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     

Studies on the inhibitory action of somatostatin in the electrically stimulated rat vas deferens.

Somatostatin (SS) inhibits in a dose-dependent manner electrically evoked contractions in the rat vas deferens. This action was not modified by yohimbine, naloxone or a mixture of antagonists containing atropine, phentolamine, methysergide, burimamide, propranolol and indomethacin, but was markedly potentiated by reducing the Ca2+ concentration of the medium from 2.5 to 1.25 mM and greatly inhibited when increasing the Ca2+ concentration of the medium from 2.5 to 5.0 mM. Clonidine (CLO), but not beta-endorphin (ENDO) was affected similarly to SS by changing the Ca2+ concentration of the medium. The contractile effect of norepinephrine in unstimulated rat vas deferens was not altered by SS. These results were taken as an indication that SS produces its inhibitory action in the rat vas deferens by interacting with specific SS and its receptors presumably located in the cell membranes of adrenergic nerve terminals. The interaction between SS and its receptors may provoke a decreased diffusion of Ca2+ ions into the nerve terminals and/or a decreased mobilisation of Ca2+ ions from intraneuronal stores thus leading to a reduction in electrically evoked release of norepinephrine.     

Prazosin selectively inhibits the responses to field stimulation in the rat vas deferens

The effects of prazosin on contractile responses of the epididymal and prostatic halves of the rat vas deferens have been studied. In the epididymal half responses to acetylcholine were not altered by hexamethonium, 100 microM, but were inhibited by atropine, 10 nM. Prazosin, 100 nM, had no effect on maximal or submaximal responses to acetylcholine or methacholine. In the presence of cocaine, 10 microM, prazosin 10 and 100 nM, reduced the magnitude of the maximal twitch and sustained responses to field stimulation in the epididymal and prostatic halves of the vas deferens. Submaximal responses at all frequencies of stimulation and to exogenously applied noradrenaline were also inhibited by prazosin. Thus all responses to field stimulation of the epididymal and prostatic halves of the vas deferens are susceptible to selective alpha 1-adrenoceptor blockade with low concentrations of prazosin.     

Noradrenergic supersensitivity in the rat vas deferens during barbital withdrawal

Three days' withdrawal from long-term barbital administration results in a shift to the left and increase in the maximum response of the concentration--effect curve to norepinephrine in the isolated rat vas deferens. The maximum response to carbachol and potassium was also increased without a leftward displacement of the concentration--effect curves. This specific increase in sensitivity to norpinephrine could indicate an adaptive alteration of the adrenoreceptors of the rat vas deferens (receptor supersentivity) as has been proposed for supersensitivity in the central nervous system during withdrawal from neuroleptics and barbiturates.     

Abolition of neurally evoked motor responses of the vas deferens by 6-hydroxydopamine

Contractions of isolated guinea-pig or rat vasa deferentia evoked by stimulation through ring electrodes were abolished after incubation with 6-hydroxydopamine?HBr. After this treatment it was no longer possible to demonstrate the presence of fluorescent nerves by the Falk-Hillarp technique and most preparations became supersensitive to noradrenaline. It is concluded that the motor innervation of rat and guinea-pig vasa deferentia is noradrenergic.     

Veratridine-induced outward transport of 3H-noradrenaline from adrenergic nerves of the rat vas deferens

Summary 1. The neuronal release by 100 mol/l veratridine of preloaded ³H-noradrenaline was studied in the rat vas deferens, the MAO, COMT and vesicular uptake of which were inhibited. To prevent any exocytotic release of the ³-Hamine, all solutions were calcium-free. Veratridine induced an early and a late peak of tritium efflux. The early peak was abolished by the presence of 1 mol/l desipramine, the late peak was abolished by 1 mol/l tetrodotoxin (administered subsequently to the first peak). The administration of veratridine plus 1 mmol/l ouabain resulted in only the early peak of efflux. 2. The peak response to veratridine plus ouabain was increased by a very early administration of veratridine plus ouabain (after 40 min of wash-out instead of the usual 130 min) (i. e., when the relative size of the axoplasmic distribution compartment was increased). However, very high axoplasmic ³H-noradrenaline levels (after loading with 37 instead of the usual 0.2 mol/l) reduced the height of the peak (when expressed as a FRL). 3. Substantially similar responses to vcratridine plus ouabain were obtained after loading with ³H-noradrenaline, ³H-adrenaline or ³H-dopamine. 4. As the second peak of veratridine-induced release is ouabain-sensitive, it appears to be caused by exhaustion of neuronal ATP stores; this, in turn, raises the intravesicular pH and induces efflux of ³H-noradrenaline from the vesicles into the axoplasm. The first peak, on the other hand, represents outward transport of ³H-noradrenaline from the axoplasmic compartment. Evidently, a pronounced vesicular distribution of ³H-noradrenaline takes place even after inhibition by reserpine of the vesicular uptake. 5. In preparations with intact vesicular uptake (MAO and COMT inhibited) a plateauresponse was obtained; in the presence of 10 mol/l Ro 4-2184 (a reserpine-like compound) a peak response was restored after loading with 0.2 mol/l³H-noradrenaline, less so after loading with 37 mol/l. 6. It is confirmed that veratridine (plus ouabain) exerts a reserpine-like effect when applied to tissues with intact vesicular uptake and intact MAO.Abbreviations COMT catechol-O-methyl transferase - DOMA dihydroxy mandelic acid - DOPEG dihydroxyphenylglycol - DOPAC dihydroxyphenylacetic acid - FRL fractional rate of loss - MAO monoamine oxidase - 5-HT 5-hydroxytryptamine with technical assistance of Marianne BablSupported by the Deutsche Forschungsgemeinschaft (Bo 521 and SFB 176) Send offprint requests to: H. Bönisch     

Release of neuropeptide Y (NPY) induced by tyramine in the isolated vas deferens of rat

Summary The effect of tyramine on the isolated vas deferens of rats was investigated. Tyramine induced a dose-dependent contraction which was blocked by phentolamine and disappeared in adrenergic denervated tissues. In the presence of an antiserum to neuropeptide Y (NPY), the contraction induced by concentrations of tyramine greater than 10 M was markedly increased. In addition to inducing the release of ³H-norepinephrine (NE), tyramine evoked a concentration-dependent efflux of NPY-like immunoreactivity (NPYLI) from synaptosomal preparations. This action was not modified either by the removal of calcium ion from the medium or by the pretreatment with tetrodotoxin (0.5 M). Desipramine suppressed the NPY-LI release induced by tyramine apparently by the inhibition of the uptake of tyramine is suggested by the significant positive correlation between the reduction of ⁴C-tyramine uptake and the inhibition of NPY-LI release induced by desipramine (r = 0.946). Therefore, we suggest that tyramine does induce the release of NPY from rat vas deferens, in addition to effecting NE secretion. Send offprint request to J. T. Cheng at the above address     

Effect of full agonists following calcium deprivation in rat vas deferens

The contractile effects of maximum doses of adrenaline, noradrenaline, methacholine, acetylcholine, serotonin and barium chloride were studied following substitution of a medium without calcium for the normal nutrient solution. Except for the last agonist, the effects fall to about 10% within the first 3 min with prompt return to normal value upon reintroduction of regular fluid. This recovery is, however, slower if the previous incubation in Ca-free solution is prolonged. When barium choride is used in a calcium-free medium, the maximum height of contractions falls exponentially at a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of about 180 min. This decay can be accelerated by giving successive 5-min doses of the agonist or by using EDTA. It is hypothesised that excitation—contraction coupling in rat vas deferens depends on at least two different calcium sources: a deep site associated with the effects of barium, and a superficial one, related to the other agonists. To explain the slow recovery after prolonged calcium lack, a third compartment in series with the latter is suggested. No indication is found that the biphasic effects of barium depend on two different calcium pools.     

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     

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.     

Prejunctional inhibitory alpha-adrenoceptors and dopaminoceptors of the rat vas deferens and the guinea-pig ileum in vitro.

The effects of α-adrenoceptor and dopaminoceptor agonists and antagonists were investigated on prejunctional receptors of the rat vas deferens and the guinea-pig ileum. The order of potency of the agonists for twitch inhibition of the rat vas deferens was clonidine > oxymetazoline > dopamine > apomorphine > noradrenaline whilst the order of potency for inhibiting the stimulated guinea-pig ileum was clonidine > oxymetazoline > noradrenaline > dopamine > apomorphine. Yohimbine readily blocked the inhibitory effects of clonidine, oxymetazoline and noradrenaline in both tissues but was less effective against dopamine and apomorphine. Pimozide selectively blocked the effects of dopamine and apomorphine on the rat vas deferens and was almost completely ineffective against clonidine, oxymetazoline and noradrenaline. However, pimozide significantly antagonized the noradrenaline-induced twitch inhibition of the stimulated guinea-pig ileum in addition to antagonising dopamine and apomorphine action. The pA₂ values for pimozide against dopamine, apomorphine and noradrenaline in both tissues were significantly different. It is concluded that the prejunctional α-adrenoceptors of the rat vas deferens are the same as those located on the terminal cholinergic neurones of the guinea-pig ileum whilst the prejunctional dopaminoceptors in these tissues appear to differ from one another.     

Autoradiographic study of the rat vas deferens incubated with 3H-noradrenaline

Summary Rat vasa deferentia were incubated with 0.2 mol/l ³H-noradrenaline for 60 min and then washed out with amine-free solution for 100 min. Autoradiography then revealed a preferential labelling of the varicosities in the immediate vicinity of the surface of the tissue. However, when tissues were obtained from reserpine-and pargyline-pretreated rats (to block vesicular uptake and monoamine oxidase), ³H-noradrenaline was able to penetrate more deeply into the tissue. These differences are in accordance with the view that the autoradiographs reflect the ³H-noradrenaline concentration gradient (within the extracellular space) generated by the neuronal uptake of the 3H-amine; the concentration gradient is steeper (and the heterogeneity of labelling is more pronounced) in tissues with intact vesicular uptake and monoamine oxidase than in tissues in which these mechanisms had been inhibited.Send offprint requests to I. Azevedo at the above adress     

Persistent enhancement of potassium-induced responses of the rat vas deferens by desipramine

Summary The effect of desipramine on the cumulative dose-response curves of noradrenaline and potassium (K⁺) was examined on the isolated rat vas deferens. An exposure of 10 min to 10^–7 M desipramine caused a leftward shift and an increase in the maximum response of cumulative dose-response curves of noradrenaline. Desipramine (10^–7 M), in contact with the tissue for 10 min, enhanced responses to cumulative additions of K⁺ without causing a consistent change in threshold concentrations. Wash-out of desipramine resulted in a rapid loss of enhanced maximum response to noradrenaline while the maximum response to K⁺ did not show any decrease for up to 120 min after wash-out of drug. One possible explanation for the persistent enhancement of K⁺-induced responses may be that desipramine causes postjunctional changes which selectively influence contractile responses of this tissue to K⁺.     

Evidence for the participation of calcium in non-genomic relaxations induced by androgenic steroids in rat vas deferens

BACKGROUND AND PURPOSE: Androgens cause non-genomic relaxation in several smooth muscle preparations. However, such an effect has not been investigated in rat vas deferens yet. Our purpose was to study the effect of testosterone and derivatives in this tissue. EXPERIMENTAL APPROACH: The influence of androgens was tested on contraction and translocation of intracellular Ca(2+) induced by KCl in rat vas deferens in vitro. KEY RESULTS: The testosterone derivative 5alpha-dihydrotestosterone produced a rapid and reversible concentration-dependent relaxation of KCl-induced contractions. Other androgens were also effective, showing the following rank order of potency: androsterone >5beta-dihydrotestosterone >androstenedione >5alpha-dihydrotestosterone >testosterone. Calcium-induced contractions were also inhibited (about 45%) by 5alpha-dihydrotestosterone (30 microM). Moreover 5alpha-dihydrotestosterone blocked the increase of intracellular Ca(2+) induced by KCl, measured by the fluorescent dye fura-2. Relaxation to 5alpha-dihydrotestosterone was resistant to the K(+) channel antagonists glibenclamide, 4-aminopyridine and charybdotoxin. It was not affected by removal of epithelium or by L-NNA (300 microM), an inhibitor of nitric oxide biosynthesis, nor by selective inhibitors of soluble guanylate cyclase, ODQ or LY 83583, indicating that nitrergic or cGMP mediated mechanisms were not involved. The androgen-induced relaxation was also not blocked by the protein synthesis inhibitor cycloheximide (300 microM) or by the classical androgen receptor flutamide (up to 100 microM), corroborating that the effect is non-genomic. CONCLUSIONS AND IMPLICATIONS: Testosterone derivatives caused relaxation of the rat vas deferens, that did not involve epithelial tissue, K(+) channels, or nitric oxide-dependent mechanisms, but was related to a partial blockade of Ca(2+) influx.     

Presence of a phosphoramidon-sensitive endothelin-converting enzyme which converts big-endothelin-1, but not big-endothelin-3, in the rat vas deferens

Summary Endothelin-1 (ET-1) enhanced field stimulation-evoked (0.1 Hz), nerve-mediated contractions of the prostatic portion of the rat vas deferens. The human precursor of ET-1, big-endothelin (1-38) (big-ET-1) was only two-fold less potent than ET-1 (pD₂ values: 7.30 and 7.49, respectively). The threshold concentrations necessary to elicit an increase of the response to electrical stimulation was lower for ET-1 (5 nmol/l) than for big-ET-1 (25 nmol/l). Endothelin-3 (ET 3) also markedly enhanced the response of the tissue to field stimulation with a potency similar to ET-1 (pD₂ value: 7.59). In contrast, the precursor of ET-3, big-endothelin (1–41) (big-ET-3), was inactive at concentrations up to 0.5 mol/l. Treatment of the preparations with phosphoramidon (50 mol/l) markedly reduced the twitch enhancement by big-ET-1 without affecting the response to ET-1. Our results suggest the presence of a specific phosphoramidon-sensitive endothelin-converting enzyme which converts big-ET-1 to ET-1 in the rat vas deferens.Abbreviation ECE Endothelin-converting enzyme Send offprint requests to P. D'Orléans-Juste at the above address     

Effect of extracellular pH on contractile responses of the guinea-pig vas deferens

1. Effects of changing the pH of the bathing solution (7.0, 7.4 and 7.8) on the contractile response of the guinea-pig isolated vas deferens to ATP, noradrenaline (NA) and ATP in the presence of NA or electrical field stimulation (EFS) were investigated. 2. Low pH tended to augment the phasic contractile response to ATP (0.01-1 mmol/L), while high pH significantly reduced the contractile response to ATP. In contrast, low pH depressed the tonic contractile response to NA (0.1-10 mumol/L), while high pH augmented the response to NA. The contractile response to 1 mmol/L ATP was markedly potentiated in the presence of 0.1-10 mumol/L NA. The potentiated contractile response to ATP in the presence of NA was unaffected by changes in pH. 3. Electrical field stimulation produced a biphasic contractile response. Low pH enhanced the initial rapid phasic contractile response to EFS, while high pH depressed the response. In contrast, the second slow tonic contractile response to EFS was unaffected by changes in pH. 4. These findings may indicate that the phasic contractile response to EFS is mainly caused by ATP while the tonic contractile response is a synergistic response to ATP and NA concomitantly released from sympathetic nerve terminals.     

The heterogeneity of the neuronal distribution of exogenous noradrenaline in the rat vas deferens

Summary After loading of the incubated rat vas deferens with 0.2 mol/l ³H-noradrenaline (followed by 100 min of wash-out with amine-free solution), the efflux of endogenous and exogenous compounds was determined by HPLC with electrochemical detection and by column chromatography with scintillation counting. Two different types of heterogeneity of labelling were found. The first one is due to the preferential labelling of varicosities close to the surface of the tissue, the second one to the preferential labelling of vesicles close to the surface of loaded varicosities. As diffusion distances within the tissue and within varicosities are then longer for endogenous than for exogenous amine and metabolites, the composition of spontaneous efflux of exogenous compounds differed from that for endogenous compounds. Because of preferential neuronal and vesicular re-uptake of endogenous noradrenaline, the percentage contribution by noradrenaline to overall efflux was: endogenous < exogenous. While ³H-DOPEG was the predominant exogenous metabolite, DOPEG and MOPEG equally contributed to the endogenous efflux.Desipramine abolished the consequences of the first heterogeneity of labelling, i.e., it increased the efflux more for endogenous than for exogenous noradrenaline; moreover it decreased the efflux of ³H-DOPEG, but increased that of ³H-MOPEG. The reserpine-like compound Ro 41284, on the other hand, abolished the consequences of the second type of heterogeneity; it reduced the specific activity of total efflux (i.e., of the sum of noradrenaline + DOPEG + MOPEG) to the specific activity of the tissue noradrenaline. The degree of heterogeneity of labelling was reduced after inhibition of monoamine oxidase and also when the tissues were loaded with 2 or 20 mol/l ³H-noradrenaline.It is proposed that the various compartments and pools of noradrenaline described in the literature reflect the two heterogeneities described here.Abbreviations COMT catechol-O-methyl transferase - DOMA dihydroxymandelic acid - DOPEG dihydroxyphenylglycol - FRL fractional rate of loss (= rate of efflux/tritium content of tissue measured at onset of collection period) - HPLC high performance liquid chromatography - MAO monoamine oxidase - MOPEG methoxyhydroxyphenylglycol - NMN normetanephrine - VMA vanillylmandelic acid Send offprint requests to E. Schömig at the above addressThis study was supported by the Deutsche Forschungsgemeinschaft (SFB 176, Gr 490/5 and Scho 383/1). Some of the results were presented to the German Pharmacological Society (Schönfeld 1990; Trendelenburg 1990)     

Effects of α-adrenoceptor agents on norepinephrine release from vas deferens of several species including man

Unlike the situation with the vas deferens of rats, guinea-pigs, rabbits an dogs, neither phentolamine nor yohimbine enhanced electrically-evoked release of [³H]norepinephrine from the in vitro human vas deferens. However, clonidine produced, by a phentolamine-sensitive mechanism, a concentration-related inhibition of release inthe human vas deferens. The results indicate that, even though presynaptic α-adrenoceptors exist, a negative feedback regulation of release by norepinephrine, which occurs in the vas deferens of the other species, may not be functionally important in the vas deferens of the human.     

Mechanisms of the release of 3H-noradrenaline by dimethylphenylpiperazinium (DMPP) in the rat vas deferens

Summary In the rat vas deferens, DMPP is a substrate of uptake, (K_rn = 11.5 mol/I). After block of vesicular uptake, monoamine oxidase and catechol-O-methyl transferase, after loading of the tissue with ³H-noradrenaline, and in calcium-free solution (i. e., when axoplasmic ³H-noradrenaline levels were high and when depolarization-induced exocytotic release was impossible), DMPP induced a pronounced outward transport of ³H-noradrenaline. On the other hand, when, in similar experiments, vesicular uptake and monoamine oxidase were intact (i.e., when axoplasmic ³H-noradrenaline levels were low), DMPP induced very little outward transport of ³H-noradrenaline. This discrepancy indicates that DMPP has little ability to mobilize vesicularly stored ³H-amine.When the medium contained calcium (catechol-O-methyl transferase inhibited, all other mechanisms intact), 100 (but not 10) mol/l DMPP induced a hexamethonium-sensitive release of ³H-noradrenaline of short duration. Hence, in the presence of extracellular calcium, 100 mol/l DMPP elicits exocytotic release via activation of hexamethonium-sensitive nicotinic acetylcholine receptors.DMPP inhibits the monoamine oxidase of rat heart homogenate with an IC₅₀ of about 100 mol/l.Abbreviations COMT catechol-O-methyl transferase - DMPP dimethylphenylpiperazinium - DOMA dihydroxymandelic acid - DOPEG dihydroxyphenylglycol - MAO monoamine oxidase - NMN normetanephrine - OM-fraction column chromatographic fraction containing all O-methylated ³H-metabolites - OMDA fraction containing O-methylated and deaminated metabolites Supported by the Deutsche Forschungsgemeinschaft (SFB 176) Send offprint requests to U. Trendelenburg at the above address