Pharmacological characterization of alpha-adrenoreceptor subtypes in rat isolated thoracic aorta

Contractions induced by phenylephrine and noradrenaline are differently affected by endothelium-dependent relaxation in rat aorta

In rings of rat aorta precontracted with phenylephrine (10 μm) or noradrenaline (10 μm), addition of carbachol (10 μm) produced an endothelium-dependent relaxation. However, regardless of the concentration of agonist tested, both the intensity and duration of the relaxation were significantly less when noradrenaline, rather than phenylephrine, was used as the precontracting agent. The different responses observed do not appear to be related to destruction of endothelium-derived relaxing factor by autoxidation of noradrenaline since neither EDTA (30 μm) nor superoxide dismutase (30 units mL−1) improved the relaxation to carbachol. In addition, in endothelium-free rings, the noradrenaline (1 μm)-induced contraction was less sensitive than the phenylephrine (1 μm)-induced contraction to sodium nitroprusside (0·1 μm) or to 8-Br-cGMP (300 μm). With phenylephrine-, but not noradrenaline-, induced contraction, the relaxation triggered by carbachol was significantly reduced by pretreatment of the aortic rings with chloroethylclonidine (50 μm), which inactivates a subpopulation of α1-adrenoceptors. Thus, the results confirm that both alkylation sensitive and resistant α1-adrenoceptors exist in rat aorta and indicate that EDRF may discriminate between these two α1-adrenoceptor subtypes which are differently affected by phenylephrine and noradrenaline.

Controlling bleeding from superficial wounds by the use of topical alpha adrenoreceptor agonists spray. A randomized, masked, controlled study

In an attempt to determine whether alpha adrenergic agonists sprayed directly over the wound are able to reduce a superficial bleeding, phenylephrine (0.25%), oxymetazoline (0.05 and 0.25%) and saline (0.9%) were tested in a rat model. The study was randomized, controlled and quantitative. A total of four incisions were made in each rat, and each solution was sprayed directly on the incision according to a specific protocol. The bleeding times were measured and summed up. Biases were minimized by the fact that each rat received all four solutions, including the control, in all possible combinations. The mean bleeding time after spraying phenylephrine (0.25%) was significantly shorter than the mean bleeding time after spraying saline (1.90 +/- 0.14 min versus 4.80 +/- 0.43 min, respectively, P < 0.001) and significantly shorter than the mean bleeding time after spraying oxymetazoline (0.05 or 0.25%: 4.46 +/- 0.54 and 5.50 +/- 0.58 min, respectively, P < 0.001). No statistically significant difference was found between the mean bleeding time after spraying oxymetazoline (0.05 or 0.25%) compared with saline. We conclude that sprayed phenylephrine (0.25%) can be used for reducing superficial bleedings. This method is simple, cost-effective, does not cause further trauma to the tissue, and can be used to treat several bleedings simultaneously (especially abrasions and lacerations) with a single application, without the need for direct physical contact with the bleeding sites. The method is apparently safe, but further studies are needed to test the systemic effect of the sprayed solution.

Lack of involvement of pertussis toxin-sensitive G-proteins in norepinephrine-induced vasoconstriction of rat aorta smooth muscle

Several studies have shown that stimulation of pertussis toxin (PTX)-sensitive G-proteins amplified alpha-adrenoceptor (alpha-AR) agonist-induced vasoconstriction in small muscular and resistance arteries. The aim of this study was to assess the potential involvement of PTX-sensitive G-proteins in norepinephrine (NE)-induced constriction in a large diameter artery, the rat aorta. PTX (1 microg/mL, 2 hr; 3 microg/mL, 4 hr) did not modify concentration-response curves to NE in endothelium-denuded aortic rings. However, several lines of evidence suggested that aortic smooth muscle cells (SMC) had a PTX-sensitive G-protein pathway. [alpha-(32)P]ADP-ribosylation of G(i/o)-proteins by PTX (3 microg/mL, 4 hr) was demonstrated in situ in the intact aorta without endothelium. alpha(i/o) subunits were identified in vitro by both immunoblotting and ADP-ribosylation experiments in rat aorta SMC membranes. The measurement of G(i/o)-specific GTPase activity evidenced an effective coupling between NE receptors and G(i/o)-proteins, as NE induced an increase in basal G(i/o)-specific GTPase activity (20.7 +/- 2.8 vs 7.2 +/- 2.2 pmol P(i)/mg protein at 5 min; P < 0.05 vs basal). Co-immunoprecipitation revealed the in vitro coupling between alpha(1D)-ARs and G(i)-protein in rat aorta SMC membranes. In conclusion, we identified a PTX-sensitive G(i/o)-protein pathway in rat endothelium-denuded aorta. We showed an effective coupling between NE receptors and G(i)-proteins via alpha(1D)-ARs. Since PTX has no effect on NE-induced vasoconstriction, the PTX-sensitive G(i)-protein pathway does not play a predominant role in NE-induced responses in rat aorta SMC in contrast to small diameter muscular and resistance arteries.

Alpha-adrenoceptors and vascular regulation: molecular, pharmacologic and clinical correlates

This manuscript is intended to provide a comprehensive review of the alpha-adrenoceptors (ARs) and their role in vascular regulation. The historical development of the concept of receptors and the division of the alpha-ARs into alpha 1 and alpha 2 subtypes is traced. Emphasis will be placed on current understanding of the specific contribution of discrete alpha 1- and alpha 2-AR subtypes in the regulation of the vasculature, selective agonists and antagonists for these receptors, the second messengers utilized by these receptors, the myoplasmic calcium pathways activated to initiate smooth muscle contraction, as well as the clinical uses of agonists and antagonists that work at these receptors. New information is presented that deals with the molecular aspects of ligand interactions with specific subdomains of these receptors, as well as mRNA distribution and the regulation of alpha 1- and alpha 2-AR gene transcription and translation.

Enhanced endothelial nitric oxide activity contributes to the reduced responsiveness of vascular alpha1-adrenoceptors following aortic barodenervation

We have recently shown that short-term aortic barodenervation diminishes constrictor responses to activation of alpha1-adrenoceptors in rat aortic smooth muscle. This study investigated the potential role of vascular endothelium and its derived vasoactive substances, nitric oxide and prostaglandins, in the reduced alpha1-adrenoceptor responsiveness after aortic barodenervation. Exposure of isolated aortic rings from aortic barodenervated and sham-operated rats 48 h after surgery to cumulative addition of phenylephrine (alpha1-adrenoceptor agonist, 3 x 10(-8) - 1 X 10(-4) M) resulted in concentration-related contractions that were significantly (P < 0.05) smaller in rings of denervated rats. Removal of the endothelium increased phenylephrine-mediated contractions in rings obtained from aortic barodenervated rats to near sham-operated levels as demonstrated by the similar contractile responses and slopes of the regression lines of the concentration-response curves. Pretreatment with indomethacin (cyclooxygenase inhibitor, 1 x 10[-5] M) had no effect on contractile responses to phenylephrine in rings from both groups of rats. In contrast, N(G)-nitro-L-arginine (nitric oxide synthase inhibitor, 3 x 10[-5] M) elevated basal vascular tone and significantly (P < 0.05) increased alpha1-adrenoceptor responsiveness, effects that were more evident in rings from denervated compared with sham-operated rats. N(G)-nitro-L-arginine produced significantly (P < 0.05) greater increases in the slopes of the regression lines (136.1 +/- 22% vs. 73.0 +/- 8.6% mg tension/mg tissue/log molar concentration) and maximum contractile response (Emax) to phenylephrine (161.2 +/- 8.2% vs. 76.7 +/- 6.1%) in rings from denervated compared with sham-operated rats suggesting an enhanced nitric oxide activity in aortas of denervated rats. This notion is further supported by the finding that cumulative i.v. administration of N(G)-nitro-L-arginine (1, 2, 4 and 8 mg/kg) elicited significantly (P < 0.05) greater pressor responses in conscious barodenervated compared with sham-operated rats. These results suggest that the endothelium plays a major role in the reduced constrictor responses to alpha1-adrenoceptor activation that occurs shortly after aortic barodenervation. This effect of the endothelium appears to involve, at least in part, enhancement of endothelial nitric oxide activity.

Short-term aortic barodenervation diminishes alpha 1-adrenoceptor reactivity in rat aortic smooth muscle

Our previous studies have shown that aortic baroreceptor denervation elicits acute increases in blood pressure and significant elevations of sympathetic activity and peripheral vascular resistance. This study investigated the short-term (3 and 48 h) effect of aortic barodenervation and associated sympathetic hyperactivity on the functional activity of alpha 1-adrenoceptors in rat aortic smooth muscle. Compared with sham operation, aortic barodenervation caused acute rises in blood pressure and heart rate and reductions in baroreflex sensitivity. Blood pressure and heart rate remained elevated when measured in conscious aortic barodenervated rats 3 h after surgery but subsided to sham-operated levels at 48 h; the baroreflex sensitivity, however, remained attenuated. Hexamethonium (0.5-4 mg/kg, i.v.) elicited significantly (P < 0.05) greater depressor responses in conscious aortic barodenervated rats than in sham-operated rats at both 3 and 48 h, suggesting a higher sympathetic activity in denervated rats. Exposure of aortic rings from aortic barodenervated and sham-operated rats to cumulative addition of phenylephrine (alpha 1-adrenoceptor agonist, 3 x 10(-8)-1 x 10(-4) M) resulted in concentration related contractile responses that were similar in the two groups of rats at 3 h in contrast to significantly (P < 0.05) smaller contractions in rings from denervated rats at 48 h. The maximum contraction developed (Emax) at 48 h showed approximately 50% reduction in rings from aortic barodenervated compared with sham-operated rats (239 +/- 16 vs. 558 +/- 15 mg tension/mg tissue). The pA2 value for prazosin (alpha 1-adrenoceptor antagonist) was not altered by aortic barodenervation at 3 h but showed significant (P < 0.05) increases, compared with sham-operated values, at 48 h. It is concluded that short-term aortic barodenervation results in an elevation of sympathetic activity that coincides with reduced responsiveness of aortic smooth muscle to alpha 1-adrenoceptor activation. The aortic barodenervation-induced alpha 1-adrenoceptor desensitization is not a result of decreased receptor affinity but may involve an alteration of receptor density or in the post-receptor activation events.

Mediation of noradrenaline-induced contractions of rat aorta by the alpha 1B-adrenoceptor subtype

1. The subtypes of alpha 1-adrenoceptor mediating contractions to exogenous noradrenaline (NA) in rat aorta have been examined in both biochemical and functional studies. 2. Incubation of rat aortic membranes with the irreversible alpha 1B-adrenoceptor antagonist, chloroethylclonidine (CEC: 10 microM) did not change the KD of [3H]-prazosin binding in comparison to untreated membranes, but reduced by 88% the total number of binding sites (Bmax). 3. Contractions of rat aortic strips to NA after CEC (50 microM for 30 min) incubation followed by repetitive washing, showed a marked shift in the potency of NA and a partial reduction in the maximum response. The residual contractions to NA after CEC incubation were not affected by prazosin (10 nM). 4. The competitive antagonists prazosin, terazosin, (R)-YM-12617, phentolamine, 5-methylurapidil and spiperone inhibited contractions to NA with estimated pA2 values of 9.85, 8.54, 9.34, 7.71, 7.64 and 8.41, respectively. 5. The affinity of the same antagonists for the alpha 1A- and alpha 1B- adrenoceptors was evaluated by utilizing membranes from rat hippocampus pretreated with CEC, and rat liver, respectively. 5-Methylurapidil and phentolamine were confirmed as selective for the alpha 1A-adrenoceptors, whereas spiperone was alpha 1B-selective. 6. A significant correlation was found between the pA2 values of the alpha 1-adrenoceptor antagonists tested and their affinity for the alpha 1B-adrenoceptor subtype, but not for the alpha 1A-subtype. 7. In conclusion, these findings indicate that in rat aorta most of the contraction is mediated by alpha 1B-adrenoceptors, and that the potency (pA2) of an antagonist in this tissue should be related to its antagonistic effect on this subtype of the alpha 1-adrenoceptor population.

Calcium antagonists and vasodilatation

Calcium antagonists comprise a diverse group of chemically unrelated agents that interact with voltage-operated calcium channels (L-type) and thereby inhibit smooth muscle contractility. They are used to treat several major cardiovascular disorders, including hypertension and angina pectoris; they are also studied in congestive heart failure and in atherosclerosis. The current view is that their therapeutic action is related to vasodilatation. This view is an oversimplification, as will be shown in this review. It will also be illustrated that all calcium antagonists are not identical pharmacological agents.

The role of endothelial cells on the alterations in vascular reactivity induced by insulin-dependent diabetes mellitus: effects of insulin treatment

1. In this study, the maximal contractile responses to noradrenaline (NA) were significantly increased but pD2 values (sensitivity) were not significantly altered in aortas with endothelium from insulin-dependent (ID)-diabetic rats when compared to their controls. 2. Removal of the endothelium resulted in a significant increase in the maximum response of control and ID-diabetic aortas and also loss of the difference in maximum contractile response to NA between ID-diabetic aortas and their corresponding controls. 3. Endothelium-dependent relaxation stimulated by acetylcholine (ACh), methacholine, histamine, ATP and insulin, in aortic rings precontracted with NA were significantly attenuated but unchanged by A23187 in ID-diabetic vessels relative to controls. 4. Relaxations produced by sodium nitroprusside (SNP) in ID-diabetic aortas were similar to those in control vessels. 5. The sensitivity of ID-diabetic aortas to relaxant substances were not changed when compared with controls. 6. These results demonstrate that ID-diabetes-induced specific changes in vascular reactivity and endothelial cells have an important role on the maintenance of vascular tonus. This feature may be responsible for the major complications of diabetes such as macroangiopathy, vasospasm and high blood pressure in late stage of the disease.

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.

Effects of endothelial cell removal on alpha-adrenoceptor-mediated responses of aortae of pregnant rats

1. The possibility that changes in sensitivity of the aorta occurring during pregnancy in the rat are due to changes in uptake mechanisms, alpha- and/or beta-adrenoceptor-mediated responses was investigated. 2. Thoracic aortic rings from 20 day pregnant rats showed increased sensitivity to the constrictor effects of phenylephrine, with increased maximum responses when compared with those from non-pregnant animals. Removal of endothelium caused leftward shifts of the log concentration-response curves with further increases in maxima to the same extent in rings of both non-pregnant and 20 day pregnant rats. 3. Propranolol, beta-oestradiol, and nisoxetine did not significantly alter the CR-curves to phenylephrine of endothelium-intact or denuded thoracic aortic rings obtained from either non-pregnant or 20 day pregnant rats. 4. BHT-920 failed to cause marked constriction of endothelium-intact rings. After removal of endothelium, significant constrictor responses to BHT-920 occurred which were of similar magnitude for rings from both non-pregnant and 20 day pregnant rats. 5. Relaxant responses to BHT-920 of endothelium-intact rings preconstricted with phenylephrine were not significantly different between those from non-pregnant and 20 day pregnant rats. Removal of endothelium resulted in rightward shifts of the curves together with decreased maximum responses. 6. These results support earlier suggestions that the endothelium plays a role in controlling vascular reactivity to phenylephrine. The increased maximum response of thoracic aortic rings to phenylephrine seen during pregnancy does not appear to be due to marked changes in amine uptake mechanisms or beta- or alpha 2-adrenoceptor-mediated responses.

Do both adrenaline and noradrenaline stimulate cardiac alpha-adrenoceptors to induce positive inotropy of rat atria?

1. The positive inotropic responses of rat paced left atria to adrenaline and noradrenaline were recorded. Desmethylimipramine (DMI, 1 microM) and metanephrine (10 microM) were initially present throughout. 2. The positive chronotropic responses of spontaneously beating right atria to adrenaline were used as a reference. In these, pindolol, in increasing concentrations, caused progressive shift of the concentration-response curves to the right, which yielded a pA2 value (8.15) compatible with antagonism of beta-adrenoceptors. 3. The left atrial tension responses to adrenaline showed an initial progressive displacement by pindolol (up to 3 microM) which gave an unexpectedly low pA2 value (6.48). However, with further increases in pindolol concentration there was no additional shift of the curve. In the presence of pindolol (3 microM), prazosin (0.1 microM) displaced the curve to the right but the pA2 value derived from this shift (7.75) was less than expected for alpha 1-adrenoceptor antagonism. 4. When the experiments in the presence of pindolol (3 microM) were repeated in the absence of DMI, prazosin displaced the concentration-response curves for adrenaline-induced left atrial tension to a greater extent and the pA2 value (8.76) was now compatible with adrenaline stimulating typical alpha 1-adrenoceptors. 5. The concentration-response curves for noradrenaline-induced left atrial tension were also progressively displaced to the right by pindolol (0.1, 0.3 and 1.0 microM). These concentrations yielded a Schild plot of unity slope and a pA2 value of 7.94 +/- 0.04. This was not significantly different from the pA2 value of 8.02 +/- 0.07 determined for pindolol against isoprenaline in the left atria, which indicates a normal interaction of noradrenaline with beta-adrenoceptors in the absence and presence of low concentrations of pindolol. 6. A further increase in the concentration of pindolol to 3 microM failed to induce an additional shift of the noradrenaline curves, whether a 'before and after' antagonist or a 'naïve tissue' design was adopted. Similarly, the rightwards shift of the concentration-response curves by timolol reached a limit as the concentration was increased. In all cases the limit of shift occurred at a noradrenaline EC50 value of 5-10 microM. 7. At the limit of beta-adrenoceptor antagonism, prazosin and dibenamine did not displace the noradrenaline curves further. The residual inotropic response to noradrenaline therefore appeared to be mediated via neither alpha- nor beta-adrenoceptors. 8. DMI, in the absence of beta-blockade, produced the potentiation of adrenaline and noradrenaline expected of a neuronal uptake inhibitor. However, in the presence of pindolol, there was no potentiation of the right atrial rate response to adrenaline while its left atrial tension responses were antagonized. This suggested that DMI was acting as an alpha-adrenoceptor antagonist. It also explained the less-than-expected shift by prazosin of the adrenaline responses in the presence of both pindolol and DMI, the latter drug already exerting some alpha-blocking activity. In contrast, the left atrial tension responses to noradrenaline in the presence of pindolol (1 microM) were neither potentiated nor antagonized by DMI. 9. When the effects of prazosin upon left atrial tension responses to noradrenaline in the presence of pindolol (10 microM) were examined in the presence of a lower concentration of DMI (O.1 microM) or cocaine (1O microM), again there was no further shift of the curve. However, when the effect of prazosin) The Macmillan Press Ltd 1989 598 K.L. WILLIAMSON & K.J. BROADLEY was examined in the absence of DMI, but in the presence of pindolol (1 and 1O microM) or timolol (3 microM), there was a small shift of the curves by prazosin (0.1 microM). This yielded pA2 values of 7.19, 7.34 +/- 0.1 and 7.66 +/- 0.09, which were at least one order of magnitude less than literature values and that obtained with adrenaline (8.76 +/- 0.18), and are not consistent with noradrenaline stimulating an alpha 1-adrenoreceptor in the presence of beta-adrenoceptor blockade, the increase in left atrial tension by noradrenaline does not appear to be mediated by beta l- or typical alpha-adrenoceptors. This is in contrast to adrenaline which in these conditions stimulates typical alpha 1-adrenoceptors.

The modulatory role of vascular endothelium in the interaction of agonists and antagonists with alpha-adrenoceptors in the rat aorta

1. We have examined the effect of endothelium on the antagonistic action of prazosin, doxazosin, yohimbine and phentolamine against phenylephrine, clonidine and noradrenaline. 2. The action of prazosin against phenylephrine was similar to that earlier reported against noradrenaline, acting as a non-competitive antagonist in the presence of endothelium and as a competitive antagonist in the absence of endothelium. Prazosin also acted as a non-competitive antagonist against clonidine in the absence of endothelium. 3. Doxazosin behaved in a similar way to prazosin against noradrenaline, phenylephrine and clonidine acting as a non-competitive antagonist in the presence of endothelium and as competitive antagonist after removal of endothelium. In contrast, yohimbine and phentolamine acted as competitive antagonists both in the presence and in the absence of endothelium. 4. Analysis of the concentration-response curves for noradrenaline, phenylephrine and clonidine in the presence and in the absence of endothelium showed that the affinity for all three agonists was the same but not the efficacy and the receptor reserve, both of which were lower in the presence than in the absence of endothelium. 5. The rank order of agonist potency in the absence of endothelium was noradrenaline greater than phenylephrine greater than clonidine. The rank order of antagonist potency was prazosin greater than or equal to doxazosin greater than phentolamine greater than yohimbine. 6. The results show that vascular endothelium modulates the contractile response to alpha-adrenoceptor agonists and also modifies the action of the antagonists prazosin and doxazosin but not that of yohimbine and phentolamine. This effect of endothelium was related to a change in agonist efficacy and receptor reserve. These results also suggest that the alpha-adrenoceptors of the isolated aorta of the rat are predominantly, if not exclusively of the alpha 1-subtype.

Activities of octopamine and synephrine stereoisomers on alpha-adrenoceptors

1. The activities of the (-)- and (+)-forms of m- and p-octopamine and m- and p-synephrine on alpha 1-adrenoceptors from rat aorta and anococcygeus and alpha 2-adrenoceptors from rabbit saphenous vein were compared with those of noradrenaline (NA). 2. The rank order of potency of the (-)-forms on alpha 1-adrenoceptors from rat aorta and alpha 2-adrenoceptors was NA greater than m-octopamine = m-synephrine greater than p-octopamine = p-synephrine. The two m-compounds were 6 fold less active than NA on alpha 1-adrenoceptors from rat aorta and 150 fold less active on alpha 2-adrenoceptors. The two p- compounds were 1,000 fold less active than NA on both alpha 1-adrenoceptors from rat aorta and alpha 2-adrenoceptors. The rank order of potency of the (-)- forms on alpha 1-adrenoceptors from rat anococcygeus was NA = m-synephrine greater than m-octopamine greater than p-octopamine = p-synephrine. m-Octopamine was 4 fold less active than NA and (-)-m-synephrine. The two p- compounds were 30 fold less active than NA. 3. The rank order of potency of the (+)- forms was NA greater than m-octopamine greater than m-synephrine greater than p-octopamine greater than p-synephrine on both alpha 1- and alpha 2-adrenoceptors. The potency of each (+)- form was 1-2 orders of magnitude less than that of the (-) counterpart, the differences being greater for the stereoisomers of synephrine than for those of octopamine on both alpha 1- and alpha 2-adrenoceptors. 4. The yohimbine diastereoisomer antagonists, rauwolscine and corynanthine, were tested against (-)-NA and (-)-m-octopamine-induced contractions in both preparations. Based upon the known selectivities of these isomers for alpha-adrenoceptor subtypes, it is concluded that the rat aorta contains only alpha 1-adrenoceptors while the rabbit saphenous vein possesses predominantly alpha 2-adrenoceptors. 5. Ligand binding data for the octopamine and synephrine stereoisomers at alpha 1- and alpha 2-binding sites from rat cerebral cortex was also obtained. (-)-Forms were more active than (+)-forms. The rank order of affinity of the (-)-forms for both alpha 1- and alpha 2-binding sites was NA greater than m-octopamine = m-synephrine greater than p-synephrine greater than p-octopamine. The relative affinities of the members of the series against alpha 1-binding sites were very similar to their relative functional activities on rat aorta. However, the affinities of both m- and p-compounds relative to that of ( -)-NA were much greater at the x2-binding sites than were the relative activities in rabbit saphenous vein, possibly suggesting low intrinsic efficacy. Functional antagonist responses to NA by the (-)-octopamine and synephrines could not, however, be demonstrated on rat aorta or rabbit saphenous vein. 6. The activities of m-octopamine and m-synephrine were not significantly different from each other on either a,-adrenoceptors from rat aorta or x2-adrenoceptors; however, m-synephrine is more active than m-octopamine on a,-adrenoceptors from rat anococcygeus. Both m-octopamine and msynephrine can be considered to be naturally occurring x,-selective amines. However, if m- and poctopamine are co-released with NA in amounts proportional to their concentration, it is concluded that their activities on m,- and x2-adrenoceptors are too low to be physiologically significant.

Role of cyclic GMP in the modulation by endothelium of the adrenolytic action of prazosin in the rat isolated aorta

The effect of endothelium on the adrenolytic action of prazosin was studied in the rat isolated aorta. Prazosin showed a non-competitive type of antagonism in preparations with intact endothelium while in preparations where endothelium had been removed, prazosin at concentrations between 0.3 nM-10 nM acted as a competitive antagonist. Methylene blue, used to decrease tissue levels of guanosine 3':5'-cyclic monophosphate (cyclic GMP), converted prazosin from a non-competitive antagonist into an apparently competitive antagonist in the presence of endothelium. Increasing tissue levels of cyclic GMP by incubation with 8-bromo-cyclic GMP converted prazosin from an apparently competitive antagonist into a non-competitive antagonist in the absence of endothelium. Analysis of concentration-response curves for noradrenaline in the presence and absence of endothelium showed that the affinity for noradrenaline was the same but the efficacy, measured by estimating the receptor reserve, was not; it was lower in the presence than in the absence of endothelium. It was concluded that the change in the mode of antagonism of prazosin after endothelium removal could be related to an alteration in the efficacy of the agonist, brought about by a change in the tissue levels of cyclic GMP.

A study on the postjunctional excitatory alpha-adrenoreceptor subtypes in the mesenteric arterial bed of the rat

The nature of the postsynaptic adrenoreceptor subtypes which mediate vasoconstriction in the mesenteric arterial bed of the rat was investigated using mixed and selective alpha 1, alpha 2 and beta-agonists and antagonists. Phenylephrine (PE) an alpha 1-selective agonist and noradrenaline (NA) a mixed alpha1 and alpha 2-agonist, produced a rise in perfusion pressure (vasoconstriction). The responses to NA remained stable with time whereas responses to PE considerably increased. UK14304 an alpha 2-selective agonist at low doses (10(-8)-10(-7) moles), caused small, slow contractions in most preparations. Repeated administration of these doses or slightly higher ones, densensitized the tissue to this compound but not to NA or PE. Finally, UK14304 given simultaneously with NA or PE, at doses higher than 5 X 10(-7) moles, reduced contractions to the latter compounds and this effect was not altered by 10(-7) M rauwolscine, an alpha 2-selective antagonist. Prazosin, an alpha1-selective antagonist, as expected, reduced contractions to NA considerably at 10(-10)-10(-8) M and abolished contractions to UK14304 at 2 X 10(-9) M. Rauwolscine, at 10(-8) M, potentiated contractions to NA and at 10(-6) M reduced contractions to both NA and PE (when compared to time controls). When propranolol (10(-6) M), a beta-antagonist was included in the perfusion fluid, rauwolscine no longer potentiated responses to NA but reduced them at all concentrations. Under the same conditions rauwolscine affected the responses to PE in a similar direction to that observed in the absence of propranolol. These results suggest that in the rat mesenteric arterial bed: rauwolscine exerts an effect additional to alpha2-adrenoreceptor antagonism.(ABSTRACT TRUNCATED AT 250 WORDS)