Inhibitory presynaptic effect of noradrenaline on the hypogastric ganglion of the guinea pig

The effect of noradrenaline on the hypogastric ganglion of the guinea pig was studied to establish the role of catecholamine-containing sites in the sympathetic ganglia with the hypogastric nerve-vas deferens preparation. In these experiments the ganglion was separated pharmacologically from the muscle by a partition. An application of noradrenaline, adrenaline and isoproterenol each to the ganglion inhibited a twitch response of the vas deferens to preganglionic nerve stimulation at low frequencies. The drug effect decreased with increasing frequencies of stimulation. The inhibitory action of these drugs was antagonized by phentolamine, but not by propranolol. Clonidine also reduced the response to the nerve stimulation in a similar manner as did catecholamines, and the inhibitory action was antagonized by phentolamine. In contrast, phenylephrine had no significant action. The inhibitory effect of noradrenaline and clonidine was antagonized by the alpha-2 antagonist yohimbine, but not by such alpha-1 antagonists as prazosin and E-643. Noradrenaline did not affect the ganglion-stimulating action of acetylcholine. These results suggest that the inhibitory action of noradrenaline may be mediated by alpha-2 adrenoceptors on presynaptic cholinergic nerve endings of the guinea-pig hypogastric ganglion. These data are discussed in relation to the presence of noradrenaline-containing sites in this ganglion.

Biochemical and functional properties of large and small dense-core vesicles in sympathetic nerves of rat and ox vas deferens

A comparative study of the noradrenaline storing vesicles in vas deferens from ox and rat was performed. Microsomal fractions were subjected to density gradient centrifugation. In rat, noradrenaline and dopamine beta-hydroxylase were mainly present in the upper fractions of the gradient, which is consistent with the predominance of light (small dense-core) vesicles in this species. In ox, noradrenaline, dopamine and dopamine beta-hydroxylase were found in the gradient in a bimodal distribution. This is consistent with the presence of about equal numbers of small and large dense-core vesicles in this species. On the other hand, chromogranin A, immunologically related proteins and enkephalin-like immunoreactivity were only present in the dense (large dense-core) vesicle population. In order to study the capability of light and dense vesicles to synthesize noradrenaline we "pulse-labelled" ox vasa deferentia with [3H]tyrosine. Already 3.5 min after the pulse both types of vesicles contained [3H]noradrenaline and [3H]dopamine. During longer "chase" periods the amount of [3H]dopamine gradually declined. We conclude that dense (large dense-core) vesicles contain chromogranin A, immunologically related proteins and enkephalin-like immunoreactivity whereas light (small dense-core) vesicles are devoid of these components. Both types of vesicles contain dopamine beta-hydroxylase and can synthesize noradrenaline from dopamine under in vivo conditions.

Control of transmitter release in guinea-pig vas deferens by prejunctional P1-purinoceptors

The specific P1-purinoceptor agonist 2-chloroadenosine (3 X 10(-7) M to 3 X 10(-6) M) reduced the magnitude of excitatory junction potentials (e.j.p.s) recorded from guinea-pig vas deferens in response to field stimulation of the sympathetic nerves, but did not have any direct effect on the resting membrane potential. Trains of pulses (2-16 Hz) for 20 s produced a biphasic contractile response, both phases of which were reduced by 2-chloroadenosine (10(-7) to 10(-5) M) by up to 45%. In contrast, the same concentrations of 2-chloroadenosine enhanced by about 20% the contractile response of the vas deferens to exogenously applied adenosine 5'-triphosphate (ATP) and noradrenaline (NA). The specific P1-purinoceptor antagonist 8-phenyltheophylline (8-PT) reversed the inhibitory effect of 2-chloroadenosine on e.j.p. magnitude, partially reversed the inhibitory action of 2-chloroadenosine on both phases of the contractile response to nerve stimulation, and partially reversed the enhancing effect of 2-chloroadenosine on responses to exogenous ATP and NA. We propose that release of ATP and NA (as cotransmitters) from sympathetic nerves can be modulated by prejunctional P1-purinoceptors.

Post-natal change in the effect of denervation on the rat vas deferens

The vas deferens of 14-54 d old rats was denervated, isolated after 7 d, divided into prostatic and epididymal halves, and denervation supersensitivity to noradrenaline, acetylcholine, methacholine and tetramethylammonium (TMA) was examined. The supersensitivity to any of these agonists did not appear in rats younger than 21 d of age. Thereafter a leftward shift of the dose-response curve and an increase in the maximum contraction to noradrenaline were observed. The maximum contraction became progressively greater with development in the prostatic half but was approximately constant in the epididymal half. Supersensitivity to methacholine distinctly developed with age in the epididymal half. Only a subsensitivity to TMA was observed, suggesting that TMA acts on the nerve. The effect of denervation on the response to acetylcholine in the epididymal half resembled that to methacholine, and in the prostatic half resembled that to TMA. The developmental change in the effect of denervation on noradrenaline was discussed in relation to the full contractile ability of the vas halves, and that to cholinergic drugs was discussed in relation to nicotinic and muscarinic receptor population.

The outward transport of axoplasmic noradrenaline induced by a rise of the sodium concentration in the adrenergic nerve endings of the rat vas deferens

The adrenergic nerve endings of the rat vas deferens were loaded with 3H-(-)-noradrenaline; COMT was inhibited by the presence of 10 mumol/l U-0521, and all experiments were carried out with calcium-free solution. After 100 min of wash-out a neuronal efflux of tritium was obtained which remained constant with time (when expressed as fractional rate of loss; FRL); it contained more DOPEG than noradrenaline. The in vitro administration of reserpine-like drugs (reserpine and Ro 4-1284) increased the FRL of tritium, presumably because of an increase in the leakage of noradrenaline from storage vesicles; the efflux of DOPEG increased more than that of noradrenaline, and the ratio NA/DOPEG declined. Inhibition of the membrane ATPase (by omission of potassium from the medium or by the presence of 3 mmol/l ouabain) increased the FRL of tritium, presumably because of an increase in the net leakage of noradrenaline from the storage vesicles (as a consequence of the fall in the concentration of free axoplasmic noradrenaline; see below). Veratridine also increased the FRL of tritium, partly because of its known reserpine-like effect (Bönisch et al. 1983); in the presence of 1 mumol/l veratridine, the efflux of DOPEG increased. Irrespective of the presence or absence of reserpine or Ro 4-1284, inhibition of the membrane ATPase or the presence of veratridine (agents or procedures which increase the axoplasmic sodium concentration) always resulted in a brisk increase of the efflux of noradrenaline that was accompanied by a simultaneous decrease in the efflux of DOPEG (see above for one exception).(ABSTRACT TRUNCATED AT 250 WORDS)

Transmitter secretion from individual varicosities of guinea-pig and mouse vas deferens: highly intermittent and monoquantal

A modification of "classical" electrophysiological techniques was used to characterise the secretory activity of individual release sites of the sympathetic nerves of guinea-pig and mouse vas deferens. The rising phases of the intracellularly recorded excitatory junction potentials of a smooth muscle cell, were electrically differentiated, and fluctuations in the rate of rise recorded as phasic peaks (in the dV/dt of excitatory junction potentials), termed "discrete events". Experimental factors which may influence discrete events were examined systematically, and criteria established to recognize a discrete event as the "image" of transmitter secreted from a particular release site. To determine the quantal content of a stimulus-evoked discrete event, it was compared with discrete events occurring spontaneously in the same cell. Furthermore, the frequency dependence of the probability of occurrence of discrete events was compared with that of the evoked fractional secretion of tritium-labelled noradrenaline, to find out if release sites from which noradrenaline is secreted share the characteristics of those secreting the (still unknown) transmitter causing discrete events. The results obtained permit the following tentative conclusions: Both in guinea-pig and mouse vas deferens stimulus-induced secretion of transmitter from a single varicosity of the sympathetic nerves is highly "intermittent". Transmitter secretion from a varicosity is basically monoquantal. Spontaneous secretion of transmitter quanta occurs from "random" sites, but a nerve impulse causes secretion of a quantum from a "preferred site" of a varicosity: during a stimulus train quanta are secreted in "complementary pairs". Secretion of the first quantum in a "pair" does not lead to "autoinhibition" of the site from which it was released, but induces a short-lasting facilitation. (5) Some characteristic features of the secretion of the neurotransmitter causing discrete events, seem to apply to the secretion of noradrenaline from noradrenergic nerves also.

Alpha-adrenoceptor antagonism by apoyohimbine and some observations on the pharmacology of alpha-adrenoceptors in the rat anococcygeus and vas deferens

alpha-Adrenoceptor antagonism of several test drugs was assessed against adrenergic contractile responses to field stimulation in rat vas deferens and anococcygeus, the prejunctional inhibitory effect of xylazine in vas deferens and the contractile effects of alpha-adrenoceptor agonists in anococcygeus. Against the adrenergic nerve-induced contraction in vas deferens, the potency series was WB 4101 greater than prazosin greater than apoyohimbine greater than corynanthine greater than yohimbine greater than rauwolscine. Against the inhibitory effect of xylazine in vas deferens the potency series was apoyohimbine greater than rauwolscine = yohimbine greater than WB 4101 much greater than prazosin and corynanthine. In anococcygeus, against the contractile responses to adrenergic nerve stimulation or to the agonists amidephrine, noradrenaline and xylazine, the potency series was apoyohimbine greater than corynanthine greater than rauwolscine. These results show that apoyohimbine is more potent than the yohimbine sterioisomers as an antagonist at alpha 1- and alpha 2-adrenoceptors but is not more selective. The assay methods employed confirm the current classification of 'alpha'-receptors and drugs.

Angiotensin III-induced modulation of neurogenic responses in the rabbit vas deferens and portal vein

The effects of prostaglandin synthesis inhibitors on the presynaptic actions of angiotensin (ang) II and III were examined in isolated rabbit vasa deferentia and portal veins. Ang II caused dose-dependent potentiation of low frequency, nerve stimulation in vasa deferentia and portal vein. Indomethacin (26 microM) enhanced the electrically-induced contractions in the vasa deferentia only but did not alter the potency of ang II in either preparation. In contrast, ang III decreased contractions in vasa deferentia induced by nerve stimulation by up to 36% (10(-6) M) and potentiated these contractions at concentrations higher than 10(-6) M. The inhibitory action of ang III on vasa deferentia was converted to potentiation by pretreatment with indomethacin or mepacrine. Exogenous PGE2 blocked low frequency nerve stimulation and not responses to norepinephrine. This prostaglandin appeared to mimic ang III in the vas deferens. No effects of ang III were observed if the contractions were induced by exogenous alpha adrenergic agonists. [Sar1, Ala8] ang II antagonized all responses to the angiotensins, whereas [Sar1, Cys-CH3(8)] ang II selectively antagonized the angiotensin-induced potentiation. Responses in field-stimulated portal veins were potentiated by ang III and this response was unaffected by indomethacin. This investigation strongly suggests the existence of at least two ang receptors in the vas deferens and demonstrates for the first time selective responses to the octa- and heptapeptides in the same effector organ.

The time course of GABA action on the crayfish stretch receptor: evidence for a saturable GABA uptake

The conductance increase induced by bath application of GABA has been measured in voltage-clamped stretch-receptor neurones of crayfish. A rapid conductance increase was obtained only at GABA concentrations above 3.3 X 10(-4) M. The response to lower GABA concentrations (between 10(-4) and 10(-6) M) developed slowly over 30-60 min. Repetitive application of intermediate GABA concentrations induced postsynaptic conductance changes which were progressively enhanced in their onset and magnitude. In the presence of nipecotic acid or in Na+-free Ringer solutions, the response to all GABA concentrations was rapid and constant for each concentration. The time course of inhibitory postsynaptic currents was unaffected by nipecotic acid. These results suggest the presence of a saturable GABA uptake system which limits the access of bath-applied GABA to postsynaptic receptors. This system has little if any effect on the termination of response to synaptically released GABA.

Mechanisms of chloride transport in crayfish stretch receptor neurones and guinea pig vas deferens: implications for inhibition mediated by GABA

Since activation of GABAA receptors is believed to open an associated Cl- channel, the intracellular Cl- activity (aiCl) must be lower than that predicted from a passive distribution, to account for hyperpolarizing responses, or higher, to account for depolarizing responses. The physiological and pharmacological properties of the implied Cl- extruding and accumulating mechanisms have been investigated by direct measurements of aiCl. A coupled K+-Cl- co-transport has been found in crayfish stretch receptor neurones and a predominating Cl(-)HCO3(-) exchange in guinea pig vas deferens. From the different ionic mechanisms involved in Cl- extrusion and accumulation, it is proposed that drugs which affect Cl- transport mechanisms will reduce GABA responses of both polarities only if their action is via interference with the Cl- recognition site, but not if it is via interference with the co- or counter-ion recognition site.

Post-natal development of functional neurotransmission in rat vas deferens

Responses of the rat vas deferens to drugs and to field stimulation were examined in sexually immature rats. The vasa from immature rats often exhibited spontaneous contractions and displayed greater sensitivity to the contractile effects of alpha-adrenoceptor agonists. The responses of the vasa from immature rats to single pulse field stimulation lacked the adrenergic component of the response although the non-adrenergic component was present. The responses were antagonized by alpha 2-adrenoceptor agonists. In the presence of cocaine, an adrenergic component of the response did appear. During trains of pulses the pre- and postjunctional effects of adrenergic transmission which are found in adult rats were absent in vasa from immature rats. Electron microscopic studies showed no qualitative differences in adrenergic innervation in vasa from immature and adult rats. It is concluded that a state of 'pre-innervation supersensitivity' associated with a lack of functional adrenergic transmission exists in the vas deferens of immature rats. The supersensitivity disappears and functional transmission develops during the period in which testosterone secretion increases in the rat. The reason for the lack of functional transmission at a time when the innervation appears to be morphologically mature is not clear but may be due to the noradrenaline release mechanism not being fully operative.

Inhibition of excitatory junction potentials in guinea-pig vas deferens by alpha, beta-methylene-ATP: further evidence for ATP and noradrenaline as cotransmitters

Excitatory junction potentials (e.j.p.s) and spontaneous excitatory junction potentials (s.e.j.p.s) recorded from guinea-pig vas deferens were greatly reduced or abolished by alpha, beta-methylene-ATP, the stable analogue of adenosine 5'-triphosphate (ATP). Brief (20-50 ms) local application of ATP by pressure ejection from a micropipette produced a transient depolarisation comparable to the e.j.p. Noradrenaline (NA) applied in a similar manner produced no such response. The depolarisation produced by the local application of ATP was also inhibited by alpha, beta-methylene-ATP. Superfusion of the tissue with ATP or NA produced depolarisation of muscle cells; in the presence of alpha, beta-methylene-ATP the depolarisation produced by ATP was almost abolished, whereas that produced by NA was not reduced. These results are consistent with the hypothesis that the e.j.p.s in the guinea-pig vas deferens are mediated by ATP, acting as a cotransmitter with NA in the sympathetic nerves supplying this organ.

The vas deferens as a suitable preparation for the study of alpha-adrenoreceptor molecular mechanisms

The responses of the isolated guinea pig vas deferens to phenylephrine in the absence or presence of phenoxybenzamine were studied. The occurrence of a homogenous receptor population was suggested by the finding that the dissociation constant (KA) values for the agonist did not change following varying degrees of receptor inactivation by different concentrations of phenoxybenzamine in the organ bath. In addition, constant pD'2 values were obtained for the antagonistic effect of phenoxybenzamine in these circumstances. Different values for KA and median effective concentration indicated a full agonistic effect for phenylephrine. The presence of spare receptors was revealed by comparing theoretical curves for receptor occupancy and concentration-effect curves for phenylephrine. It is suggested that the isolated guinea pig vas deferens may be a suitable preparation for the study of alpha-adrenergic mechanisms and their regulation.

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.

Release of noradrenaline and dopamine by nerve stimulation in the guinea-pig and rat vas deferens

Spontaneous and nerve stimulated release of noradrenaline and dopamine from rat and guinea-pig vas deferens have been measured electrochemically after separation by high performance liquid chromatography (h.p.l.c.). In the absence of nerve stimulation both noradrenaline (NA) and dopamine were released into the bathing fluid in the rat but in the guinea-pig only noradrenaline could be detected. Drugs which block neuronal and extraneuronal uptake of catecholamines had little effect on spontaneous overflow but both tetraethylammonium and phenoxybenzamine increased overflow. Transmural nerve stimulation (5-10 Hz) increased catecholamine overflow in both species and dopamine release was now measurable from the guinea-pig vas. In the rat, the proportion of dopamine to NA was unchanged from that released spontaneously. The release of both amines was little affected by drugs that block neuronal and extraneuronal uptake and a monoamine oxidase inhibitor, but was inhibited by tetrodotoxin 0.2 microgram ml-1. In the guinea-pig tetraethylammonium 10 mM doubled overflow and phenoxybenzamine 10(-5)M increased it by five times but the dopamine percentage remained constant and equal to the control. Following nerve stimulation the amount of dopamine released expressed as a percentage of total catecholamine release was 6% for the rat and 1.3% for the guinea-pig. These values were considerably higher than the comparable figures for dopamine: NA content of the two tissues (2% and 0.5% respectively). Repeated periods of stimulation depleted these tissue stores and the depletion of dopamine was significantly greater than that of NA. 6 Our interpretation of these results is that both dopamine and NA are released from a common store during normal noradrenergic transmission. While all or most of the axonal dopamine is contained in this releasable pool, most of the axonal NA lies in a second, less readily released pool.

[3H]Xylamine accumulation by two sympathetically innervated tissues

The accumulation of [3H]xylamine [( ring-G-3H]-N-2'-chloroethyl-N-ethyl-2-methylbenzylamine; [3H]XYL) by rabbit thoracic aorta and rat vas deferens was studied in vitro. [3H]XYL uptake in both tissues saturated at the same concentrations and displayed the same time course as that for maximal inhibition of [3H]noradrenaline [( 3H]NA) accumulation by XYL. Hydrolysis of [3H]XYL or coincubation with Na2S2O3 reduced tissue accumulation of tritium by 80%. In aorta and vas deferens, about 45% and 65%, respectively, of the total [3H]XYL accumulation was blocked by 100 mumol/L desmethylimipramine (DMI), l-NA, or bretylium. In the absence of sodium ion, the uptake of [3H]XYL was reduced by approximately these same amounts. In both tissues nearly 70% of the [3H]XYL taken up was protein-bound when measured as trichloroacetic acid-precipitated tritium. Of this radioactivity, the same proportion was sensitive to 100 mumol/L DMI or the absence of sodium ion as found for total tissue accumulation of [3H]XYL. Hydrolysis of [3H]XYL prior to incubation with vas deferens reduced the protein-bound tritium by more than 95%. The studies indicate that [3H]XYL interacts with NA uptake carrier in both rabbit aorta and rat vas deferens and that a substantial portion of the resulting protein-bound radioactivity is carrier-dependent.

High-pressure liquid chromatographic-fluorometric detection of adenosine and adenine nucleotides: application to endogenous content and electrically induced release of adenyl purines in guinea pig vas deferens

To avoid some of the disadvantages associated with using radiolabeling to investigate adenyl purine content and release from excitable tissues, a reverse-phase high-pressure liquid chromatographic method utilizing fluorescence detection for the measurement of picomole amounts of endogenous ATP and its 6-amino purine analogs has been developed. This procedure has been used to determine the content of adenyl purines in the guinea pig vas deferens and that released from the tissue following stimulation of adrenergic nerves. The total tissue content was measured to be 1.6, 0.76, and 0.10 mumol/g of ATP, ADP, and AMP, respectively. However, adenosine could not be detected. Hypoxia caused a significant decrease in ATP content concomitant with an increase in adenosine content to 0.04 mumol/g. Following transmural electrical stimulation of the guniea pig vas deferens, the release of the following purine compounds was detected: ATP (0.106 nmol/g), ADP (0.242 nmol/g), AMP (0.035 nmol/g), and adenosine (0.454 nmol/g).

Pharmacological evidence that adenosine triphosphate and noradrenaline are co-transmitters in the guinea-pig vas deferens

The contractile response of the guinea-pig vas deferens to tetanic nerve stimulation was biphasic. The first phase was mimicked by exogenously applied ATP. The second more tonic phase was mimicked by exogenously applied noradrenaline (NA). Intracellular micro-electrodes were used to record the electrical response of the vas deferens to nerve stimulation and to exogenously applied ATP and NA. Local application of ATP (10(-5) to 10(-3)M), by pressure ejection from a micropipette, produced a depolarization similar in magnitude and time course to the excitatory junction potential (e.j.p.). NA produced no such response. Superfusion of the vas deferens with ATP and NA (10(-6) to 10(-4)M) produced a depolarization. The depolarization produced by NA was more gradual than that produced by the same concentration of ATP. The ATP-receptor antagonist ANAPP3 (arylazido aminopropionyl-ATP) preferentially antagonized the first component of the neurogenic contractile response and also antagonized the e.j.p. The alpha-receptor antagonist prazosin preferentially antagonized the second phase of the neurogenic contractile response and enhanced the e.j.p. Similar results were obtained using the irreversible alpha-receptor antagonists phenoxybenzamine and dibenamine. Cocaine (10(-6) and 10(-5)M) enhanced the second phase of the contractile response to nerve stimulation, but reduced the first phase. Lidocaine (10(-5) and 10(-4)M) had no such effect. Cocaine (10(-6) and 10(-5)M) reduced the magnitude of e.j.p.s. at all stimulation frequencies from 1 to 8 Hz. In the presence of the selective alpha 2-receptor antagonist yohimbine (10(-7)M), both phases of the contractile response to nerve stimulation were enhanced to the same degree. This concentration of yohimbine also increased the magnitude of e.j.p.s. In the presence of 10(-7) M-yohimbine, cocaine (10(-6) and 10(-5)M) still enhanced the second phase of the contractile response, but no longer reduced the initial phase of the contraction or e.j.p.s to the same degree. In vas deferens from animals pre-treated with reserpine (2 mg/kg.day), the second phase of the contractile response to nerve stimulation was reduced but neither the first phase of the contraction nor the e.j.p.s was blocked. These results suggest that the first phase of the neurogenic contractile response of the vas deferens and the e.j.p. are mediated by ATP acting on P2-purinoreceptors, whereas NA mediates phase two, via alpha 1-adrenoceptors. The results also suggest that release of ATP and NA is influenced by a negative feed-back mechanism involving presynaptic alpha 2-adrenoceptors.

Characteristics of neuronal release of ATP

Recent studies have described a transmitter-like release of ATP in brain. Once released, extraneuronal ATP is rapidly metabolized to adenosine by ecto-ATPase and nucleotidase. Adenosine, through actions at specific receptors, inhibits neuronal firing in the brain. ATP shares these inhibitory actions, presumably by forming adenosine extraneuronally. Caffeine and theophylline probably exert CNS stimulation by antagonizing adenosine's inhibitory actions in the brain. Extracellular ATP occasionally excites quiescent neurons in the cortex. A possible role for ATP as a sensory neurotransmitter is suggested by its excitatory actions on a subpopulation of dorsal horn cells. ATP release has also been described from sensory nerves in the periphery, motor nerves, nerves of the myenteric plexus, bladder, vas deferens, and from adrenal chromaffin cells and platelets. The possibility that ATP might function as a transmitter, co-transmitter or modulator in the peripheral nervous system is discussed.

Frequency dependent intermittency and ionic basis of impulse conduction in postganglionic sympathetic fibres of guinea-pig vas deferens

Some characteristic features of the functional innervation of guinea-pig vas deferens have been determined. Both ganglionic transmission from the hypogastric nerves and impulse propagation in proximal regions (main branch bundles within about 15 mm from the prostatic end of the organ) of the majority of single postganglionic sympathetic fibres of vas deferens nerve, had a high safety factor. Failure at these levels cannot account for the intermittent pattern of electrically-evoked secretion of transmitter from the individual varicosity of the terminals of vas deferens nerves, observed under identical experimental conditions. The shape of the extracellular single fibre action potential recorded by small calibre suction electrodes remained constant in proximal regions of vas deferens nerve, when the frequency of stimulation was varied between 0.5 and 8 Hz. Therefore, frequency-dependent facilitation of transmitter secretion in this tissue cannot be explained by frequency-dependent growth in the amplitude of nerve action potentials, as earlier assumed. However, when recordings were made in distal regions of vas deferens nerve (in small axon bundles, close to their points of insertion into the substance of the epididymal end of the organ), on two occasions fibres were found in which the safety factor for impulse conduction was low and frequency-dependent. The possibility is discussed that this feature, which was an exception in these non-terminal regions of vas deferens nerve, may be shared by the majority of fibres as they proceed distally towards the terminals. Clearly, if this is the case, intermittent failure of transmitter secretion from the individual varicosity may be due, at least in part, to intermittent failure of conduction of the nerve impulse to the terminals. Some useful qualitative information on the ionic basis of the extracellular nerve action potential, that might underlie a proximo-distal decline in the safety factor for impulse conduction in these nerves, was obtained by determining the effects on the shape of the signal, caused by varying the ionic composition of the medium (sodium, calcium), and by local addition of agents with known actions on sodium (tetrodotoxin), potassium (tetra-ethyl ammonium, 4-aminopyridine, rubidium, barium) and calcium channels (cobalt, manganese, lanthanum, nickel, D-600). By these criteria, the action potential that was shown to be a "normal" sodium-potassium spike, in proximal regions of vas deferens nerve, was found to have a different "pharmacological profile", in distal regions of the nerve, in a manner suggesting that here nerve impulse conduction had become somehow "calcium-dependent".(ABSTRACT TRUNCATED AT 400 WORDS)

Tetrodotoxin-sensitive and -resistant effects of veratridine on the noradrenergic neurone of the rat vas deferens

The veratridine-induced release of 3H-noradrenaline from noradrenergic neurones was examined in the isolated vas deferens of either untreated or reserpine plus pargyline-pretreated rats. The rat vas deferens, whose catechol O-methyltransferase was inhibited, was first incubated with 0.4 mumol/l 3H-(-)noradrenaline (30 min) and then washed repeatedly with amine-free solution. After 120 min (i.e., well after the efflux of tritium from the tissue had reached a steady level and was predominantly of neuronal origin), washout was continued in the presence of veratridine for further 10-15 min. In vasa deferentia of untreated rats, veratridine (1-100 mumol/l) caused a concentration-dependent increase in the efflux of tritium. At high concentrations of the drug (30 or 100 mumol/l), this increase in efflux was peak-like during the first 3 min ("peak response") and then fell to a plateau ("plateau response"). In the presence of veratridine, unchanged 3H-noradrenaline accounted for about 75% of the tritium efflux (the rest being represented by deaminated 3H-catechol metabolites). The "peak response" to veratridine (100 mumol/l) was abolished by tetrodotoxin (TTX; 1 mumol/l) or the absence of external Ca2+. Cocaine (10 mumol/l) affected neither the "peak response" as such nor the contribution by 3H-noradrenaline to the efflux of tritium during that response. Hence, the "peak response" was due to exocytotic release of 3H-noradrenaline from the neurone. The "plateau response" to veratridine (100 mumol/l) was unaffected by the absence of external Ca2+, largely resistant to TTX (1 mumol/l) and moderately reduced by cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by sulfur-containing amino acids and GABA of sympathetic neurotransmission in guinea-pig vas deferens

Electrical stimulation produced a contraction in the isolated guinea-pig vas deferens. This response was blocked by tetrodotoxin, guanethidine and bretylium but not by atropine. The magnitude of the contractile response to electrical stimulation depended on the concentration of the external calcium. Sulfur-containing amino acids and GABA inhibited the electrically induced contraction but not that caused by noradrenaline and ATP. The order of potency for inhibition of the contraction at a concentration of 10(-4) M being GABA greater than or equal to cysteine greater than methionine greater than cysteic acid greater than taurine. The contractile response to electrical stimulation was also inhibited by EGTA, this inhibition being similar to that by cysteic acid and taurine but weaker than that by methionine, cysteine and GABA at a concentration of 10(-4) M. The inhibitory action of sulfur-containing amino acids and GABA was abolished by increasing the calcium concentration in the medium. The results suggest that sulfur-containing amino acids and GABA reduce transmitter release from the sympathetic nerve terminals by inhibiting calcium availability for the transmitter secretion process.