Existence of alpha-adrenoceptor subtypes in isolated human gastroepiploic and omental arteries

Adrenergic Response of Splanchnic Arteries from Cirrhotic Patients: Role of Nitric Oxide, Prostanoids, and Reactive Oxygen Species

Peripheral and splanchnic vasodilatation in cirrhotic patients has been related to hyporesponsiveness to vasoconstrictors, but studies to examine the vascular adrenergic response provide contradictory results. Hepatic arteries from cirrhotic patients undergoing liver transplantation and mesenteric arteries from liver donors were obtained. Segments 3 mm long from these arteries were mounted in organ baths for testing isometric adrenergic response. The concentration-dependent contraction to noradrenaline (10−8 to 10−4 M) was similar in hepatic and mesenteric arteries, and prazosin (α 1-adrenergic antagonist, 10−6 M), but not yohimbine (α 2-adrenergic antagonist, 10−6 M), produced a rightward parallel displacement of this contraction in both types of arteries. Phenylephrine (α 1-adrenergic agonist, 10−8 to 10−4 M) and clonidine (α 2-adrenergic agonist, 10−8 to 10−4 M) also produced concentration-dependent contractions that were comparable in hepatic and mesenteric arteries. The inhibitor of cyclooxygenase meclofenamate (10−5 M), but not the inhibitor of nitric oxide synthesis Nw-nitro-l-arginine methyl ester (l-NAME, 10−4 M), potentiated the response to noradrenaline in hepatic arteries; neither inhibitor affected the response to noradrenaline in mesenteric arteries. Diphenyleneiodonium (DPI; 5 × 10−6 M), but neither catalase (1000 U/ml) nor tiron (10−4 M), decreased the maximal contraction for noradrenaline similarly in hepatic and mesenteric arteries. Therefore, it is suggested that, in splanchnic arteries from cirrhotic patients, the adrenergic response and the relative contribution of α 1- and α 2-adrenoceptors in this response is preserved, and prostanoids, but not nitric oxide, may blunt that response. Products dependent on NAD(P)H oxidase might contribute to the adrenergic response in splanchnic arteries from control and cirrhotic patients.

Evidence of alpha1-adrenoceptor functional changes in omental arteries of patients with end-stage renal disease

1 Alpha1-Adrenoceptor (alpha1-AR) subtypes were characterized in isolated omental arteries obtained after abdominal surgery in patients with end-stage renal disease (ESRD) or with Diabetes Mellitus type 2 plus ESRD (ESRD-DM). 2 Omental arteries from patients with ESRD and ESRD-DM elicited a significant increase in sensitivity to phenylephrine with a pD(2) (-log EC50) of 6.7 and 6.6, respectively, vs. the control (5.8, P < 0.001). 3 Stimulation with phenylephrine was conducted in the presence or absence of selective alpha1-AR competitive antagonists: 5-methylurapidil (alpha1A-), AH11110A (1-[biphenyl-2-yloxy]-4-imino-4-piperidin-1-yl-butan-2-ol; alpha1B-) and BMY7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro [4.5] decane-7,9-dione; alpha(1D)-). The relative abundance of mRNA for all three alpha(1)-ARs was determined. 4 The maximal contractile responses to phenylephrine were: E(max) 1.59 +/- 0.17, 1.48 +/- 0.08 and 1.55 +/- 0.14 g for the ESRD, ESRD-DM and control groups, respectively. 5 Functionally, there was an increment in the affinity for the alpha(1A)-AR antagonist (pA2: control 7.45, ESRD 8.36, ESRD-DM 8.0; P < 0.01), and a reduction in the alpha1B-AR antagonist affinity (8.3 for controls, 7.6 for ESRD and 7.3 for ESRD-DM; P < 0.01) associated with renal disease. The affinities for the alpha1D-AR antagonist were similar among the studied groups (8.5 for the controls, 8.7 for the ESRD and 8.1 for the ESRD-DM groups). 6 Renal disease increased mRNA expression of alpha(1B)-ARs and reduced both alpha1A- and alpha(1D)-ARs subtypes in ESRD and ESRD-DM patients. 7 The results suggest that human omental arteries exposed to chronic uraemia show vascular hypersensitivity to phenylephrine, because of functional alpha1-AR changes.

Vasoconstrictor responses are normal but prostanoid-mediated vasodilatation is enhanced in human cirrhotic mesenteric arteries

BACKGROUND AND AIMS

The mechanisms responsible for mesenteric vasodilatation in cirrhosis have not been fully elucidated. The aim of the present study was to examine whether there is altered intrinsic vascular reactivity of human mesenteric vessels in cirrhosis, which might contribute to vasodilatation in vivo.

METHODS

Ten mesenteric arteries from six cirrhosis patients undergoing liver transplantation were compared with 11 arteries from six control patients. Vasoconstrictor responses to potassium, norepinephrine and methoxamine were determined. Endothelium-dependent vasodilatation responses to acetylcholine and substance P were determined both before and after inhibition of nitric oxide (NO) and prostanoid synthesis.

RESULTS

Cirrhotic vessels responded normally to potassium depolarization and did not differ to control vessels with respect to sensitivity and maximal response to norepinephrine. In cirrhotic vessels, inhibition of NO synthesis had significantly less effect on substance P-induced vasorelaxation than in controls (% Relaxation: cirrhosis 70.3 +/- 9.6; control 34.9 +/- 9.5; P = 0.03). However, after inhibition of both NO and prostanoid synthesis, vasodilatory responses were eliminated in both groups.

CONCLUSIONS

The findings of the present study indicate that intrinsic hyporesponsiveness to vasoconstrictors does not play a pathogenetic role in the mesenteric vasodilatation in human cirrhosis. Furthermore, vasodilator prostanoids might make a significant contribution in mediating enhanced endothelium-dependent vasorelaxation in the mesenteric circulation.

Neurogenic double-peaked vasoconstriction of human gastroepiploic artery is mediated by both alpha1- and alpha2-adrenoceptors

1. The contribution of postjunctional P2X receptors and subtypes of alpha-adrenoceptors to vasoconstrictor responses following periarterial electrical nerve stimulation (PNS, 30 s trains of pulses at a frequency of 2, 4 or 8 Hz) was investigated in human gastroepiploic arteries. 2. The vasoconstrictor response to PNS at a stimulation of 4 or 8 Hz was a two-peaked response, whereas at a frequency of 2 Hz it appeared only as a late peak. All vasoconstrictions evoked by PNS were abolished by phentolamine, a nonselective alpha-adrenoceptor inhibitor, but not by alpha,beta-methylene ATP, a P2X receptor-desensitizing agent. 3. The early peak to PNS at 4 or 8 Hz was abolished by prazosin, an alpha1-adrenoceptor antagonist, while the late one still remained, although it was markedly inhibited. The responses remaining after prazosin were blocked by rauwolscine. The vasoconstrictor response to PNS at 2 Hz was not affected by prazosin (0.1 microM), but was abolished by rauwolscine (0.1 microM), an alpha2-adrenoceptor antagonist. 4. OPC-28326 (10 microM), a newly developed vasodilator, which preferentially exerts its antagonistic actions on the alpha2B- and alpha2C-adrenoceptors, significantly reduced the noradrenaline-induced vasoconstriction in the absence or presence of prazosin. OPC-28326 had a greater inhibitory effect on the late peak evoked by PNS than the early one. The neurogenic responses remaining after OPC-28326 were abolished by prazosin. 5. The present results suggest that sympathetic vasoconstriction of the human gastroepiploic artery is mediated by both alpha1- and alpha2-adrenoceptors postjunctionally, but not by P2X receptors. The alpha2-adrenoceptors may be preferentially activated at a low frequency of stimulation, which induces a constriction more slowly than that by alpha1-adrenoceptors. The existence of alpha2-adrenoceptors may cause an enhancement of alpha1-adrenoceptor-induced responses.

Purinergic and adrenergic cotransmission in canine isolated and perfused gastroepiploic arteries

1. The vasoconstrictor responses of canine gastroepiploic artery to periarterial electrical nerve stimulation (PNS; 30 s trains of pulses at a frequency of 2, 4 or 8 Hz) were observed in a frequency dependent manner. The PNS-induced vasoconstrictions were abolished by tetrodotoxin (1 micromol/L) and mostly depressed but not completely by guanethidine (10 micromol/L). 2. Vasoconstrictor responses to administered noradrenaline were antagonized significantly by prazosin (0.1 micromol/L), an alpha1-adrenoceptor antagonist, but were not significantly affected by suramin (100 micromol/L), a P2 purinoceptor antagonist, or alpha,beta-methylene ATP (1 micromol/L), a P2X receptor desensitizing agent. Exogenous ATP-induced responses were clearly depressed by suramin or alpha,beta-methylene ATP, but were not significantly affected by prazosin. 3. The vasoconstrictor responses to PNS at a low frequency (2 and 4 Hz) of stimulation were markedly inhibited by suramin (100 micromol/L) and by alpha,beta-methylene ATP (1 micromol/L). The remaining responses after suramin or alpha,beta-methylene ATP were abolished by subsequent application of prazosin (0.1 micromol/L). At a high frequency (8 Hz) of stimulation, the vascular response was not significantly inhibited by suramin or alpha,beta-methylene ATP, but it was abolished by prazosin. 4. Injection of xylazine (0.3-30 nmol/L), an alpha2-adrenoceptor agonist, did not induce any clear vasoconstriction. The exposure of tissues to rauwolscine (0.1-0.3 micromol/L), an alpha2-adrenoceptor antagonist, dose-dependently increased PNS-induced vasoconstrictions at all frequencies tested. 5. The present results indicate that ATP acts as a cotransmitter with noradrenaline and is responsible for post-junctional vasoconstrictor responses at low frequencies of sitmulation, whereas the effect of noradrenaline is dominant at high-frequency stimulation in canine gastroepiploic artery. Prejunctional alpha2-adrenoceptor autoinhibition may modulate the release of either noradrenaline or ATP from sympathetic nerve terminals.