Pharmacological characterization of the postsynaptic alpha-adrenoceptors in human uterine artery

Identification of a novel snake peptide toxin displaying high affinity and antagonist behaviour for the α2-adrenoceptors

BACKGROUND AND PURPOSE Muscarinic and adrenergic G protein-coupled receptors (GPCRs) are the targets of rare peptide toxins isolated from snake or cone snail venoms. We used a screen to identify novel toxins from Dendroaspis angusticeps targeting aminergic GPCRs. These toxins may offer new candidates for the development of new tools and drugs. EXPERIMENTAL APPROACH In binding experiments with (3) H-rauwolscine, we studied the interactions of green mamba venom fractions with α(2) -adrenoceptors from rat brain synaptosomes. We isolated, sequenced and chemically synthesized a novel peptide, ρ-Da1b. This peptide was pharmacologically characterized using binding experiments and functional tests on human α(2)-adrenoceptors expressed in mammalian cells. KEY RESULTS ρ-Da1b, a 66-amino acid peptide stabilized by four disulphide bridges, belongs to the three-finger-fold peptide family. Its synthetic homologue inhibited 80% of (3) H-rauwolscine binding to the three α(2)-adrenoceptor subtypes, with an affinity between 14 and 73 nM and Hill slopes close to unity. Functional experiments on α(2A) -adrenoceptor demonstrated that ρ-Da1b is an antagonist, shifting adrenaline activation curves to the right. Schild regression revealed slopes of 0.97 and 0.67 and pA(2) values of 5.93 and 5.32 for yohimbine and ρ-Da1b, respectively. CONCLUSIONS AND IMPLICATIONS ρ-Da1b is the first toxin identified to specifically interact with α(2)-adrenoceptors, extending the list of class A GPCRs sensitive to toxins. Additionally, its affinity and atypical mode of interaction open up the possibility of its use as a new pharmacological tool, in the study of the physiological roles of α(2)-adrenoceptor subtypes.

Pharmacological characterization of alpha 1-adrenoceptors in porcine uterine artery

This study was undertaken to ascertain if changes in the affinity to alpha 1-adrenoceptor agonists and antagonists could explain the increase in uterine artery vasoconstriction to adrenergic stimuli during the luteal phase of the estrous cycle in pigs. We also sought to determine the subtype(s) of adrenoceptor (alpha 1A and alpha 1B) in the porcine uterine artery. When phenylephrine was the agonist, uterine artery pA2 values for prazosin were 8.98, 9.04 and 9.10 in the luteal and follicular phases and in early pregnancy, respectively. The KA (dissociation constant) values for phenylephrine were 6.5, 3.7 and 4.4 (microM) in the luteal and follicular phases and in early pregnancy, respectively. The use of the putative alpha 1A-adrenoceptor WB4101 (2-[[[2-(2,6-dimethoxyphenoxy)ethyl]amino]methyl]-1,4-benzodioxane ) and the alpha 1B-adrenoceptor antagonist chloroethylclonidine indicated that both alpha 1A- and alpha 1B-adrenoceptors are present in the porcine uterine artery and that a similar magnitude of inhibition of responses to noradrenaline by each of the antagonists occurred in arteries from the luteal and follicular phases and in early pregnancy. This suggests that the alpha 1A:alpha 1B adrenoceptor ratio does not vary significantly during the estrous cycle and in early pregnancy. This study indicates that porcine uterine artery alpha 1-adrenoceptor affinity is similar in the luteal and follicular phases of the estrous cycle and in early pregnancy.

Noradrenaline-mediated contractions of ovine uterine artery: role of inositol 1,4,5-trisphosphate

To elucidate the role of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) as a second messenger through which noradrenaline regulates contractions of the uterine artery, we present here studies designed to characterize simultaneously the noradrenaline-mediated contractions and Ins(1,4,5)P3 formation in isolated uterine arteries from near-term pregnant sheep. Noradrenaline stimulated a rapid increase of Ins(1,4,5)P3 formation with the peak at 30 second. Simultaneous measurement of noradrenaline-induced contractile responses and Ins(1,4,5)P3 formation revealed a significant linear correlation between these two events. In accordance with the contractile results, the noradrenaline-mediated inositol phosphate accumulation was blocked by prazosin (0.1 microM), but not by yohimbine (0.1 microM). Pre-treatment of tissues with pertussis toxin (200 ng/ml, 3 h) failed to block noradrenaline-induced inositol phosphate accumulation. We conclude that, in the uterine artery of late pregnancy, the alpha 1-adrenoceptor-elicited contraction, at least the initial phasic component, is predominantly mediated by the formation of Ins(1,4,5)P3, leading to release of Ca2+ from intracellular stores.

Endothelin and neuropeptide Y are vasoconstrictors in human uterine blood vessels

The control of uterine blood flow is regulated by a number of factors including vasoactive present in the endothelium and the perivascular nerves. We examined the effects of two vasoactive peptides, endothelin and neuropeptide Y, on segments of human uterine arteries and veins in vitro. We found endothelin, a potent vasoconstrictor in the uterine artery and vein, giving a slow and long-lasting contraction starting at 10(-9) mol/L. At this concentration, the established vasoconstrictor noradrenaline had almost no effect. Neuropeptide Y had no effect of its own in the human uterine artery, but increased basal tone in uterine veins, and at higher concentrations triggered spontaneous contractions. Neuropeptide Y (10(-7) mol/L) often, but not always, enhanced the contraction induced by noradrenaline (10(-6) mol/L). Neuropeptide Y (10(-7) mol/L) also enhanced the contraction induced by transmural nerve stimulation (20 Hz, 0.4 msec). We conclude that endothelin and neuropeptide Y may modulate the control of human uterine circulation.

Effects of tyramine on the human uterine artery in vitro

1. We have studied the effects of tyramine on the human uterine artery (HUA) in order to assess its site of action. 2. Tyramine (5 x 10(-5) to 10(-3) M) contracts the isolated human uterine artery. Tachyphylaxis appeared when concentration-response curves were repeated and the contraction was diminished by prazosin (10(-8), 10(-7) and 10(-6) M). The maximal contraction induced by tyramine (10(-3) M) was 25% of the maximal response to noradrenaline (10(-5) M). 3. After 2 hr of tyramine perfusion a decrease of the contractile response to KCl, 30 and 60 mM (15.0 and 12.9%) and noradrenaline 10(-6) M (83.2%) is shown. 4. However, when tyramine was previously added for 3 min to the bath, the response to KCl increased while the response to noradrenaline was lower. 5. A possible postsynaptic antagonistic effect for tyramine in the HUA is suggested in addition to its usual presynaptic effect.

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.

Characterization of alpha-adrenoceptors in the late pregnant ovine uterine artery

Ring segments of uterine artery from pregnant ewes within two weeks of term were used to study alpha-adrenoceptor-mediated contractions in vitro. The order of potency for the agonists giving the same maximum contractile effect was norepinephrine greater than epinephrine greater than phenylephrine much greater than methoxamine; whereas the maximum contraction to clonidine was about 42% of that to norepinephrine. Medetomidine caused inconsistent responses and B-HT 920 was ineffective. Phentolamine (10(-7) M) and prazosin (10(-7) M), significantly (P less than 0.005) inhibited epinephrine-induced contractions, whereas yohimbine (10(-7) M) was ineffective. Clonidine-induced contractions were blocked by prazosin (10(-8) and 10(-7) M) but were insensitive to yohimbine (10(-7) M). The dissociation constant (KB) of the competitive antagonist prazosin was determined with phenylephrine. The slope of the Schild plot was not significantly different from unity. The pA2 value of prazosin with phenylephrine was 9.1 +/- 0.2. The dissociation constant (KA) and the relationship between receptor occupancy and response for the full agonists norepinephrine and phenylephrine were analyzed using the irreversible antagonist dibenamine. The KA values of norepinephrine and phenylephrine were 1.5 +/- 0.4 x 10(-6) M and 2.5 +/- 0.8 x 10(-6) M, respectively. The occupancy response relationship was nearly linear and the half-maximal response to norepinephrine or phenylephrine was obtained with a 50% receptor occupancy. These results suggest that contraction to alpha-adrenoceptor agonists in the late pregnant ovine uterine artery is mediated primarily by alpha 1-adrenoceptors, and there appears to be no substantial alpha 1-adrenoceptor reserve in this tissue. The mediation of contraction via alpha 2-adrenoceptors is not prominent in this tissue.

Intravenous clonidine hydrochloride toxicity in pregnant ewes

Administration of intravenous clonidine hydrochloride has been advocated to rapidly control blood pressure in severe preeclampsia. To examine clonidine's acute maternal and fetal effects were intravenously injected 300 micrograms clonidine in eight chronically prepared normotensive near term ewes. Unlike intravenous saline solution injection, clonidine produced significant toxicity--intraamniotic pressure increased 97 +/- 27% (p less than 0.05), uterine blood flow decreased 55 +/- 7% (p less than 0.001), maternal and fetal serum glucose increased 158 +/- 23% and 249 +/- 91%, respectively (p less than 0.001), and maternal and fetal Po2 decreased to 44 mm Hg +/- 4 mm Hg and 13 mm Hg +/- 1 mm Hg, respectively (p less than 0.05). Maternal and fetal blood pressure and serum cortisol were unaffected by clonidine, whereas heart rate decreased. No adverse maternal or fetal effects were noted with serum clonidine concentrations less than 1.0 ng/ml. Direct fetal infusion of clonidine did not lower fetal arterial Po2 levels, although heart rates decreased and serum glucose levels increased. The multiple effects of clonidine infusion are best explained by actions on alpha 2-adrenergic receptors. These results suggest that intravenous administration of clonidine may adversely affect the fetus by direct actions and by alterations in maternal physiology.