Effect of moderate cooling on contractile responses in mouse vas deferens and its relation to calcium

Isolated mouse vas deferens preparations were used to study the effect of temperature on noradrenaline-induced contractions. Preparations were suspended in the organ bath containing Krebs-Henseleit solution for isometric tension recording. Contractile responses to noradrenaline were investigated in the mouse vas deferens after moderate cooling from 37 to 26 or 22° C. A significant increase of the phasic contractions to noradrenaline was observed at 26 or 22°C compared with responses obtained at 37° C (about 12.3 and 35.6% increase at 26 and 22° C, respectively). The secondary noradrenaline-induced sustained contraction was also significantly enhanced after moderate cooling to 26° C. The potentiation of noradrenaline-induced contraction at 26° C remained in a Ca²⁺-free EGTA (1 mM)-containing solution. However, sustained contraction was suppressed after removal of the calcium from the medium at 37 and 26°C. Contraction to caffeine was significantly enhanced at 22° C compared with 37°C. By contrast, barium chloride-induced contraction of the vas deferens was markedly decreased after moderate cooling to 22° C. In the presence of ouabain (0.1 mM), the noradrenaline-induced peak contraction was significantly increased at 37°C. However, potentiation of the noradrenaline response at 22° C was unaffected by the Na⁺/K⁺ pump inhibitor. Noradrenaline-induced peak contractions were depressed in the presence of vanadate (1 mM) and cyclopiazonic acid (10 M), two Ca²⁺-ATPase inhibitors, at 37° C and also at 22° C. These results suggest that temperature-induced hyperreactivity is partly due to an increase of the amount of calcium released from intracellular stores. The inhibition of the Na⁺/K⁺ pump due to cooling may participate in this effect whereas Ca²⁺-ATPase inhibition does not seem to be involved.     

The effect of extracellular sodium ion concentration on the action of opiates to inhibit potassium-evoked released of [H]noradrenaline from the mouse vas deferens

Opiates depress the potassium-induced efflux of [³H]noradrenaline from the mouse vas deferens in a concentration-dependent (the IC₅₀ for normorphine was 1.5 μM), stereospecific and naloxone-reversible manner. As the concentration of sodium in the extracellular fluid was reduced, the inhibitory action of opiates was also reduced. This attenuation of opiate action is the converse of that predicted by the ‘sodium-shift’ observed in opiate binding studies in which lowering the sodium concentration potentiates opiate agonist binding. The relevance of sodium to the pharmacological actions of opiates is discussed.     

P2-purinoceptor-mediated autoinhibition of sympathetic transmitter release in mouse and rat vas deferens

Effects of drugs acting at P₂-purinoceptors on the release of newly taken up [³H]-noradrenaline were studied in slices of mouse and rat vas deferens. The slices were superfused and stimulated electrically, in most experiments by trains of 60 pulses/8 Hz.In mouse vas deferens, the P₂-purinoceptor antagonists reactive blue 2 (1.8–100 M) and brilliant blue G (10–300 M) increased the stimulation-evoked overflow of tritium in a concentration-dependent manner as shown previously for suramin. Reactive blue 2, which preferentially blocks the P_2Y-subtype, was the most potent compound and the compound with highest maximal effect, an increase by 104%. Pyridoxalphosphate-6-azophenyl-2,4-disulfonic acid (PPADS), in contrast, caused a small increase only at a single concentration (30 M). The effects of reactive blue 2, brilliant blue G and suramin were not additive. The P₂ agonist adenosine 5-O-(3-thio)-triphosphate (ATPS) reduced the evoked overflow of tritium. As shown previously for suramin, reactive blue 2 30 M and brilliant blue G 100 M antagonized the effect of ATPS. From the shift of the ATPS concentration-response curve to the right, an apparent pK_B value of 5.3 was estimated for reactive blue 2 and an apparent pKB of 4.5 for brilliant blue G. In rat vas deferens, reactive blue 2 (3–30 M), brilliant blue G (10 M) and suramin (30–300 M) also increased the evoked overflow of tritium. As in the mouse, reactive blue 2 was the most potent compound and the compound with highest maximal effect, an increase by 9001o. As previously demonstrated in the mouse, suramin (300 M) increased the evoked overflow of tritium only when rat vas deferens slices were stimulated by trains of 60 pulses at 1 or 8 Hz, but not when they were stimulated by trains of 6 pulses/100 Hz.The results confirm the operation of a P₂-purinoceptor-mediated prejunctional negative feedback controlling the release of noradrenaline in mouse vas deferens and demonstrate the same mechanism in rat vas deferens. The prejunctional P₂-purinoceptors are P_2Y-like in both species. They are a novel kind of autoreceptors, operating in parallel to prejunctional ₂-autoreceptors. Correspondence to: I. von Kügelgen at the above address     

Effects of carbenoxolone on syncytial electrical properties and junction potentials of guinea-pig vas deferens

The effects of the putative gap junction blocker carbenoxolone on smooth muscle syncytial properties and junction potentials were studied in guinea pig vas deferens (GPVD). Treatment with 50 μM carbenoxolone reversibly and significantly increased input resistance (R _in) (by 682.5 ± 326.0 %, P < 0.05) and abolished cable potentials within 6–7 mins of incubation, without disturbing resting membrane potential. Carbenoxolone reversibly and significantly increased the amplitude of spontaneous excitatory junction potentials (sEJPs) by 96.9 ± 35.45% (P < 0.05), shifted their amplitude distribution rightwards, and reduced their frequency of occurrence by 58.17 ± 17.7% (P < 0.05), without altering their time courses. Similarly, carbenoxolone increased the amplitude of evoked excitatory junction potentials (eEJPs) by 17.7 ± 5.88% and τ _decay by 19.43 ± 8.29% (P < 0.05). Our results indicate that carbenoxolone alters the electrical properties and junctional potentials of the GPVD by a mechanism consistent with a relatively specific block of gap junctions. These results suggest that gap junction mediated cell-to-cell communication may significantly modulate the electrical properties and junctional potentials of the GPVD and consequently the physiological functioning of this tissue.     

Opiate receptor binding studies in the mouse vas deferens exhibiting tolerance without dependence

Summary The apparent lack of dependence in highly opiatetolerant isolated mouse vas deferens may conflict with unitary theories postulating a common biochemical mechanism for both phenomena. Therefore, attention focused on binding sites of the opiate receptors which in this tissue are located presynaptically. Binding studies conducted with homogenate of highly tolerant vasa deferentia revealed no significant different results as compared to naive tissues. Further studies examined the effect of guanine nucleotide on opiate receptor interaction. Apparently, -, - and -opiate receptors in the mouse vas deferens proved resistant to the regulatory action of guanine nucleotide in naive and tolerant tissues. From these experiments, it is concluded that the binding characteristics of opiate receptors in the mouse vas deferens do not change with chronic activation. In addition, the lack of an effect of guanine nucleotide on opiate binding leads to the suggestion that binding sites are not coupled to adenylate cyclase in this tissue. Taken together, these findings draw the attention to the coupling mechanism of opiate receptors in the mouse vas deferens which may play a key role in the adaptational mechanisms following chronic opiate exposure.     

Effect of normorphine and enkephalin on spontaneous potentials in the vas deferens.

Spontaneous excitatory junction potentials were recorded intracellularly from the smooth muscle of the mouse vas deferens. Exposure of the vas deferens to either normorphine (1 muM) or methionine-enkephalin (0.5 muM) did not alter the frequency or amplitude distribution of the spontaneous potentials. The possible mechanisms by which the narcotic agonists depress the release of noradrenaline on nerve stimulation without inhibiting the spontaneous release of noradrenaline are discussed.     

The effects of chronic morphine administration to mice on the contractile activity in vitro of the vas deferens without and with morphine

Chronic treatment of mice with morphine enhances maximal responses to noradrenaline in isolated vasa deferentia. Addition of morphine to the bath further increases the responses to noradrenaline (facilitatory effect). Vasa deferentia from reserpine-pretreated mice showed increased responses to noradrenaline, but upon addition of morphine to the bath no further facilitatory response to noradrenaline. The facilitatory response was not inhibited by naloxone and it was not elicited by narcotic analgesics other than morphine. High calcium in the medium (3.8 mM) suppressed the enhanced maximal responses to noradrenaline of vasa deferentia from mice chronically treated with morphine and also suppressed the facilitatory response to noradrenaline. Low calcium in the medium (1.43 mM) had the opposite effects. Reserpine-induced changes of noradrenaline responses were not affected by alteration of the calcium concentration in the medium. In the K-depolarized vas deferens, a reduction of the depressant effect of morphine on the calcium-induced contractions was observed after chronic administration of morphine.     

Role of Ca-activated K channels and Na,K-ATPase in prostaglandin E1- and E2-induced inhibition of the adrenergic response in human vas deferens

We studied the role of K⁺ channels and Na⁺,K⁺-ATPase in the presynaptic inhibitory effects of prostaglandin E₁ (PGE₁) and PGE₂ on the adrenergic responses of human vas deferens. Furthermore, we determined the effects of increasing extracellular K⁺ concentrations ([K⁺]_o) and inhibition of Na⁺,K⁺-ATPase on neurogenic and norepinephrine-induced contractile responses. Ring segments of the epididymal part of the vas deferens were taken from 45 elective vasectomies and mounted in organ baths for isometric recording of tension. The neuromodulatory effects of PGEs were tested in the presence of K⁺ channel blockers. PGE₁ and PGE₂ (10⁻⁸ to 10⁻⁶ M) induced inhibition of adrenergic contractions. The presence of tetraethylammonium (10⁻³ M), charybdotoxin (10⁻⁷ M), or iberiotoxin (10⁻⁷ M), prevented the inhibitory effects of PGE₁ and PGE₂ on the adrenergic contraction. Both glibenclamide (10⁻⁵ M) and apamin (10⁻⁶ M) failed to antagonize PGE₁ and PGE₂ effects. Raising the [K⁺]_o from 15.8 mM to 25.8 mM caused inhibition of the neurogenic contractions. Ouabain at a concentration insufficient to alter the resting tension (10⁻⁶ M) increased contractions induced by electrical stimulation but did not alter the contractions to norepinephrine. The inhibition of neurogenic responses induced PGE₁, PGE₂ and increased extracellular concentration of K⁺ was almost completely prevented by ouabain (10⁻⁶ M). The results demonstrate that PGE₁ and PGE₂ inhibit adrenergic responses by a prejunctional mechanism that involves the activation of large-conductance Ca²⁺-activated K⁺ channels and Na⁺,K⁺-ATPase.     

Changes in electrophysiological properties and noradrenaline response in vas deferens of diabetic rats

The purpose of this study was to study changes in the sympathetic nerves of the vas deferens in 10-week-old streptozotocin-induced diabetic rats. To assess the activity of autonomic neurons, we recorded the amplitude and frequency of spontaneous junction potentials in vasa deferentia from age-matched controls and streptozotocin-induced diabetic rats. No change in the resting membrane potential of the smooth muscle of the vas deferens was found in streptozotocin-induced diabetic rats. The frequency of spontaneous junction potentials was significantly increased in the streptozotocin-induced diabetic rats and their amplitude was also markedly increased. The dose–response curve for the contractile response of the vas deferens to noradrenaline was significantly shifted to the right and the apparent affinity (pD₂ value) was significantly decreased in streptozotocin-induced diabetic rats. These results suggest that degeneration of sympathetic neurons may occur in the vas deferens of 10-week streptozotocin-induced diabetic rats and that the greater amplitude of the spontaneous junction potentials may be related to an increase in Ca²⁺ mobilization, though the increase in Ca²⁺ mobilization does not lead to an enhanced contractile response.     

Importance of frequency and pulse width in field stimulation of the mouse vas deferens: different behaviour of twitch-inhibiting drugs

Summary The effect of six twitch-inhibiting drugs on the stimulus-response relationship in the field-stimulated mouse vas deferens was compared by means of stimulus response curves which were obtained in two ways, that is, variation of frequency at constant pulse width and variation of pulse width at constant frequency. The wtich-inhibiting potency (in the range of maximal twitch responses) differed with the type of stimulation in a way permitting two groups of substances to be defined: group A (tetrodotoxin, procaine, verapamil) was more effective on frequency-response curves and group B (FK 33-824, clonidine, nifedipine) on pulse-width response curves; the latter applied also to subnormal calcium concentrations in the bath solution. The results suggest that the effect of inhibitory drugs on the fieldstimulated vas of the mouse varies greatly with the type of field stimulation.     

Replacement of K+ with Rb+ or Cs+, and its effects on the mechanical responses to norepinephrine and methacholine in the rat vas deferens

The rat vas deferens was stored overnight in cold, K⁺-free Krebs solution to deplete intracellular K⁺ then incubated in K⁺-, Rb⁺- or Cs⁺-containing Krebs solution at 37°C to load these ions inside the cells. After 4 h, the contents of K⁺ or Rb⁺ reached the level of K⁺ in the fresh vas deferens; the content of Cs⁺ was less than half that of the fresh vas deferens. Dose-response curves to norepinephrine and methacholine were determined under these conditions, and the curves in Rb⁺ or Cs⁺ solution were compared with those in K⁺ solution. The cold storage per se had little effect on the dose-response curves in K⁺ solution except that it slightly decreased the maximal response to norepinephrine. The dose-response curves in Rb⁺ solution were to the left of those in K⁺ solution. The maximal response to methacholine was greatly increased. On the other hand, the dose-response curves in Cs⁺ solution were to the right of those in K⁺ solution. The maximal responses were greatly decreased with both drugs. The results suggest that Rb⁺ but not Cs⁺ can fully substitute for K⁺ in the rat vas deferens response to norepinephrine and methacholine.     

Inhibition by the putative potassium channel opener pinacidil of the electrically-evoked release of endogenous dopamine and noradrenaline in the rat vas deferens

Summary The effect of pinacidil on the release of endogenous noradrenaline and dopamine from the sympathetic innervation of the rat vas deferens was examined. Amine release was evoked by electrical stimulation (1, 2, 5 and 10 Hz) or by depolarization with high potassium (75 mmol/l) in the medium. Dopamine and noradrenaline were measured by means of high pressure liquid chromatography with electrochemical detection.Pinacidil (1, 5, 10 and 50 mol/l) produced a concentration-dependent inhibition of the electrically stimulated (2 Hz) overflow of noradrenaline and dopamine. Only pinacidil 50 mol/l increased the spontaneous loss of dopamine and noradrenaline. The inhibitory effects of pinacidil (5 mol/l) on amine overflow were also observed at other frequencies of stimulation (1, 5 and 10 Hz). The magnitude of the inhibitory effect on noradrenaline release was approximately the same at all frequencies (63% to 56% reduction); for dopamine, the higher the frequency of stimulation, the greater the inhibitory effect of pinacidil (up to 73% reduction). When the preparations were continuously stimulated for 70 min at 2 Hz, pinacidil (5 mol/l) reduced the overflow of dopamine and noradrenaline during the first 40 or 30 min of stimulation only. The addition of phentolamine (1 mol/l) to the perifusion medium slightly reduced the inhibitory effect of pinacidil on amine overflow, but the inhibition by pinacidil remained statistically significant. Tetraethylammonium (10 mmol/l) completely abolished the inhibitory effect of pinacidil (10 mol/l). Pinacidil (5 mol/l) did not reduce the potassium-evoked release of the amines.The results demonstrate that pinacidil impairs transmitter release from the sympathetic innervation of the rat vas deferens, probably as a consequence of the opening of potassium channels. Send offprint request to P. Soares-da-Silva at the above adress     

Failure of tyramine to release neuronal ATP as a cotransmitter of noradrenaline in the guinea-pig vas deferens

Contractions, release of noradrenaline and r elease of ATP elicited by the indirectly acting sympathomimetic amine tyramine and responses elicited by exogenous noradrenaline were studied in the isolated vas deferens of the guinea pig. Release of noradrenaline was assessed as overflow of tritium after preincubation with [³H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique.In tissues pretreated with pargyline 1 mM, tyramine 300 M, when added to the superfusion medium for 2 min, elicited contraction and an overflow of tritium (mainly [³H]-noradrenaline) and ATP. Contraction and ATP overflow responses were prevented and tritium overflow was greatly reduced by desipramine 10 M Prazosin 0.3 M abolished contractions and evoked ATP overflow without changing tritium overflow. Blockade of postjunctional P₂-purinoceptors by suramin 300 M caused a marked decrease of tyramine-evoked contractions and a slight reduction of tritium overflow whereas evoked ATP overflow was markedly increased. The effect on contraction was not shared by two other P₂-purinoceptor antagonists, namely pyridoxalphosphate-6-azophenyl-2,4-disulfonic acid (PPADS) 32 M and diisothiocyanatostilbene-2,2-disulfonic acid (DIDS) 32 M: PPADS increased contractions about fourfold, whilst DIDS had no effect at all. When the vas deferens was superfused for 24 min with medium containing tyramine 300 M, evoked contractions and tritium overflow continued throughout whereas ATP overflow faded rapidly to basal values. In the presence of prazosin 0.3 M, tyramine 300 M again failed to elicit contractions as well as an overflow of ATP. Application of noradrenaline 10 M instead of tyramine also resulted in prolonged contraction and an overflow of ATP that declined rapidly.It is concluded that all ATP released by tyramine is non-neuronal in origin, secondary to the activation of postjunctional ₁-adrenoceptors by released noradrenaline. The non-neural ATP does not seem to play a functional role in smooth muscle contraction and derives from a postjunctional source which is subject to a rapid depletion upon sustained ₁-adrenoceptor activation.     

Chronic inhibition of monoamine oxidase reduces noradrenaline release in rat vas deferens and anococcygeus muscle

Summary Rats were treated once (acute) or once daily for 21 days (chronic) with clorgyline (2 mg/kg) or nialamide (50 mg/kg). (–)Deprenyl (1 mg/kg) was given for 21 days. One day after the last injection, vas deferens and anococcygeus muscles were removed and noradrenaline stores labelled with ³H-noradrenaline. Efflux of total tritium following electrical field stimulation was decreased by both acute and chronic treatment with clorgyline and nialamide, as well as by chronic treatment with deprenyl. Total tritium release from anococcygeus muscle was reduced by both acute and chronic treatment with clorgyline. Fractionation of the effluent showed that release of both free noradrenaline and metabolites was decreased by MAO inhibitor treatment in vivo, but this effect was not reproduced by in vitro incubation of the vas deferens with clorgyline (1 M). By contrast to the effect of electrical field stimulation, release of 3Hnoradrenaline induced by veratrine was increased by chronic treatment with both clorgyline and nialamide. Release of total tritium by depolarising concentrations of KCl was also increased by chronic clorgyline treatment. These results could be explained by a proportionally greater release of tritium from a cytoplasmic compartment following veratrine and KCl than electrical stimulation, since MAO inhibition increases cytoplasmic noradrenaline levels. Alternatively, release by electrical stimulation may be affected to a greater extent by presynaptic ₂-adrenoceptors, and presynaptic receptors may be stimulated by increased synaptic levels of free noradrenaline following MAO inhibition. Send offprint requests to J. P. M. Finberg at the above address     

Inhibitory effect of papaverine on the contraction induced by transmural stimulation in the isolated mouse vas deferens

The effect of papaverine on the contractions induced by adrenergic neurotransmission in the isolated mouse vas deferens was investigated. Papaverine, 10⁻⁷–10⁻⁵M, showed a dose-dependent and reversible inhibition on the induced contractions. When the frequency of stimulation was varied (2.5–20.0 Hz), the inhibitory effect tended to be marked at the lower frequencies.     

Comparison of corelease of noradrenaline and ATP evoked by hypogastric nerve stimulation and field stimulation in guinea-pig vas deferens

Contractions and overflow of tritium and ATP elicited by hypogastric nerve stimulation (HNS) and field stimulation (FS) were studied in the guinea-pig isolated vas deferens preincubated with [³H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique.HNS and FS elicited contraction, tritium overflow and ATP overflow. HNS at supramaximal current strength produced smaller responses than did FS at supramaximal current strength (210 pulses/7 Hz). Supramaximal HNS and submaximal FS were used in the remainder of the study. Prazosin (0.3 mol/l) reduced contractions and the overflow of ATP elicited by both HNS and FS; the evoked overflow of tritium was not changed (210 pulses/7 Hz). Combined administration of prazosin (0.3 mol/l) and suramin (300 mol/l) abolished contractions and reduced the overflow of ATP elicited by both HNS and FS slightly more than did prazosin alone; tritium overflow again was not changed (210 pulses/7 Hz). Contractions, tritium overflow and ATP overflow increased with the frequency of both HNS and FS (from 7 to 25 Hz; 210 pulses); the increase in ATP overflow with frequency was more marked than the increase in tritium overflow. The preferential increase of ATP overflow with the frequency of HNS and FS persisted in the combined presence of prazosin (0.3 mol/l) and suramin (300 mol/l).The study confirms for HNS, a more physiologic way of sympathetic nerve stimulation, several observations previously obtained with FS. First, HNS-evoked ATP release is detectable as an overflow of ATP into the superfusion fluid. Second, a large part of the HNS-evoked release of ATP is postjunctional in origin, due to activation of post-junctional ₁-adrenoceptors and presumably P₂-purinoceptors. Third, the average neural release of ATP per pulse facilitates with the frequency of stimulation to a greater extent than the average release of noradrenaline per pulse.     

Effects of calcitonin gene-related peptide (CGRP) on Ca2+-channel current of isolated smooth muscle cells from rat vas deferens

Summary Effects of calcitonin gene-related peptide (CGRP), a putative non-adrenergic non-cholinergic neutrotransmitter on the electrical properties of the cell membrane, were investigated in enzymically dispersed smooth muscle cells from rat vas deferens. Under current clamp conditions, CGRP (up to 10^–7 M) did not induce significant changes in membrane potentials or input resistance in the resting state. The configurations of action potentials elicited by depolarizing current pulses were also unaffected, except that a prolongation of the duration of the action potentials by a high dose (10^–7 M) of CGRP was observed in some of the cells. Under whole cell voltage clamp conditions, the transient and sustained K⁺ currents, activated by depolarizing voltage-steps, were apparently decreased in the presence of 10^–9 to 10^–7 M CGRP. The peptide increased the voltage-gated Ca²⁺ current in cells loaded with 145 mM Cs⁺ solution in order to block the K+ currents. The voltage-dependency of the peak Ca²⁺ current was not changed by CGRP. Ba²⁺ (10.8 mM) was used as a charge carrier for the Ca²⁺-channel current to clarify further the effects of CGRP on the properties of the current. CGRP (10^–8 M) delayed the inactivation time course of the Ca²⁺-channel current and slowed the recovery from inactivation. The peptide did not affect the steady-state inactivation measured by changing the holding potential. The Ca²⁺-channel current in the presence of CGRP was suppressed by nicardipine (10^–6 M) to the same extent as the current under control conditions. The results suggest that CGRP modifies the L-type Ca²⁺ channel in smooth muscle cells. Correspondence to N. Matsuki at the above address     

Actions of kurtoxin on tetrodotoxin-sensitive voltage-gated Na<Superscript>+</Superscript> currents,Na<Subscript>V</Subscript>1.6, in murine vas deferens myocytes

Kurtoxin is described as a selective inhibitor of Ca_V3.1. Using patch-clamp techniques, the modulatory effects of kurtoxin on tetrodotoxin-sensitive voltage-gated Na⁺ currents (I _Na) recorded from mouse vas deferens myocytes were investigated. Kurtoxin increased the peak amplitude of I _Na between −40 and −30 mV, whilst inhibited the peak amplitude at more positive potentials than −10 mV, thereby demonstrating a dual action on the peak amplitude of I _Na. The time to reach the peak amplitude of I _Na became significantly longer in the presence of kurtoxin in comparison with that of the controls. Kurtoxin also slowed the deactivation of I _Na at more positive membrane potentials than −30 mV. Kurtoxin enhanced the total amount of electrical charge of I _Na in a concentration-dependent manner. These results suggest that kurtoxin is a modulator of Na_V1.6 in native freshly dispersed smooth muscle cells from mouse vas deferens as well as its action on Ca_V3.1.     

Pre- and postsynaptic effects of p-tyramine and p-octopamine in the prostatic portion of the rat vas deferens

Summary The effects of p-tyramine and p-octopamine on the twitch responses of the prostatic portion of the rat vas deferens to electrical stimulation (0.025 Hz) were compared with the effects of noradrenaline. In tissues with normal monoamine oxidase (MAO) activity, the three amines increased the height and duration of the twitch contractions. When MAO activity was inhibited by pargyline (10 mol/l), p-tyramine and p-octopamine had mixed excitatory-inhibitory effects on the twitches, while noradrenaline had mostly excitatory effects along the whole range of concentrations assayed (0.158–15.8 mol/l). Selective blockade of ₁- and ₂-adrenoceptors, by corynanthine and yohimbine, respectively, showed that the excitatory effect of the amines depended on the activation of ₁-adrenoceptor and that the inhibitory action was related to the activation of ₂-adrenoceptors. Pretreatment with reserpine (5 mg/kg, 24 h; 2.5 mg/kg, 2 h before the experiment) largely prevented the effects of p-tyramine and p-octopamine, but the amines still modified the twitch responses to field stimulation. The addition of corynanthine and yohimbine to the bathing fluid revealed a considerable activation of ₁-excitatory and ₂-inhibitory adrenoceptors. Cocaine (10 mol/l) did not antagonize, but rather enhanced the inhibitory effects of p-tyramine and p-octopamine in tissues with normal contents of noradrenaline. Moreover, cocaine did not antagonize the inhibition caused by p-tyramine, and enhanced the inhibition induced by p-octopamine in the prostatic portion of the vasa deferentia from reserpine-pretreated animals. These results suggest that in this tissue, at least when MAO activity is inhibited, p-tyramine and p-octopamine behave similarly. The effects of both amines on the twitch contractions depend on the noradrenaline-releasing action of the compounds and, in addition, the compounds seem to activate adrenoceptors directly. Send offprint requests to S. M. Celuch at the above addressCareer Investigator on leave of absence from the Consejo de Investigaciones Científicas y Técnicas, ININFA, Junín 956, 5°P, RA-1113 Buenos Aires, Argentina