Alpha-adrenoceptor reserve in the canine cephalic vein

Differences between full and partial alpha-adrenoceptor agonists in eliciting phasic and tonic types of responses in the longitudinal smooth muscle of the rat portal vein

The aim of the present investigation was to study, taking into account both quantitative and qualitative differences, the influence of full and partial alpha-adrenoceptor agonists on spontaneous myogenic activity in the rat portal vein. We found that the alpha-adrenoceptor agonists cirazoline, adrenaline, noradrenaline, phenylephrine, St 587, Sgd 101/75, B-HT 920 and UK-14,304 could increase the amplitude of the phasic myogenic contractions in the rat portal vein with apparent differences in EC50 and Emax values. In addition to an increase in phasic myogenic activity, the alpha-adrenoceptor agonists cirazoline, adrenaline, noradrenaline, and phenylephrine were also able (in higher concentrations) to increase the basal tone of the rat portal vein preparation, again with apparent differences in EC50 and Emax values. Changing the extracellular Ca2+ concentration from 0.9 mmol/l to 2.5 mmol/l had no influence on the phasic character and the concentration range in which St 587 and UK-14,304 increased spontaneous myogenic activity, although changes in amplitude and frequency of the spontaneous myogenic contractions were less pronounced at a higher extracellular Ca2+ concentration (2.5 mmol/l). By the use of Schild analysis with the competitive alpha-adrenoceptor antagonists prazosin (pA2 = 8.74) and 5-methyl-urapidil (pA2 = 8.37), it was established that the contractile responses to St 587 were mediated by the same alpha 1-adrenoceptor subtype as the phasic and tonic type of contraction elicited by phenylephrine as described in a previous study. The concentration-response curve of UK-14,304 was significantly shifted to the right by low concentrations of prazosin (3 nmol/l-30 nmol/l), indicating stimulation of alpha 1-adrenoceptors by UK-14,304 in the rat portal vein.(ABSTRACT TRUNCATED AT 250 WORDS)

Postsynaptic alpha-adrenoceptors in the perfused canine saphenous vein in vitro

To study the relative localization of alpha 1 and alpha 2-adrenoceptors in relation to the intima and the adventitia of canine saphenous vein, a comparison was made of the potency of alpha 1- and alpha 2-adrenoceptor agonists applied by intraluminal and extraluminal route of perfused segments of that vessel. Noradrenaline was the most potent of the agonists used and was approximately as potent by intraluminal as by extraluminal route. Cocaine (12 mumol/l) caused supersensitivity to noradrenaline which was of about the same magnitude (threefold) irrespective of the route of administration of noradrenaline. The selective alpha 1-agonist phenylephrine was about 10 times less potent than noradrenaline and was also equieffective by both routes. The selective alpha 2-agonist UK-14,304, at concentrations lower than 0.3 mumol/l, caused very small responses and only in 3 out of 14 experiments. In all cases it caused responses at concentrations higher than 0.3 mumol/l. Cocaine did not change the sensitivity to either phenylephrine or UK-14,304. Thus, it is concluded that the results obtained with cocaine agreed with expectations for a homogeneously innervated tissue. Furthermore, alpha 1-adrenoceptors seem to predominate and to be evenly distributed throughout the media. The lack of responses to the low concentrations of UK-14,304--those selectively acting on alpha 2-adrenoceptors--was ascribed to the very low efficacy of this agonist on the distal part of the canine saphenous vein and to the tone created by the perfusion pressure which might be high enough to mask this small response.

The effectiveness of alpha 2-adrenoceptor activation increases from the distal to the proximal part of the veins of canine limbs

1. The effectiveness of alpha 1- and alpha 2-adrenoceptor activation was compared at different levels of the saphenous and cephalic vein of the dog in vitro. 2. Helically cut strips were used to determine concentration-response curves to phenylephrine, noradrenaline, UK-14,304 (5-bromo-6-(imidazoline-2-ylamino)-quinoxaline) and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetra-hydro-4H-(thiazo)-4,5-d-azepine). The effect of prazosin and yohimbine on these curves was also studied. 3. At the distal level, the maximum response to UK-14,304 amounted to 33 and 50% of those to noradrenaline in the saphenous and cephalic vein, respectively, while at the proximal level the maximum response to UK-14,304 amounted to 72 and 78% of those to noradrenaline, in the saphenous and cephalic vein, respectively. 4. In both vessels, the results obtained with B-HT 920 were very similar to those for UK-14,304. 5. The pD2 values for UK-14,304 - which were identical at the three levels of both vessels - and the pA2 values for the antagonism exerted by either prazosin or yohimbine against the responses to UK-14,304 indicate that the alpha 2-adrenoceptors are identical at the different levels of both vessels. 6. These results show that the effectiveness of alpha 2-adrenoceptor stimulation increases from the distal to the proximal regions of canine limb veins. Apparently, this is due to a greater density of alpha 2-adrenoceptors in the proximal regions. 7. Yohimbine is much more potent against phenylephrine distally than proximally in both vessels. However, after 30 nm phenoxybenzamine - a concentration which eliminates the vast majority of alpha,-adrenoceptors without affecting alpha 2-adrenoceptors - yohimbine became equally potent at both levels, suggesting that the difference existing before phenoxybenzamine depended on alpha,-adrenoceptors. Hence it is concluded that alpha,-adrenoceptors in distal and proximal portions may differ.