An attempt at selective protection from phenoxybenzamine of postjunctional alpha-adrenoceptor subtypes mediating contractions to noradrenaline in the rabbit isolated saphenous vein

Contractile responses to sumatriptan and ergotamine in the rabbit saphenous vein: effect of selective 5-HT(1F) receptor agonists and PGF(2alpha)

Contractile responses to ergotamine, sumatriptan and the novel 5-HT(1F) receptor agonists, LY334370 and LY344864 were examined using the rabbit saphenous vein. Ergotamine (pEC(50)=8.7+/-0.06) was 30 fold more potent than 5-hydroxytryptamine (5-HT) (pEC(50)=7.2+/-0.13) and 300 fold more potent than sumatriptan (pEC(50)=6.0+/-0.08) in contracting the rabbit saphenous vein in vitro. The selective 5-HT(1F) receptor agonists, LY334370 or LY344864 (up to 10(-4) M), did not contract the rabbit saphenous vein. The contractile response to ergotamine in this tissue resulted from activation of both alpha(1) and 5-HT(1B/1D) receptors based on the observation that prazosin (10(-6) M), an alpha-adrenoceptor antagonist, and GR127935 (10(-8) M) a 5-HT(1B/1D) receptor antagonist, dextrally shifted the contractile response to ergotamine. In contrast, prazosin (10(-6) M) did not alter contraction to sumatriptan whereas GR127935 (10(-8) M) was a potent antagonist (-log K(B)=10.0) suggesting that sumatriptan-induced contraction of the rabbit saphenous vein was mediated only by activation of receptors similar or identical to 5-HT(1B/1D) receptors. PGF(2alpha) (3x10(-7) M) produced a modest increase (approximately 5.0 - 10.0% maximum PGF(2alpha) contraction) in saphenous vein force. Precontraction with PGF(2alpha) (3x10(-7) M) dramatically augmented the potency and maximal contractile response to sumatriptan (pEC(50)=7.1) and modestly enhanced the contractile potency of ergotamine (pEC(50)=9.0) in the rabbit saphenous vein. However, PGF(2alpha) (3x10(-7) M) only unmasked a contraction to the 5-HT(1F) receptor agonists when concentrations exceeded 10(-5) M, concentrations considerably higher than their 5-HT(1F) receptor affinities. LY334370 (10(-6) M) pretreatment did not alter contraction to either sumatriptan or ergotamine and a higher concentration (10(-5) M) of LY334370 or LY344864 inhibited contraction to sumatriptan. Thus, activation of 5-HT(1F) receptors will not induce vascular contraction (either alone or following modest tone with PGF(2alpha)) or augment contraction to other contractile agonists in the rabbit saphenous vein.

Characterization of human recombinant alpha(2A)-adrenoceptors expressed in Chinese hamster lung cells using intracellular Ca(2+) changes: evidence for cross-talk between recombinant alpha(2A)- and native alpha(1)-adrenoceptors

1. Human alpha(2A)-adrenoceptors expressed in Chinese hamster lung (CHL) fibroblasts have been pharmacologically characterized by measuring intracellular calcium (Ca(2+)(i)) changes using the Ca(2+)-sensitive dye Fluo3-AM, in conjunction with a fluorometric imaging plate reader (FLIPR). 2. Several alpha-adrenoceptor agonists were examined including the alpha(2)-adrenoceptor agonists UK-14304, B-HT 920, dexmedetomidine and A-54741, the selective alpha(1)-adrenoceptor agonist phenylephrine and the non-selective adrenergic agonist noradrenaline. Of these only noradrenaline (mean pEC(50)=6.49) and A-54741 (6.90) evoked changes in Ca(2+)(i); A-54741 was a partial agonist relative to noradrenaline, achieving only 33% of the noradrenaline maximum. 3. Ca(2+)(i) changes induced by noradrenaline and A-54741 were antagonized by the alpha(2)-selective antagonist rauwolscine (10 nM) and by the alpha(1)-selective antagonists prazosin (0.1 nM) and doxazosin (1.0 nM). 4. Phenylephrine (100 microM) and UK-14304 (10 microM) alone were ineffective in causing Ca(2+)(i) increase. In the presence of a fixed concentration of UK-14304 (3.0 microM), phenylephrine induced concentration-dependent increases in Ca(2+)(i) (mean pEC(50)=5.33). In the presence of phenylephrine (30.0 microM) UK-14304 induced Ca(2+)(i) release (pEC(50)=6.92). The effects of phenylephrine were abolished by prazosin (1.0 nM) or rauwolscine (100 nM). 5. In saturation radioligand binding experiments using membranes of parental (non-transfected) CHL cells there was a small, specific binding of [(3)H]-prazosin (B(max)=24 fmol mg protein(-1); pK(D)=10. 24). 6. Collectively, these data suggest that alpha-adrenoceptor agonist-induced Ca(2+)(i) release in CHL fibroblasts transfected with the human alpha(2A)-adrenoceptor is dependent upon co-activation of the recombinant receptor and a native alpha(1)-adrenoceptor.

Alpha-adrenoceptor subtypes of dog saphenous vein: another unusual property

The functional relationship between vascular smooth muscle alpha1- and alpha2-adrenoceptor (AR) subtypes was investigated by simultaneous measurement of contractile and fluorescence ratio in fura-2 loaded rings of dog saphenous vein (DSV). Prazosin, as well as rauwolscine, at 0.1 microM, substantially antagonized contractions and associated cytosolic [Ca2+] rises induced by UK 14304, while rauwolscine, as well as prazosin, antagonized similar effects of phenylephrine (PE). These antagonisms were characterized by a parallel rightward shift of the concentration-response curves. In the absence of extracellular Ca2+, PE as well as UK 14304 caused simultaneous transient elevation of contractile force and cytosolic [Ca2+], although the UK 14304 responses were smaller than PE responses. We propose that DSV smooth muscle cells possess interacting alpha1- and alpha2-ARs which have overlapping functional domain sensitive to the agonists and antagonists of either alpha-AR subtype. Both alpha-AR subtypes appear to utilize similar signaling mechanisms via Ca2+ release from the same intracellular stores and Ca2+ entry across the plasma membrane.

Characterization of alpha 2-adrenoceptors mediating contraction of dog saphenous vein: identity with the human alpha 2A subtype

1. In the dog saphenous vein alpha 1- and alpha 2-adrenoceptors mediate noradrenaline-induced contractions in vitro. In order to study the alpha 2-adrenoceptor in isolation, alpha 1-adrenoceptors were inactivated by treatment of tissues with the alkylating agent phenoxybenzamine (3.0 microM for 30 min) in the presence of rauwolscine (1 microM) to protect alpha 2-adrenoceptors. 2. Noradrenaline-induced contractions of tissues treated with phenoxybenzamine were antagonized competitively by the selective alpha 2-adrenoceptor antagonist rauwolscine, pKB = 8.63 +/- 0.07 (means +/- s.e. mean; n = 3), consistent with an interaction at alpha 2-adrenoceptors. 3. Noradrenaline was a full agonist at alpha 2-adrenoceptors in dog saphenous vein. By use of the method of partial receptor alkylation and analysis of concentration-effect curve data by direct, operational model fitting methods, the affinity (pKA) and efficacy (tau) were 5.74 +/- 0.07 and 7.50 +/- 1.05, respectively (n = 6). Nine other agonists which were examined each had affinities higher than noradrenaline, but with the exception of the imidazoline, A-54741 (5,6-dihydroxy-1,2,3,4-tetrahydro-1-naphthyl-imidazoline) had relatively lower efficacies. 4. To compare the alpha 2-adrenoceptor in dog saphenous vein to the human recombinant subtypes, the affinities of twenty-one compounds were estimated in functional studies in the dog saphenous vein and in radioligand binding studies for the human alpha 2A, alpha 2B and alpha 2C receptor subtypes expressed in Chinese hamster lung (CHL) cells. 5. Of twenty-one compounds examined in ligand binding studies, only nine had greater than ten fold selectivity for one human receptor subtype over either of the other two. These compounds were A-54741, oxymetazoline, guanfacine, guanabenz, prazosin, spiroxatrine, tolazoline, WB 4101 and idazoxan. In dog saphenous vein, their affinities (pKA and pKB for agonists and antagonists respectively) were: A-54741 (pKA = 8.03 +/- 0.05), oxymetazoline (pKA = 7.67 +/- 0.09), guanfacine (pKA = 6.79 +/- 0.03); guanabenz (pKA = 7.02 +/- 0.13); prazosin (pKB = 5.19 +/- 0.08), spiroxatrine (pKB = 6.59 +/- 0.04), tolazoline (pKB = 6.21 +/- 0.07), WB 4101 (pKB = 7.42 +/- 0.09) and idazoxan (pKB = 7.11 +/- 0.08). 6. Comparisons of affinity estimates for these nine compounds at the receptor in dog saphenous vein and at the human recombinant subtypes suggest that the vascular receptor is most similar to the h alpha 2A subtype; correlation coefficients (r) were 0.82 (h alpha 2A), 0.24 (h alpha 2B) and 0.04 (h alpha 2C).

Pharmacological characterization of noradrenaline-induced contractions of the porcine isolated palmar lateral vein and palmar common digital artery

1. The aim of this study was to examine the pharmacological characteristics of alpha-adrenoceptor-mediated contractions in two porcine isolated blood vessels, the palmar lateral vein (PLV) and the palmar common digital artery (PCDA). This was carried out with noradrenaline used as the agonist throughout, and either phentolamine (non-selective alpha-adrenoceptor antagonist), prazosin and YM-12617 (selective alpha 1-adrenoceptor antagonists) or rauwolscine and CH-38083 (selective alpha 2-adrenoceptor antagonists). 2. Noradrenaline (0.003-10 microM) produced concentration-dependent contractions in both vessels, with the PCDA (pD2 = 6.33 +/- 0.07, n = 10) being approximately 10 fold less sensitive to noradrenaline compared to the PLV (pD2 = 7.39 +/- 0.09, n = 8). Also, the maximal response to noradrenaline was greater in the PCDA compared to the PLV. Phentolamine (0.03-30 microM) produced parallel rightward shifts in the CRC to noradrenaline in both tissue preparations. The pA2 values were similar and slopes of the Schild plots were not significantly different from unity, indicating an interaction between phentolamine and a single receptor in each preparation. 3. In the PCDA the alpha 1-adrenoceptor antagonists, prazosin (0.01-1 microM) and YM-12617 (0.01-1 microM) produced non-parallel rightwards shifts in the CRC to noradrenaline, with the lower 10-15% of the CRC exhibiting greater resistance to the effects of these antagonists compared to the upper part. In contrast, rauwolscine (1-10 microM) and CH-38083 (10 microM) produced parallel displacement of the CRC to noradrenaline. In the PLV, low concentrations of either alpha l- (0.01 microM) or alpha2-adrenoceptor antagonists(0.1-1 microM) produced a large shift in the CRC, but subsequent higher concentrations had only small additional effects. Based upon pKB values estimated from the effects of the lower concentrations of antagonists, the results are consistent with a large population of alpha1-adrenoceptors in the PCDA and a mixture of alpha l- and alpha2-adrenoceptors in the PLV.4. In both tissues, when an ac,- and an a2-adrenoceptor antagonist were used in combination the effect produced was greater than that with either agent alone. In contrast, the combination of the alpha1-adrenoceptor antagonists (prazosin and YM-12617 together) or the alpha2-adrenoceptor antagonists (CH-38083 and rauwolscine together) were no more effective than that produced by the individual antagonists. These findings suggest the presence of functional alpha l- and alpha2-adrenoceptors in the PLV andPCDA.5. Phenoxybenzamine (0.3-3 microM, 60min exposure) produced a concentration-dependent reduction in the maximal response to noradrenaline which was more pronounced in the PCDA than the PLV. After a 60 min exposure to a combination of phenoxybenzamine (1 microM) and rauwolscine (1 microM), the remaining NA-induced contraction after washout was resistant to prazosin (0.1 microM) and sensitive to rauwolscine(1 microM) in both tissue preparations, indicating the existence of functional alpha2-adrenoceptors in both vessels.6. Evidence suggests that post-junctional alpha l- and alpha2-adrenoceptors contribute to noradrenaline-induced contractions in the PCDA and PLV, with the latter possessing a larger population of functional alpha2-adrenoceptors.

A comparison of the effects of angiotensin II and Bay K 8644 on responses to noradrenaline mediated via postjunctional alpha 1-and alpha 2-adrenoceptors in rabbit isolated blood vessels

1. The effects of angiotensin II (AII) and Bay K 8644 on responses to noradrenaline (NA) mediated via postjunctional alpha 1- and/or alpha 2-adrenoceptors have been compared in three isolated venous preparations from the rabbit, the lateral saphenous vein, the left renal vein and the ear vein. 2. A similar action of AII and Bay K 8644 was observed only in the lateral saphenous vein; each potentiated responses to NA after isolation of a homogeneous population of postjunctional alpha 2- adrenoceptors. However, even in this preparation the mechanism of action for these agents was not identical. The sensitivity of KCl-induced contraction to changes in extracellular calcium ions (reflecting activation of voltage-dependent Ca2+ channels) was enhanced by Bay K 8644 but reduced by AII. 3. All produced a selective facilitation of responses mediated via postjunctional alpha 2-adrenoceptors. In the lateral saphenous vein it reduced the effectiveness of prazosin and facilitated responses after isolation of alpha 2-adrenoceptors with phenoxybenzamine and rauwolscine. It directly enhanced responses to NA in the ear vein, where only alpha 2-adrenoceptors are involved. In contrast, AII did not influence responses mediated via postjunctional alpha 1-adrenoceptors in the left renal vein (even after the receptor reserve had been removed with phenoxybenzamine) nor the 'rauwolscine-resistant' component of responses to NA in the saphenous vein. 4. Bay K 8644 enhanced contractile responses to NA mediated both via alpha 2-adrenoceptors, in the lateral saphenous vein, and via alpha 1-adrenoceptors in the left renal vein. Thus, unlike angiotensin II, no preferential effect was apparent. 5. Bay K 8644 was inactive against responses to NA in the rabbit isolated ear vein. Since the sustained component of responses to NA in this preparation is dependent upon the influx of extracellular Ca2 , these observations suggest that the influx of Ca2+ stimulated by NA is mediated via receptor-operated (1,4-dihydropyridine-resistant) Ca2 + channels.

The effects of nifedipine on alpha 2-adrenoceptor-mediated contractions in several isolated blood vessels from the rabbit

1. The effects of the dihydropyridine calcium channel blocker, nifedipine, on noradrenaline-induced contractile responses have been examined in several isolated blood vessels from the rabbit, with particular emphasis on responses mediated via postjunctional alpha 2-adrenoceptors. 2. In the isolated renal vein, ear vein, distal saphenous artery, saphenous vein and plantaris vein, 0.1 microM and 1 microM nifedipine reduced responses elicited by 54 mM KCl by more than 70%. The remaining responses were abolished by alpha-adrenoceptor blockade, suggesting the involvement of noradrenaline released from neurones activating a dihydropyridine-resistant mechanism. 3. In the renal vein (alpha 1-), ear vein (predominantly alpha 2-), distal saphenous artery (alpha 1- greater than alpha 2-), saphenous vein and plantaris vein (alpha 2- greater than alpha 1-), 0.01 microM and 0.1 microM nifedipine produced concentration-related reductions in the maximum response to noradrenaline. However, 1 microM nifedipine was no more effective than 0.1 microM nifedipine and the reduction in the maximum varied from 10-25% of the control response. Thus, a sizeable component of the alpha-adrenoceptor-mediated response in all blood vessels is resistant to dihydropyridine calcium channel blockers and this appears to be unrelated to the alpha-adrenoceptor subtype involved. 4. Following irreversible inactivation of alpha 1-adrenoceptors and isolation of functional alpha 2-adrenoceptors in the saphenous vein, plantaris vein and distal saphenous artery (the latter requiring the presence of angiotensin II), the effect of nifedipine on responses to noradrenaline was increased. However, a component of the alpha 2-adrenoceptor response in each preparation was present even after the concentration of nifedipine was increased to 1 microM. 5. In the saphenous vein, a preparation in which it has been demonstrated previously that alpha 2-adrenoceptor-mediated responses are highly dependent upon the presence of extracellular calcium ions, partial depolarization with 20mM KCl failed to increase the inhibitory effect of 0.1 microM nifedipine. This suggests the involvement of dihydropyridine-resistant Ca2+ channels. The possible relationship between these dihydropyridine-resistant Ca2+ channels, alpha-adrenoceptor subtypes and 'receptor-operated' Ca2 + channels is discussed.

Postjunctional alpha-adrenoceptors in the rabbit isolated distal saphenous artery: indirect sensitivity to prazosin of responses to noradrenaline mediated via postjunctional alpha 2-adrenoceptors

1. Under normal experimental conditions, the rabbit isolated distal saphenous artery appears to contain a homogeneous population of postjunctional alpha 1-adrenoceptors. Prazosin competitively antagonized responses to noradrenaline (NA) with a pA2 value of 8.6, while a relatively high concentration of rauwolscine (1 microM), produced only a 2 fold rightward displacement of the NA cumulative concentration-response curve (CCRC). 2. Despite the fact that angiotensin II (AII) was without effect on responses to NA or phenylephrine in this preparation, this peptide made responses to NA less susceptible to the antagonistic action of prazosin. This was particularly evident on the lower portion of the CCRC for NA. These results suggest that in the presence of AII, NA produces contractile responses by an action mediated through a prazosin-resistant adrenoceptor. 3. An attempt was made to isolate a homogeneous population of postjunctional alpha 2-adrenoceptors by use of a receptor protection procedure involving the combination of rauwolscine and phenoxybenzamine. After the protection protocol no responses were observed to the alpha-adrenoceptor agonists NA, phenylephrine or UK-14304. In the presence of angiotensin II however, concentration-dependent contractions were observed to each of these agonists. Under these conditions the rank order of potency, UK-14304 greater than NA greater than phenylephrine, is consistent with that of an effect mediated through postjunctional alpha 2-adrenoceptors. 4. The responses to NA, after the protection protocol, in the presence of AII, were susceptible to the selective alpha 2-adrenoceptor antagonist, rauwolscine (1 microM), but resistant to the selective alpha 2-adrenoceptor antagonist prazosin (0.1 microM). Furthermore, the combination of rauwolscine (1 microM) and prazosin (0.1 I microM) was no more effective in blocking responses to NA than was rauwolscine (1 microM) alone. These results are consistent with the presence of a homogeneous population of postjunctional alpha 2-adrenoceptors. 5. Inducing a small degree of tone with a low concentration of the selective alpha 1-adrenoceptor agonist, phenylephrine, markedly increased the threshold sensitivity to the selective alpha 2-adrenoceptor agonist UK- 14304, in a manner analogous to that seen with All. 6. The results in the present study indicate that responses mediated via postjunctional alpha 2-adrenoceptors in the rabbit isolated distal saphenous artery are dependent upon a degree of vascular smooth muscle stimulation by some other receptor system. It is hypothesized that under normal experimental conditions, this function is fulfilled by stimulation of alpha l-adrenoceptors, while after alpha 1-adrenoceptor blockade the necessary positive influence can be provided by stimulation of All receptors. The implications for such an interaction between postjunctional alpha-adrenoceptor subtypes in demonstrating prazosin-resistant, rauwolscine- or yohimbine-sensitive responses in isolated blood vessels is discussed.

Alpha-adrenoceptor subtypes in the coronary circulation

The pathophysiological role of sympathetic coronary innervation in myocardial ischemia is not clear, probably due to the complexities of adrenergic vascular control. In the canine coronary bed in vivo under beta-adrenergic blockade, alpha 1- as well as alpha 2-adrenoceptor-mediated constrictions can be elicited with predominance of the former in the epicardial conductance arteries, and of the latter in coronary resistance vessels. However, this distribution of functional responsiveness cannot indicate distribution of receptor density and cannot remain unchanged under differing conditions. First, each of these two classes of alpha-adrenoceptors consists of a mixture of different, interacting subtypes; second, the smooth muscular responsiveness to these two classes of alpha-adrenoceptors is differently modulated by contractile preactivation, by beta 2-blockade, and by the influence of sympathetic cotransmitters; third, alpha-adrenoceptors on endothelial cells and on sympathetic nerve endings can substantially modulate sympathetic coronary constriction. Thus, this neurogenic coronary control possesses a great functional plasticity, which is not yet fully evaluated with the presently available pharmacological tools.

Influence of angiotensin II on the alpha-adrenoceptors involved in mediating the response to sympathetic nerve stimulation in the rabbit isolated distal saphenous artery

Under normal experimental conditions, sympathetic nerve-mediated responses to electrical field stimulation in the isolated distal saphenous artery of the rabbit are sensitive to prazosin (0.1 microM) and so, by definition, are mediated by alpha 1-adrenoceptors. In the presence of angiotensin II (A II, 0.05 microM) however, a component of the response to nerve stimulation became resistant to prazosin. This 'uncovered' response was virtually abolished by the selective alpha 2-adrenoceptor antagonist rauwolscine (1 microM), a concentration that in the absence of A II had enhanced nerve-mediated responses. Exposure to A II therefore, allows the clear demonstration of a role for postjunctional alpha 2-adrenoceptors in mediating the contractile response to sympathetic nerve stimulation in this arterial preparation.

Actions of non-peptide ergot alkaloids at 5-HT1-like and 5-HT2 receptors mediating vascular smooth muscle contraction

This report describes the actions of the non-peptide ergot alkaloids methysergide, methylergometrine and ergometrine at two types of 5-HT receptor mediating vascular contraction; the well established 5-HT2 receptor in rabbit aorta and a non-5-HT2 receptor in rabbit saphenous vein which resembles the 5-HT1-like receptor in dog saphenous vein. In the rabbit aorta ergometrine (1 mumol/l) and methylergometrine (0.3 mumol/l), but not methysergide, produced small contractions (14% and 7% respectively of the maximal response to 5-HT). This contraction was not related to activation of 5-HT2 receptors since it was resistant to blockade by ketanserin (0.3 mumol/l). When examined as antagonists of 5-HT-induced contractions of rabbit aorta, each ergot displayed nanomolar affinity at the 5-HT2 receptor but only methysergide behaved as a simple competitive antagonist (pKB = 8.25). Methylergometrine and ergometrine produced surmountable blockade which was accompanied by a non-parallel displacement of the 5-HT concentration-effect curves. The selective 5-HT1-like receptor agonist GR43175 (less than or equal to 30 mumol/l) was devoid of affinity at the 5-HT2 receptor in rabbit aorta. In the rabbit saphenous vein each of the ergots produced concentration-dependent contractions which resulted in overtly biphasic concentration-effect curves. Only the first phase of contraction mimicked the effects of 5-HT and GR43175 since contractions were not blocked by MDL 72222 (1 mumol/l), but were surmountably antagonised by methiothepin (10 nmol/l), ketanserin (0.3 mumol/l) and spiperone (0.3 mumol/l).(ABSTRACT TRUNCATED AT 250 WORDS)

An examination of the sources of calcium for contractions mediated by postjunctional alpha 1- and alpha 2-adrenoceptors in several blood vessels isolated from the rabbit

1. The roles of intracellular and extracellular-derived Ca2+ in alpha-adrenoceptor-mediated contractions to noradrenaline (NA) have been investigated in several isolated blood vessels from the rabbit by examining responses in the presence of a modified Krebs-Henseleit saline with 2.5 mM Ca2+ and a Ca2(+)-buffered saline with 0.1 microM free Ca2+. 2. NA was tested in preparations of the abdominal aorta, distal saphenous artery, renal vein, lateral saphenous vein, plantaris vein and ear vein exposed to a Ca2(+)-buffered saline with 0.1 microM [Ca2+]. A concentration of NA which was maximally effective in modified Krebs-Henseleit saline, produced an initial transient contraction (ITC) followed by a relaxation towards baseline. This is evidence that alpha-adrenoceptor-mediated responses in all these blood vessels depend upon calcium from both sources. 3. The ITC was particularly pronounced in the arteries and was associated more closely with the alpha 1-receptor subtype. In the abdominal aorta, distal saphenous artery and renal vein the ITC can almost exclusively be attributed to an alpha 1-adrenoceptor (prazosin-sensitive, rauwolscine-resistant). In the ear vein, and to a lesser extent the plantaris vein, the ITC was mediated in part by an alpha 2-adrenoceptor (prazosin-resistant, rauwolscine-sensitive). 4. alpha 2-Adrenoceptors in the lateral saphenous vein largely account for the response to NA in modified Krebs-Henseleit saline, but alpha 1-adrenoceptors mediate the ITC in Ca2(+)-buffered saline. After selective inactivation of alpha 1-adrenoceptors with a combination of phenoxybenzamine and rauwolscine, responses to NA in modified Krebs-Henseleit saline are slow in onset and there is no ITC in Ca2(+)-buffered saline. 5. The possible significance of the coupling of postjunctional alpha 2-adrenoceptors to dual sources of Ca2 + is discussed in relation to the interaction between alpha-adrenoceptor subtypes and the ease of demonstrating functional alpha 2-adrenoceptors in isolated blood vessels.

Effects of basal and acetylcholine-induced release of endothelium-derived relaxing factor on contraction to alpha-adrenoceptor agonists in a rabbit artery and corresponding veins

1. The effects of an endothelium-dependent (acetylcholine) and an endothelium-independent (sodium nitroprusside) relaxant against noradrenaline-induced contractions were compared in three isolated superficial blood vessels of the rabbit, the lateral saphenous vein, plantaris vein and distal saphenous artery. Both produced concentration-related relaxations of all three vessels and were more effective against submaximal than maximal contractions to noradrenaline. Transient contractions to high concentration of acetylcholine occurred only in endothelium-intact preparations of saphenous vein and were inhibited by flurbiprofen. 2. In endothelium-denuded preparations sodium nitroprusside was 3 times more effective than in endothelium-intact preparations, while acetylcholine (less than 3 microM) was inactive. Sensitivity was similar for each relaxant: lateral saphenous vein greater than or equal to plantaris vein greater than distal saphenous artery. The similar profile of sodium nitroprusside and acetylcholine suggests that differences in susceptibility to endothelium-derived relaxing factor (EDRF) are caused by inter-vessel variations in the excitation-coupling process for noradrenaline. 3. Haemoglobin inhibited acetylcholine-induced relaxations in the endothelium-intact preparation of the lateral saphenous vein and distal saphenous artery, which suggests a similar EDRF in each preparation and the likelihood that this is a single substance, presumably nitric oxide. 4. The influence of basal, spontaneously released EDRF on alpha-adrenoceptor function was tested either by mechanical disruption of the endothelium or by adding haemoglobin to endothelium-intact segments. Endothelial disruption slightly reduced contractions to noradrenaline (NA) in distal saphenous artery but increased response size of lateral saphenous and plantaris veins, in the latter also increasing sensitivity to NA: haemoglobin mimicked endothelial disruption. Thus, basal release of EDRF like acetylcholine and nitroprusside was more effective in the veins than in the corresponding artery. 5. In lateral saphenous vein responses to phenylephrine were enhanced by endothelial disruption, but without change in sensitivity: responses to UK-14304, B-HT 920 and cirazoline, which had a relatively slow speed of onset of contraction were not affected. There was no correlation between enhancement and alpha-adrenoceptor sub-type although the agonists which were enhanced all activate alpha 1-adrenoceptors. Competitive antagonists failed to reveal an alpha-adrenoceptor subtype enhanced by endothelial disruption. However, effects of phenoxybenzamine suggest that alpha 1-adrenoceptors are necessary for the influence of basal EDRF.

Postjunctional alpha 2-adrenoceptors mediate vasoconstriction in human subcutaneous resistance vessels

1. In vitro studies have been performed on human medium-sized muscular arteries (internal diameter 1-4 mm) in a classical organ bath and with human subcutaneous resistance arteries (internal diameter 103-626 microns) in a microvascular myograph. 2. Although the medium-sized muscular arteries showed no response to either of the alpha 2-agonists B-HT 933 or UK 14304 in concentrations up to 10 microM, the subcutaneous resistance arteries from all regions examined showed well-pronounced and concentration-dependent responses to B-HT 933, the pD2 (-log EC50) being 5.11 +/- 0.09. 3. In the resistance arteries the alpha 2-antagonist yohimbine caused a parallel shift to the right of the B-HT 933 concentration-response curve; the yohimbine pA2 for the B-HT 933 receptor was 7.86 +/- 0.12. 4. There was an inverse relationship between the maximum response to B-HT 933 and the calibre of the resistance vessels. 5. These results indicate the presence of a postjunctional alpha 2-adrenoceptor in human subcutaneous resistance arteries and not in medium sized muscular arteries.

Expression of functional postjunctional alpha 2-adrenoceptors in rabbit isolated distal saphenous artery--a permissive role for angiotensin II?

In the rabbit isolated distal saphenous artery, the population of postjunctional adrenoceptors is of the alpha 1 variety under normal in vitro experimental conditions, based on the potency order of selective agonists and on the effects of the antagonists prazosin and rauwolscine against responses to UK-14304. Angiotensin II (A II, 0.05 microM) however, without affecting resting baseline tension, markedly enhanced responses to UK-14304, particularly at low concentrations. This previously unseen component of the response to UK-14304 was resistant to prazosin (0.1 microM) but susceptible to rauwolscine (1 microM). A II would therefore appear to have a permissive role for the expression of a quiescent population of postjunctional alpha 2-adrenoceptors in the rabbit distal saphenous artery.