Modulation by noradrenaline and yohimbine of noradrenergic transmission in the guinea-pig mesenteric artery

Selective modulation of noradrenaline release by alpha 2-adrenoceptor blockade in the rat-tail artery in vitro

The effects of blocking alpha(2)-adrenoceptors on noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves have been investigated in rat-tail artery in vitro. Continuous amperometry was used to measure NA release and intracellularly recorded excitatory junction potentials (e.j.p.'s) were used to measure ATP release. Application of the alpha(2)-adrenoceptor antagonist, idazoxan (1 microm), increased the amplitude of NA-induced oxidation currents evoked by trains of 10 stimuli at 1 and 10 Hz. In cells deep in the media, idazoxan (1 microm) had no effect on the amplitude of e.j.p.'s evoked by trains of 10 stimuli at 1 and 10 Hz. In cells close to the adventitial - medial border, idazoxan produced a small increase in the amplitude of e.j.p.'s evoked at the end of trains of 10 stimuli at 1 Hz. In tissues pretreated with the neuronal NA uptake inhibitor, desmethylimpramine (0.3 microm), idazoxan (1 microm) markedly increased the amplitude of e.j.p.'s in cells deep in the media. The alpha(2)-adrenoceptor agonist, clonidine (0.5 microm), produced similar reductions in the amplitudes of both NA-induced oxidation currents and e.j.p.'s evoked by 10 stimuli at 1 Hz. These effects of clonidine were reversed by the subsequent addition of idazoxan (1 microm). The release of both NA and ATP is inhibited to a similar extent by activation of prejunctional alpha(2)-adrenoceptors by clonidine. In contrast, endogenously released NA more markedly inhibits NA release. These findings provide further support for the differential modulation of NA and ATP release.

Presynaptic dopamine DA2-receptors in rabbit jejunal arteries. An electrophysiological study

Excitatory junction potentials (e.j.ps) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of rabbit isolated jejunal arteries. LY 171555 1 mumol/l, SKF 38393 10 mumol/l, dopamine 10 mumol/l and clonidine 0.1 mumol/l depressed all e.j.ps in the train. The percentage inhibition was inversely related to the number of pulses. S- and R-sulpiride, 10 mumol/l, domperidone 1 mumol/l, SCH 23390 1 mumol/l and rauwolscine 1 mumol/l did not change, or even depressed the first e.j.ps. Of these compounds only S- and R-sulpiride, 10 mumol/l and rauwolscine 1 mumol/l facilitated the late e.j.ps. The percentage facilitation increased with the number of pulses until a maximum was reached; rauwolscine 1 mumol/l had the largest effect. S- and R-sulpiride, 10 mumol/l, as well as domperidone 1 mumol/l antagonized the action of LY 171555 1 mumol/l. S-Sulpiride was more potent than its R-isomer. SCH 23390 1 mumol/l and rauwolscine 1 mumol/l blunted the effect of SKF 38393 10 mumol/l. Rauwolscine 1 mumol/l slightly reduced the inhibition by dopamine 10 mumol/l; S-sulpiride 10 mumol/l was antagonistic only in the presence of rauwolscine 1 mumol/l. When rauwolscine 1 mumol/l, prazosin 0.1 mumol/l, propranolol 1 mumol/l and cocaine 10 mumol/l was added to the medium, dopamine 10 mumol/l continued to produce the same depression of e.j.ps, as in the absence of these compounds. Under such conditions S-sulpiride 10 mumol/l also counteracted dopamine 10 mumol/l. Rauwolscine 1 mumol/l prevented the effect of clonidine 0.1 mumol/l. The antagonists were not absolutely selective against only one type of agonist. We suggest that both presynaptic DA2- and postsynaptic DA1-receptors are present in rabbit jejunal arteries. The activation of either receptor-type may depress the e.j.ps. Dopamine interferes with neuroeffector transmission due to alpha 2-adrenoceptor agonist properties; its DA2-effect is unmasked only after alpha 2-adrenoceptor blockade. There was no evidence for a co-transmitter function of dopamine.

Bunazosin, an alpha 1-adrenoceptor blocker, differentially releases co-transmitters in dog mesenteric vessels

The effects of bunazosin on the electrical and mechanical responses of smooth muscle cells elicited by exogenously applied noradrenaline (NA) and by perivascular nerve stimulation were studied in the isolated mesenteric artery and vein of the dog. NA (above 10(-7) M in the artery and above 3 X 10(-8) M in the vein) depolarized the membrane. Perivascular nerve stimulation evoked an excitatory junction potential (e.j.p.) and slow depolarization in both vessels. Bunazosin and prazosin inhibited the NA-induced depolarization and slow depolarization in the artery but not in the vein. The NA actions in the vein were inhibited by yohimbine. Bunazosin (above 10(-6) M) increased the amplitude of the e.j.p. but decreased the outflow of NA during nerve stimulation. The amplitude and conduction velocity of the compound action potential of perivascular nerves were inhibited by higher concentrations of bunazosin (above 10(-5) M). The results provide evidence that bunazosin has selective inhibitory actions at alpha 1-adrenoceptors. This drug exerted differential effects on the release of co-transmitters which generate the e.j.p. and the slow depolarization, as bunazosin increased the former and decreased the latter. This suggests that e.j.p. is generated by a substance other than NA.

Comparative effects of verapamil and D-600 on the contractile responses produced by neurotransmitters and depolarizing agents in the rat isolated seminal vesicle

1. The comparative effects of verapamil and D-600, on isometric contractions produced by noradrenaline, acetylcholine, phenylephrine, clonidine, carbachol and high potassium, were studied in rat isolated seminal vesicle. 2. The aim was to see if verapamil and D-600 differed, in their selectivity, of blocking calcium, channels, and hence inhibition of isometric contractions produced in this preparation. 3. Verapamil (1 microM) and D-600 (9.5 microM), selectively, reduced the isometric contractions, greater reductions occurred in the KCl-induced contractions (control EC50 19 +/- 1.5 mM, in verapamil 58 +/- 1.3 mM, and in D-600 37 +/- 2.0 mM, means +/- SE, n = 8, P less than 0.001, less than 0.01, respectively). 4. It was concluded that KCl-induced contractions were voltage-dependent, whereas noradrenaline operated via both voltage and receptor-mediated mechanisms. 5. The clinical relevance of these results was discussed, and it may help our current understanding of the pharmacological management of sexual dysfunction.