The actions of cirazoline on the rat vas deferens

Investigation of postjunctional alpha1- and alpha2-adrenoceptor subtypes in vas deferens from wild-type and alpha(2A/D)-adrenoceptor knockout mice

1. The subtypes of alpha(1)- and alpha(2)-adrenoceptor mediating contractions of vas deferens have been examined in wild-type and alpha(2A/D)-adrenoceptor knockout mice. 2. Maximum contractions to noradrenaline but not phenylephrine were significantly greater in vas from wild-type. The alpha(1A)-adrenoceptor antagonist RS100329 (5-methyl-3-[3-[4-[2-(2,2,2,-trifluoroethoxy)phenyl]-1-piperazinyl]propyl]-2,4-(1H)-pyrimidinedione) (10 nM) significantly shifted the potency of noradrenaline. The alpha(2D)-adrenoceptor antagonist BRL 44408 (2-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-2,3-dihydro-1-methyl-1H-isoindole) significantly reduced the maximum contraction to noradrenaline in wild-type but not in knockout. 3. Following prazosin (0.1 micro M), a component of the contraction to noradrenaline in wild-type but not in knockout was sensitive to the alpha(2)-adrenoceptor antagonist yohimbine. 4. Nifedipine (10 micro M) or suramin (100 micro M) reduced the contraction to 10 Hz stimulation for 4 s to an early peak and small maintained response. The peak was abolished by the alpha(1)-adrenoceptor antagonist prazosin. 5. RS100329 or prazosin inhibited 10 Hz stimulation evoked contractions with a U-shaped concentration-response curve: inhibiting responses up to 0.1 micro M, with a reversal of inhibition above this concentration. In the presence of suramin or nifedipine, the reversal of inhibition by high concentrations of prazosin was reduced. 6. The alpha(1D)-adrenoceptor selective antagonist BMY7378 (8-[2-(4-(2- methoxyphenyl)piperazin-1-yl)ethyl]-8-azaspiro[4,5]decane-7,9-dione) produced inhibition of 10 Hz evoked contractions only in high concentrations. 7. In conclusion, contractions to nerve stimulation in mouse vas deferens involve largely alpha(1A)-adrenoceptors and purinoceptors. alpha(1)-Adrenoceptor antagonists in high concentrations increase the purinergic response presumably by blocking prejunctional alpha(2)-adrenoceptor-mediated inhibition. In the presence of nifedipine, responses are predominantly alpha(1)-adrenoceptor mediated. Contractions to exogenous noradrenaline involved both alpha(1A)- and alpha(2A/D)-adrenoceptors in wild-type mice.

Analysis of agonism at functional prejunctional alpha 2-adrenoceptors of rat vas deferens using operational and null approaches

The alpha 2-adrenoceptors located prejunctionally on the postganglionic neurons that innervate the smooth muscle of the prostatic portion of the rat vas deferens were examined. For this purpose, three imidazolidine derivatives (structurally related to clonidine) were studied for their effects on twitch contractions elicited by electrical field stimulation of this tissue. In this study, operational model-fitting and the nested hyperbolic method were used to analyse the effects of irreversible receptor alkylation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on the alpha 2-adrenoceptor-mediated effects of clonidine (2-[2,6-dichlorophenylimino]imidazolidine) in stimulated vas deferens. The operational model provided an estimate of KA for clonidine which was not significantly different from the estimate obtained by using the nested hyperbolic method (null approach). The data indicate a large receptor reserve at prejunctional alpha 2-adrenoceptors for clonidine. The estimates of apparent affinity for St-587 (2-[2-chloro-5-trifluoromethylphenylimino]imidazolidine) and St-591 (2-[2-chloro-5-methylphenylimino]imidazolidine) did not depend on the method of calculation as the 'null' method and the 'operational' method gave similar answers. Further, estimates of the ratio of tau values for these partial agonists with respect to clonidine were numerically the same as those of their relative efficacies. Therefore, no limitations in the ability of the operational model to fit experimental data and provide reproducible estimates of affinity and efficacy have been revealed for agonists acting at prejunctional alpha 2-adrenoceptors.

Suramin: a selective inhibitor of purinergic neurotransmission in the rat isolated vas deferens

The effects of the putative P2 purinoceptor antagonist suramin on contractile responses of the rat isolated vas deferens to electrical field stimulation and exogenously applied drugs (alpha,beta-methylene ATP and noradrenaline) were investigated. Suramin was devoid of agonist activity in the rat vas deferens. The response of the vas deferens to single pulse field stimulation was characteristically biphasic with a large first component peaking between 260-300 ms after the stimulus followed by a second smaller component peaking at about 650 ms post-stimulus. Suramin (100 nM-1 mM) selectively impaired the first, purinergic phase of the response to single pulse field stimulation but was without effect on the second, noradrenergic component. The response of the vas deferens to trains of electrical field stimuli (10 Hz for 10 s) was also biphasic. Suramin (1 microM-1 mM) reduced the first (less than 1 s) phase of the response by 30%, the second (greater than 5 s) plateau phase by 50% and inhibited the intermediate (2-4 s) phase by 80%. Dose-contact periods of 20 or 30 min respectively were sufficient to achieve equilibration of the inhibitory effects of suramin against the responses to single pulse field stimulation or trains of pulses. Following 30 min incubation with 1 mM suramin, the remaining first and second phase components of the response to trains of pulses were impaired and subsequently abolished by the alpha 1-adrenoceptor antagonist WB4101 establishing their noradrenergic origin. Suramin (300 microM) abolished responses of the vas deferens to alpha,beta-methylene ATP but was without effect on those to noradrenaline. Suramin (30 microM) induced a rightward shift in the concentration-response relationship to alpha,beta-methylene ATP that was associated with a significant 40% increase in the maximum response, but did not modify responses to noradrenaline. The inhibitory effects of suramin (3-300 microM) on responses of the vas deferens to approximate EC50 concentrations of alpha,beta-methylene ATP were reversible on repeated washout for 40-60 min. These results reveal suramin to be a useful pharmacological tool for the study of purinergic neurotransmission in rodent vasa deferentia.

Effects of St-587 on the alpha-adrenoceptors in the bisected rat vas deferens

The effects of the alpha-adrenoceptor agonist St-587 have been studied on the twitch responses induced by field stimulation in the prostatic portion of rat vas deferens. Moreover the drug's influence on the unstimulated prostatic and epididymal halves of rat vas deferens has also been determined. Alone and after addition of yohimbine (0.3 microM) it enhanced in a concentration-dependent manner the twitch responses in the prostatic half. Prazosin competitively antagonized (pA2 = 8.41 +/- 0.03) this effect. The enhancing effect of St-587 was not reduced in reserpinized animals. These results suggest that post-synaptic alpha 1-adrenoceptors are involved in the potentiation of twitch responses induced by St-587. When alpha 1-adrenoceptors were blocked by prazosin (0.1 microM), St-587 partially inhibited the twitch responses of the prostatic portion of rat vas deferens (Emax = 49.5 +/- 3.5%). Yohimbine completely reversed the inhibitory effects of both St-587 and clonidine. Furthermore St-587 antagonized the inhibitory effects of clonidine on twitch responses. Thus it appears that St-587 also behaves as a partial agonist of presynaptic alpha 2-adrenoceptors in this portion of rat vas deferens, but it did not induce contractions in the unstimulated prostatic half of the vas deferens. However, it competitively antagonized the alpha 1-adrenoceptor agonist phenylephrine by acting as an antagonist of prostatic postsynaptic alpha 1 adrenoceptors. These alpha 1-adrenoceptors are probably different from those that mediate the twitch enhancing response to St-587 in that portion. On the other hand, St-587 was a partial agonist of alpha 1-adrenoceptors in the epididymal half.(ABSTRACT TRUNCATED AT 250 WORDS)

Pertussis toxin and pre-junctional alpha 2-adrenoreceptors in rat heart and vas deferens

1. The effects of pretreatment with pertussis toxin (50-200 micrograms kg-1, i.p.) on the pre-junctional inhibitory actions of the alpha 2-adrenoreceptor agonist xylazine were examined by employing the pithed rat and rat isolated vas deferens. 2. In pithed rats, pertussis toxin attenuated the pressor response to xylazine, but did not alter the pre-junctional cardio-inhibitory actions of xylazine. In epididymal portions of rat vas deferens, pretreatment with pertussis toxin did not alter the pre-junctional inhibitory action of xylazine. 3. These data lend no support for the view that alpha 2-adrenoreceptor-mediated prejunctional inhibition involves inhibition of adenylate cyclase through the Ni regulatory protein, at least in the tissues examined.

A study of the post-tetanic twitch depression at the prostatic end of the rat vas deferens

The effects of different drugs on the post-tetanic twitch responses of the prostatic end of the isolation rat vas deferens were examined. Following repetitive field stimulation (4-5 Hz for 40-60 s), the post-tetanic twitches (0.2-0.5 Hz) were depressed relative to the steady-state pretetanic twitches. Maximum post-tetanic twitch depression (PTD) occurred 15-30 s after the tetanic stimulation. The PTD was not reversed but deepened and prolonged by the presynaptic alpha 2-adrenoceptor antagonists idazoxan (RX781094) or yohimbine (10(-7) M) alone or combined with the neuronal uptake blocker, desipramine (10(-6) M). The PTD was insensitive to the beta-adrenoceptor blocker, propranolol (10(-6) M), the P1-purinoceptor antagonist, 8-phenyltheophylline (10(-5) M) or the prostaglandin E synthesis inhibitor, indomethacin (10(-6) M). Pre- and post-tetanic contractions to superfused noradrenaline (NA), adenosine 5'-triphosphate (ATP), or beta, gamma-methylene ATP (beta, gamma-mATP) (bolus injections into superfusate, 1-8 X 10(-5) M) were not significantly (p less than 0.01) different. The PTD was absent in high magnesium (6.2-11.2 mM) solutions. These results are consistent with the PTD having an intrinsic presynaptic origin. They are discussed in relation to stimulation-induced modulation of transmitter release.

Age-related change in serotonin-mediated prejunctional inhibition of rat vas deferens

The effect of age on the serotonin (5-HT)-induced prejunctional inhibition of twitch contractions induced in rat vas deferens by single-pulse field stimulation at 0.1 Hz was studied. The inhibitory effect of 5-HT gradually decreased with increasing age of the rats (4-15 weeks old) and was no longer detectable in preparations from rats over 15 weeks old. Moreover, on a further increase in age to 45 weeks, 5-HT conversely enhanced the twitch response of the vas deferens. The 5-HT2 antagonists ketanserin and mianserin potentiated the 5-HT-induced inhibition of contractions of the vas deferens of young rats (8-15 weeks old), and attenuated the amplifying effect of 5-HT on the responses of preparations from old rats (95 weeks old). Thus unmasking of the inhibition depended on the age of rats. This change occurred earlier in life in the prostatic portion of the vas deferens than in the epididymal portion. A functional decrease in 5-HT-mediated prejunctional inhibition and the resultant increase in amplification of the twitch response by 5-HT are probably responsible for the age-related decrease in prejunctional inhibition.

Differential blocking actions of idazoxan against the inhibitory effects of 6-fluoronoradrenaline and clonidine in the rat vas deferens

The prejunctional inhibitory effects of clonidine and 6-fluoronoradrenaline (6-FNA) have been evaluated in the isolated prostatic segment of the rat vas deferens, against the twitch response evoked by low frequency (0.1 Hz) field stimulation. The inhibitory potency of 6FNA was significantly increased in the presence of cocaine (1 microM) or pargyline (10 microM), but was not modified in the vas deferens from rats pretreated with reserpine when the endogenous levels of noradrenaline (NA) were decreased by 97%. Clonidine was significantly more potent than 6-FNA as an inhibitory agonist, and the potency of clonidine was not modified after cocaine, pargyline or reserpine. The alpha 2-adrenoceptor blocking agent idazoxan, was a competitive antagonist against the inhibitory effects of clonidine under all experimental conditions. In contrast, the only antagonism shown by idazoxan against the inhibitory effects of 6-FNA was in the presence of cocaine (1 microM), and this antagonist effect of idazoxan was not concentration-related. Low concentrations of 6-FNA caused concentration-dependent facilitatory effects on the twitch response, which were significantly greater after treatment with idazoxan (1 microM) in reserpine-treated vas deferens. These facilitatory effects of 6-FNA were always observed in the presence of prazosin (300 nM) and also after treatment of the preparations with phenoxybenzamine (10 microM), a concentration which abolished the inhibitory actions of both clonidine and 6-FNA. The facilitatory effects on the twitch response induced by low concentrations of 6-FNA are therefore unlikely to be due to either alpha 1- or alpha 2-adrenoceptor stimulation. In conclusion, the failure of idazoxan to block the inhibitory effects of 6-FNA, while exerting a potent competitive antagonism of clonidine-induced inhibitory effects, supports the proposal that alpha 2-adrenoceptors may in fact be subdivided into two subclasses, involving imidazoline and phenylethylamine recognition sites.

Further examination of the inhibitory actions of alpha 1-adrenoceptor agonists in rat vas deferens

The inhibitory actions of alpha 1-adrenoceptor agonists were examined in the isolated bisected vas deferens of the rat. The calcium entry facilitator Bay K 8644 markedly potentiated the isometric contraction to a single stimulus pulse in epididymal portions of rat vas deferens: subsequent amidephrine produced an inhibition which was antagonised by the alpha 1-adrenoceptor antagonist, prazosin, but not by the alpha 2-adrenoceptor antagonist, yohimbine. The alpha 1-adrenoceptor agonists amidephrine and cirazoline failed to inhibit the transmitter overflow to trains of pulses at a frequency of 2 Hz in epididymal portions, but also failed to abolish the nifedipine-resistant adrenergic contraction to trains of pulses at 2 Hz in epididymal portions. It is concluded that alpha 1-adrenoceptor agonists have inhibitory effects which may be by action at presynaptic alpha 1-adrenoceptors.

Further examination of the effects of ageing on the adrenoceptor responsiveness of the rat vas deferens

Age-related changes in presynaptic alpha 2-adrenoceptors were investigated in prostatic and epididymal portions of vasa deferentia from young adult (2-3 months) and old (24-29 months) Sprague-Dawley rats, using the alpha 2-selective agonists xylazine and clonidine. In prostatic portions the inhibitory effects of clonidine against the isometric contractions to single pulse field stimulation were complicated by a postsynaptic action in old animals, but in epididymal portions in the presence of nifedipine both xylazine and clonidine were 3 times less potent in old rats. However, there were no significant differences between young and old in the potency of xylazine at inhibiting the overflow of tritium or the isometric contraction evoked by 5 Hz stimulation for 3 min in tissues pre-incubated with [3H]noradrenaline. It is suggested that there is reduced responsiveness of presynaptic alpha 2-adrenoceptors in the vas deferens of old rats, but that this can only be demonstrated using a sensitive measure of the presynaptic potency of agonists: the isometric contraction to a single stimulus.

Cirazoline, an alpha 2-adrenoceptor antagonist in guinea-pig ileum

Prejunctional effects of cirazoline have been investigated in guinea pig ileum. At high concentrations cirazoline has been shown to have antimuscarinic activity, pA2 5.25 +/- 0.29. At concentrations below those producing blockade of acetylcholine, cirazoline blocks the prejunctional alpha 2-adrenoceptor activity of clonidine, pA2 6.81 +/- 0.22, and alpha-methylnoradrenaline. Results are discussed in the light of controversial evidence for the activity of cirazoline on alpha-adrenoceptors.

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.