1.5 X 10(-4) M) increased the recovery of 3H label in the venous blood following the close-arterial infusion of [3H]-(-)-noradrenaline indicating that the drug inhibits uptake of the amine. 4. Both labetalol (3.8 X 10(-5) M) and piperoxan (7.4 X 10(-6) M) produced parallel shifts to the right of the dose-response curves to noradrenaline and oxymetazoline in isolated strips of cat splenic capsule. In this preparation both drugs acted as competitive postsynaptic alpha-adrenoceptor blocking agents. 5. Labetalol (3.3 X 10(-5) M) increased the transmitter overflow following stimulation of the splenic nerves with 200 impulses at 10 Hz. The overflow could be further increased by subsequent addition of piperoxan (7.2 X 10(-6 M). Piperoxan (5.7 X 10(-6) M) alone produced a marked increase in transmitter overflow which could be further increased by subsequent addition of desmethylimipramine (DMI; 3.2 X 10(-5) M). Cocaine (1.5 X 10(-5) M) or DMI (5.4 X 10(-5 M) produced a small increase in transmitter overflow which was not further increased by addition of labetalol (2.8 X 10(-5) M). 6. Labetalol produced a biphasic effect on the responses of the isolated blood perfused spleen of the cat to nerve stimulation. With low doses (up to 10(-4) M) vascular responses were potentiated and with high doses (greater than 10(-4) M) inhibited. The potentiation was related to uptake blockade and the inhibition to decreased transmitter overflow and postsynaptic alpha-adrenoceptor blockade. 7. Labetalol appears to act as a postsynaptic alpha-adrenoceptor antagonist in the isolated blood perfused spleen of the cat with little effect on presynaptic alpha-adrenoceptors. The moderate elevation of transmitter overflow by the drug is related to the inhibitory effect of the drug on neuronal uptake rather than on presynaptic alpha-adrenoceptors.

An uptake mechanism for L-noradrenaline in the cat spleen, associated with the nerves but distinct from uptake

1. The characteristics of the uptake of [(3)H]L-noradrenaline from 1 mug pulses injected close arterially to the isolated blood perfused cat spleen are described.2. The spleen took up 355.0+/-16.0 ng from a first pulse and 321.1+/-43.6 ng from a second.3. Uptake from a second pulse was 254.0+/-30.3 ng in the presence of the Uptake(1) inhibitor desmethylimipramine (DMI) (3.3 x 10(-5)M) and the Uptake(2) inhibitor 17-beta-oestradiol (17beta0) (1.8 x 10(-4)M).4. Uptake from small 10 ng pulses was also insensitive to DMI and 17beta0.5. Uptake from pulses was abolished by surgical denervation of the spleen, or pre-treatment with phenoxybenzamine (PBA) (8.8 x 10(-5)M).6. The pulse uptake process was impaired by omission of red cells from the perfusate.7. Stimulation of the splenic nerves at 3Hz halved the pulse uptake.8. Uptake from infusion in the presence of DMI and 17beta0 occurred only during the first few minutes of the infusion.9. It is concluded that an uptake process for L-noradrenaline distinct from Uptakes(1) and (2) is present in the spleen and this might be important physiologically.

The effects of pargyline on overflow of transmitter and uptake of noradrenaline in the cat spleen

1. Twenty minutes after the addition of pargyline (5 x 10(-4)M) to blood perfusing the isolated spleen of the cat, the overflow of transmitter resulting from stimulation of the sympathetic nerves increased 2.3-3-fold. Lower doses of pargyline did not significantly affect overflow.2. Monoamine oxidase activity, measured with either radioactively labelled tyramine or noradrenaline as substrate, was almost completely inhibited by doses of pargyline in the range of 10(-4)M to 5 x 10(-4)M. Inhibition of enzyme activity was not correlated with the effect on overflow. Pargyline had only a slight inhibitory effect on catechol-O-methyl transferase.3. Uptake of a 1 mug injection (pulse) of labelled noradrenaline, following pargyline (5 x 10(-4)M), was increased to 199.1% of that found in control experiments.4. Pargyline significantly reduced the vascular responses to nerve stimulation but had no significant effect on capsular responses.5. The inhibitor had no effect on resting overflow of labelled noradrenaline from the spleen but doubled the overflow of labelled noradrenaline following nerve stimulation.6. It is suggested that the effect of pargyline on overflow is due to increased release of transmitter during nerve stimulation.7. The possible clinical significance of these findings is discussed.

The noradrenaline content of the vas deferens of the guinea-pig

Noradrenaline is present in the guinea-pig vas deferens at a concentration some 15 to 20 times greater than in the cat spleen, but this level is far less susceptible to change. Neither stimulation nor decentralization of the adrenergic innervation of the guinea-pig vas deferens produces any alteration in the noradrenaline content. This is in marked contrast to the cat spleen in which similar procedures have previously been shown to cause significant changes in noradrenaline levels.

Perfusion of the spleen with blood containing prostaglandin E1: transmitter liberation and uptake

The cat’s spleen perfused with homologous blood containing prostaglandin E 1 provides an admirable preparation for the study of the liberation and uptake of the sympathetic transmitter. Obstruction of the circulation with platelet thrombi is eliminated; blood flow is high, and overflow of transmitter when the nerves are stimulated is higher than in other preparations. The high overflow of transmitter is attributed to adequate perfusion of a large mass of splenic tissue. There is no evidence that the amount of transmitter liberated per unit mass of tissue is increased. Uptake of liberated transmitter and of infused L-noradrenaline is no different from that observed in other preparations of the spleen.

Uptake and re-use of sympathetic transmitter in the cat's spleen

The output of transmitter resulting from stimulation of the sympathetic nerves of the cat’s spleen, perfused with homologous blood, is described and compared with similar results obtained from the spleen in situ . Transmitter overflows are somewhat lower in the spleen perfused by the present method, but the relationship between overflow and frequency of stimulation is the same as in the spleen in situ . Noradrenaline added to the perfusate at rates as high as 6 μ g/min is taken up by the spleen which shows no sign of saturation even at these high levels of presentation. Loss of transmitter from the spleen causes a progressive decline in the overflow of transmitter produced by successive trains of stimuli. Evidence is presented to support the hypothesis that the continued effectiveness of a sympathetic nerve depends upon the uptake and re-use of the transmitter that it liberates.

The responses of the spleen to nerve stimulation in relation to the frequency of splenic nerve discharge

The response of the spleen is shown to be made up of at least three distinct components, 2 vascular and 1 capsular. Although the splenic capsular response is maximal at frequencies of nerve stimulation well below 10 impulses per second, the response of the splenic vascular bed is less than 70% maximum with stimulation at 10 impulses per second (imp. /s). In the decerebrate cat the splenic nerve fibres are normally active at between 0 and 5.6 imp. /s and in asphyxia or following vertebral occlusion this activity can rise as high as 10 imp. /s.