A61603-induced vasoconstriction in porcine carotid vasculature: involvement of a non-adrenergic mechanism

A61603-induced contractions of the porcine meningeal artery are mediated by alpha1- and alpha2-adrenoceptors

It has recently been shown that A61603 (N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydro-naphthalen-1-yl]methane sulphonamide), a potent alpha(1A)-adrenoceptor agonist, decreased carotid artery conductance in anaesthetized pigs by a novel non-adrenergic mechanism. In this study, we set out to pharmacologically characterize A61603-induced contractions of the porcine isolated meningeal artery. While the maximum contractile responses of the artery were similar, A61603 (E(max): 183 +/- 23% of 100 mM KCl; pEC(50): 7.25 +/- 0.18) was more potent than noradrenaline (E(max): 156 +/- 16%; pEC(50): 5.75 +/- 0.17) or phenylephrine (E(max): 163 +/- 20%; pEC(50): 5.63 +/- 0.02). Prazosin (pA(2): 9.36 +/- 0.23) and, to a lesser extent, rauwolscine (pK(b): 6.36 +/- 0.38) and yohimbine (pK(b): 7.30 +/- 0.15) antagonised the contractions to A61603. The 5-HT(1B) (GR127935; N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide) and 5-HT(2) (ritanserin) receptor antagonists failed to affect the responses to A61603, but methiothepin, which, in addition, has a high affinity for alpha-adrenoceptors, proved an effective antagonist. The A61603-induced responses were suppressed by the cAMP stimulator forskolin, but not by the protein kinase C inhibitor chelerythrine. Our results suggest that the contraction of porcine isolated meningeal artery by A61603 is mediated via mainly alpha(1)-(probably alpha(1A)) and, to a lesser extent, alpha(2)-adrenoceptors, involving the adenylyl cyclase, but not the diacylglycerol pathway.

Bradykinin potentiation by ACE inhibitors: a matter of metabolism

1. Studies in isolated cells overexpressing ACE and bradykinin type 2 (B(2)) receptors suggest that ACE inhibitors potentiate bradykinin by inhibiting B(2) receptor desensitization, via a mechanism involving protein kinase C (PKC) and phosphatases. Here we investigated, in intact porcine coronary arteries, endothelial ACE/B(2) receptor 'crosstalk' as well as bradykinin potentiation through neutral endopeptidase (NEP) inhibition. 2. NEP inhibition with phosphoramidon did not affect the bradykinin concentration-response curve (CRC), nor did combined NEP/ACE inhibition with omapatrilat exert a further leftward shift on top of the approximately 10 fold leftward shift of the bradykinin CRC observed with ACE inhibition alone. 3. In arteries that, following repeated exposure to 0.1 microM bradykinin, no longer responded to bradykinin ('desensitized' arteries), the ACE inhibitors quinaprilat and angiotensin-(1-7) both induced complete relaxation, without affecting the organ bath fluid levels of bradykinin. This phenomenon was unaffected by inhibition of PKC or phosphatases (with calphostin C and okadaic acid, respectively). 4. When using bradykinin analogues that were either completely or largely ACE-resistant ([Phe(8)psi(CH(2)-NH)Arg(9)]-bradykinin and [deltaPhe(5)]-bradykinin, respectively), the ACE inhibitor-induced shift of the bradykinin CRC was absent, and its ability to reverse desensitization was absent or significantly reduced, respectively. Caveolar disruption with filipin did not affect the quinaprilat-induced effects. Filipin did however reduce the bradykinin-induced relaxation by approximately 25-30%, thereby confirming that B(2) receptor-endothelial NO synthase (eNOS) interaction occurs in caveolae. 5. In conclusion, in porcine arteries, in contrast to transfected cells, bradykinin potentiation by ACE inhibitors is a metabolic process, that can only be explained on the basis of ACE-B(2) receptor co-localization on the endothelial cell membrane. NEP does not appear to affect the bradykinin levels in close proximity to B(2) receptors, and the ACE inhibitor-induced bradykinin potentiation precedes B(2) receptor coupling to eNOS in caveolae.

Vasoconstriction is determined by interstitial rather than circulating angiotensin II

1. We investigated why angiotensin (Ang) I and II induce vasoconstriction with similar potencies, although Ang I-II conversion is limited. 2. Construction of concentration-response curves to Ang I and II in porcine femoral arteries, in the absence or presence of the AT(1) or AT(2) receptor antagonists irbesartan and PD123319, revealed that the approximately 2 fold difference in potency between Ang I and II was not due to stimulation of different AT receptor populations by exogenous and locally generated Ang II. 3. Measurement of Ang I and II and their metabolites at the time of vasoconstriction confirmed that, at equimolar application of Ang I and II, bath fluid Ang II during Ang I was approximately 18 times lower than during Ang II and that Ang II was by far the most important metabolite of Ang I. Tissue Ang II was 2.9+/-1.5% and 12.2+/-2.4% of the corresponding Ang I and II bath fluid levels, and was not affected by irbesartan or PD123319, suggesting that it was located extracellularly. 4. Since approximately 15% of tissue weight consists of interstitial fluid, it can be calculated that interstitial Ang II levels during Ang II resemble bath fluid Ang II levels, whereas during Ang I they are 8.8 - 27 fold higher. Consequently at equimolar application of Ang I and II, the interstitial Ang II levels differ only 2 - 4 fold. 5. Interstitial, rather than circulating Ang II determines vasoconstriction. Arterial Ang I, resulting in high interstitial Ang II levels via its local conversion by ACE, may be of greater physiological importance than arterial Ang II.