General
In addition to the implications discussed below, our findings show that i.v. pretreatment with 100 μg/kg ritanserin (a dose devoid of α
1-adrenoceptor blockade in pithed rats [
15]) is a conditio sine qua non for demonstrating the blockade produced by prazosin alone (and the role of α
1-adrenoceptors) in the DHE vasopressor responses. In keeping with this view: (i) in animals without ritanserin-pretreatment the vasopressor responses to DHE remained unchanged after 30 μg/kg prazosin (Fig.
2c), a dose that very potently blocks the α
1-adrenoceptors mediating vasopressor responses in pithed rats [
15]; and (ii) a component of these vasopressor responses (particularly at 310, 1000 and 3100 μg/kg DHE) is mediated by 5-HT
2 receptors in view of the blockade produced by 100 μg/kg ritanserin (Fig.
3), whereas the ritanserin-resistant component is mediated by other receptors. In this respect, our findings showing that the remaining vasopressor responses to DHE after ritanserin-pretreatment were attenuated by 10 and 30 μg/kg prazosin (Fig.
4a) and that they were markedly blocked by 100 and 300 μg/kg rauwolscine (Fig.
4b) establish the involvement of rauwolscine-sensitive α
2-adrenoceptors and, to a lesser extent, of prazosin-sensitive α
1-adrenoceptors. In agreement with our findings, Roquebert and Grenié [
6] reported that 500 μg/kg prazosin (i.v.) failed to block the vasopressor responses to DHE in pithed rats without pretreatment with a 5-HT
2 receptor antagonist. Accordingly, this apparent failure by 30 μg/kg prazosin (Fig.
2c) or 500 μg/kg prazosin [
6] implies that activation of vascular 5-HT
2 receptors by higher doses of DHE, which displays a high affinity for 5-HT
2A receptors (pK
i = 8.54) [
7], may have masked the blockade of α
1-adrenoceptors by prazosin. Certainly, prazosin has higher affinity (approximately 1 to 2 logarithmic units) than DHE for α
1-adrenoceptors (Table
1). However, the affinity (pK
i) of prazosin for 5-HT
2 receptors (if any) is <<4 [
22], whereas that of DHE is 8.54 (see above). Therefore, it is highly unlikely that prazosin is blocking 5-HT
2 receptors. This suggestion is reinforced when considering that the blockade produced by the combination 30 μg/kg prazosin plus 300 μg/kg rauwolscine in the absence of ritanserin was more pronounced than that produced by rauwolscine alone (Fig.
2e). This line of reasoning can also account for the higher potency of blockade by rauwolscine in ritanserin-pretreated rats (Fig.
4b) as compared to that in animals without ritanserin pretreatment (Fig.
2d). These findings, taken together, may suggest that DHE-induced vasopressor responses involve the sum of a combination of effects mediated by activation of 5-HT
2A receptors, α
1-adrenoceptors and α
2-adrenoceptors.
In addition, our experimental approach with ritanserin pretreatment further suggests that the vasopressor responses to DHE could be mainly mediated by α
1- (probably α
1A, α
1B and α
1D) and α
2- (probably α
2A, α
2B and α
2C) adrenoceptors, although some caution should be exerted when interpreting the “subtype selectivity” of the compounds used (see below and Table
1), as these responses were blocked by the antagonists: (i) 5-methylurapidil (α
1A), L-765,314 (α
1B) or BMY 7378 (α
1D) (Fig.
5); and (ii) BRL44408 (α
2A), imiloxan (α
2B) or JP-1302 (α
2C) (Fig.
6).
Involvement of α1-and α2-adrenoceptors in the vasopressor responses to DHE
DHE displays affinity for a wide variety of receptors [
1], with the same nanomolar affinity for rat α
1-adrenoceptors (pK
i: 8.0) and rat α
2-adrenoceptors (pK
i: 8.0) [
7]. Interestingly, DHE can also interact with all α
1- and α
2-adrenoceptor subtypes (see Table
1). These findings may help explain, within the context of our study, the complex interactions of DHE. Within the bounds of adrenergic mechanisms in our study using ritanserin-pretreated rats, the functional role of α
1- and α
2-adrenoceptors in the vasopressor responses to DHE is clearly established, as these responses were: (i) blocked by prazosin (10–30 μg/kg; Fig.
4a) or by rauwolscine (100–300 μg/kg; Fig.
4b); and (ii) further blocked (particularly the response to 3100 μg/kg DHE) by the combination of prazosin plus rauwolscine (Fig.
4c). Certainly, in pithed rats, 30 μg/kg prazosin and 300 μg/kg rauwolscine are doses high enough to completely block the vasopressor responses mediated by, respectively, α
1-adrenoceptors [
15] and α
2-adrenoceptors [
13]. Nonetheless, there were some important differences in the profile of blockade produced by these antagonists. Indeed, the partial blockade of the DHE responses by 30 μg/kg prazosin, being slightly more pronounced than that produced by 10 μg/kg prazosin (Fig.
4a) may suggest that it was already a supramaximal dose that, in addition to completely blocking α
1-adrenoceptors, could have weakly blocked α
2-adrenoceptors (particularly the α
2B and α
2C-adrenoceptor subtypes, for which it displays a moderate affinity; Table
1). In contrast, the marked blockade by 300 μg/kg rauwolscine, being more pronounced than that by 100 μg/kg rauwolscine (Fig.
4b), may suggest (although does not directly prove) a major role of α
2-adrenoceptors (as compared to α
1-adrenoceptors). This suggestion may help partly explain why Roquebert and Grenié [
6] could show the role of α
2-adrenoceptors, but not of α
1-adrenoceptors, in the DHE responses in Wistar rats without 5-HT
2 receptor blockade. Admittedly, Roquebert and Grenié [
6]: (i) did not analyse the effects of the combination prazosin + yohimbine as we did with the combination prazosin plus rauwolscine in animals without ritanserin pretreatment (Fig.
2e); and (ii) used older rats (300–350 g) anaesthetised with ether. Certainly, the functional expression of rat vascular α
1-adrenoceptor subtypes depends on several factors, including age [
25].
Interestingly, the failure of the combination prazosin plus rauwolscine to abolish (although markedly blocked) the DHE responses in ritanserin-pretreated rats (Fig.
4c) cannot categorically exclude the possible role of additional (although negligible) mechanisms, including an enhanced synthesis of proconstrictor prostaglandins by DHE, as reported by Müller-Schweinitzer [
26].
The possible role of the different α1- and α2-adrenoceptor subtypes in the responses to DHE
As suggested above, the vasopressor responses to DHE in ritanserin-pretreated rats are mainly mediated by rauwolscine-sensitive α
2-adrenoceptors and, apparently to a lesser extent, by prazosin-sensitive α
1-adrenoceptors. Nevertheless, these antagonists do not display selective affinities for distinguishing amongst their corresponding α
1- and α
2-adrenoceptor subtypes (Table
1). Hence, the effects of relatively more selective antagonists for the α
1-adrenoceptor subtypes (i.e. 5-methylurapidil [α
1A], L-765,314 [α
1B] and BMY 7378 [α
1D]) and the α
2-adrenoceptor subtypes (i.e. BRL44408 [α
2A], imiloxan [α
2B] and JP-1302 [α
2C]) (Table
1) were further investigated in an attempt to identify the subtypes involved.
The fact that the DHE responses were blocked after administration of each of these antagonists for α
1- (Fig.
5) and α
2-adrenoceptors (Fig.
6) basically suggests the involvement of, respectively, the α
1A/α
1B /α
1D subtypes and the α
2A/α
2B/α
2C subtypes. Importantly, the doses used of these antagonists have previously been shown: (i) to completely block the vasopressor responses mediated by the α
1A/α
1B/α
1D subtypes and the α
2A/α
2B/α
2C subtypes in pithed rats [
10,
13]; and (ii) to correlate with the affinities for their respective subtypes [
27] (see Table
1). Notwithstanding, the differences in the profile of blockade produced by each of the above antagonists deserve further considerations.
On the one hand, 30–100 μg/kg of 5-methylurapidil (Fig.
5a) and BMY 7378 (Fig.
5c) dose-dependently blocked the DHE responses and display very high affinity for, respectively, the α
1A (pK
i: 9.0) and α
1D (pK
i: 9.0) subtypes, but they also display moderate affinity for the other α
1 subtypes (with pK
i’s between 7.0 and 8.0; Table
1). Hence, one could imply that the high potency of these antagonists to block the DHE responses may be due to a marked blockade of their receptors, with partial blockade of the other α
1 subtypes. However, Zhou and Vargas [
28] showed in pithed rats that: (i) 500 μg/kg 5-methylurapidil blocked the vasopressor responses to the α
1A-adrenoceptor agonist (R)A-61603; and (ii) 100–1000 μg/kg BMY 7378, which dose-dependently blocked the vasopressor responses to phenylephrine, failed to block those to (R)A-61603. Thus, it would seem logical to suggest that 5-methylurapidil (Fig.
5a) and BMY 7378 (Fig.
5c) are reasonably selective for blocking the α
1A- and α
1D-subtypes, respectively, as suggested by Willems et al. [
27]. In contrast, the fact that only 100 μg/kg L-765,314 significantly blocked the DHE responses (Fig.
5b): (i) apparently matches with its slightly lower -but still high- affinity (pK
i: 8.3) for the α
1B subtype and its moderate affinity for the α
1D subtype (Table
1); and (ii) implies a minor role of the α
1B subtype (relative to that of the α
1A- and α
1D- subtypes) in the systemic vasculature, as suggested by Daly et al. [
29].
On the other hand, as to the role of the α
2-adrenoceptor subtypes, BRL44408 and JP-1302 are “relatively selective” for, respectively, the α
2A (pK
i: 8.7) and α
2C (pK
i: 7.6) subtypes (Table
1). Thus, the high potency of BRL44408 (100–300 μg/kg; Fig.
6a) and the lower potency of JP-1302 (only at 1000 μg/kg; Fig.
6c) to block the DHE responses might suggest a major role of the α
2A subtype and a less predominant role of the α
2C subtype mediating vasopressor responses, as suggested by Gavin and Docherty [
30]. However, the affinities of these antagonists for the α
1- adrenoceptor subtypes have not been determined (Table
1). Interestingly, in pithed rats (
n = 5), the vasopressor responses to i.v. bolus injections of 0.1, 0.3, 1, 3, 10 and 30 μg/kg phenylephrine (14 ± 2, 19 ± 2, 24 ± 2, 39 ± 5, 66 ± 7 and 115 ± 7 mmHg, respectively): (i) remained unaltered after an i.v. bolus of 100 μg/kg BRL44408 (16 ± 2, 20 ± 2, 25 ± 3, 40 ± 6, 69 ± 9 and 107 ± 12 mmHg); and (ii) were attenuated (at the highest doses) after an i.v. bolus of 300 μg/kg BRL44408 (12 ± 2, 12 ± 1, 18 ± 3, 29 ± 7, *54 ± 12 and *93 ± 16 mmHg; *
P < 0.05) (unpublished observations). The latter finding may explain why the blockade produced by BRL4408 (Fig.
6a): (i) did not significantly differ (
P > 0.05) from that produced by the combination prazosin plus rauwolscine (Fig.
4c); and (ii) was more pronounced than that produced by rauwolscine alone (Fig.
4b). In contrast, the affinity of imiloxan for the α
1-adrenoceptor subtypes is very low (pK
i < 4; which excludes its interaction with these receptors), but its affinity for the α
2B (pK
i: 7.3) and α
2C (pK
i: 6.0) subtypes (Table
1) leaves very little room for in vivo selectivity, particularly at the doses used (Fig.
6b). Indeed, the blockade of the DHE responses by 1000 and 3000 μg/kg imiloxan being practically identical (Fig.
6b) seems to suggest a minor role of the α
2B (and probably also of the α
2C) adrenoceptor subtype. Hence, we considered it unnecessary to explore the effect of more antagonist combinations.
Clearly, the above findings cannot be simply explained in terms of pure antagonism at a single receptor subtype in view of: (i) the nature of our pithed rat model (in which we cannot reach equilibrium conditions, nor can we categorically exclude the role of pharmacokinetic factors); (ii) the relative “selectivity” of the antagonists used (determined in vitro; Table
1); and (iii) the limited selectivity of these compounds when given i.v. in pithed rats.
Potential clinical implications of the present results
Admittedly, the relative “selectivity” of the α
1- and α
2-adrenoceptor antagonists used in this study (see Table
1) would seem rather limited in view of the i.v. (systemic) administration of compounds and the additional role of pharmacokinetic factors (which cannot be completely ruled out in pithed rats). Consistent with these views, other studies performed in vivo with these compounds have also shown limited selectivity [
31]. Notwithstanding, the pithed rat model is predictive of (cardio)vascular side effects [
11,
12] and provides information that cannot be obtained from in vitro studies [
32]. Moreover, from a clinical perspective, our findings may help understand the pharmacological profile of the adverse vascular side-effects (i.e. systemic vasoconstriction) produced by DHE (present results) and ergotamine [
10], even when the pharmacological profile of the α-adrenoceptor subtypes mediating systemic vasoconstriction in rodents and humans is not identical [
25].
On the other hand, although the vasoconstrictor responses to DHE mediated by α
1- and α
2-adrenoceptors are less pronounced (i.e. after ritanserin pretreatment; compare Fig.
3 with Figs.
4,
5 and
6), their effects gain importance in view of the long-lasting vasoconstriction induced by DHE, as previously reported [
23,
24]. These findings are even more relevant from a clinical perspective in view of the already increased cardiovascular risk in migraine patients [
33,
34]. Certainly, there are other drugs for the acute treatment of migraine [
2,
35,
36], including the triptans (which produce selective cranial vasoconstriction) and calcitonin gene related peptide (CGRP) receptor antagonists and antibodies (which block the cranial vasodilatation produced by trigeminal release of CGRP). Regarding CGRP receptor antagonists and antibodies, they are clearly devoid of direct vasoconstrictor effects; notwithstanding, since CGRP may play a vasodilator protective role during ischemic (cerebral and cardiac) events, CGRP blockade could transform transient ischemic events into lethal infarcts [
36]. Thus, the pharmacological analysis of the systemic vasoconstriction induced by the classical antimigraine agent DHE is of particular relevance for the further development of antimigraine drugs devoid of direct, as well as indirect, vascular side effects.