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

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

Interaction between alpha(1)- and alpha(2)-adrenoreceptors contributes to enhanced constrictor effects of norepinephrine in mesenteric veins compared to arteries

Mesenteric veins are more sensitive than arteries to the constrictor effects of sympathetic nerve stimulation and alpha-adrenoceptor agonists. We tested the hypothesis that alpha(1)- and alpha(2)-adrenoceptors interact to enhance adrenergic reactivity of mesenteric veins. We studied neurogenic and agonist-induced constrictions of mesenteric veins and arteries in vitro. Norepinephrine concentration-response curves were left-shifted in veins compared to arteries. UK 14,304 (0.01-1 microM, alpha(2)-adrenoceptor receptor agonist) did not constrict arteries or veins but enhanced constrictions and Ca(2+) signals mediated by alpha(1)-adrenoceptor stimulation in veins. Yohimbine (alpha(2)-adrenoceptor receptor antagonist) and MK912 (alpha(2C)-adrenoceptor receptor antagonist), but not alpha(2A)- or alpha(2B)-adrenoceptor antagonists, produced rightward shifts in norepinephrine concentration-response curves in veins. Pharmacological studies revealed that alpha(1D)-adrenoceptors mediate venous constrictions. Norepinephrine responses in veins from alpha(2C)-adrenoceptor knock-out (KO) mice were not different from wild type veins. Yohimbine inhibited norepinephrine constrictions in alpha(2C)-adrenoceptor KO veins suggesting that there is upregulation of other alpha(2)-adrenoceptors in alpha(2C)-KO mice. These data indicate that alpha(1D)- and alpha(2C)-adrenoceptors interact in veins but not in arteries. This interaction enhances venous adrenergic reactivity. Mesenteric vein-specific alpha(2)-adrenoceptor linked Ca(2+) and perhaps other signaling pathways account for enhanced venous adrenergic reactivity.

Modulation of noradrenergic neurotransmission in isolated rat radial artery

The present study was designed to characterize the neurogenic contraction of rat radial artery. Electrical field stimulation (EFS) evoked frequency-dependent contraction that was abolished by tetrodotoxin (neuronal Na(+) channel blocker), guanethidine (sympathetic neuron blocker), or phentolamine (alpha-adrenoceptor blocker). The alpha(1)-adrenoceptor antagonist prazosin inhibited endothelium-independent contractions to EFS, noradrenaline (NA), and the alpha(1)-adrenoceptor agonist phenylephrine. Rauwolscine, an alpha(2)-adrenoceptor antagonist, augmented nerve-mediated contractions and reduced sensitivity to NA and the alpha(2)-adrenoceptor agonist BHT-920. The beta-adrenoceptor antagonist propranolol diminished EFS-elicited contractions, while sensitivity to NA was enhanced by propranolol. Relaxations evoked by isoproterenol, a beta-adrenoceptor agonist, were abolished by propranolol. N(G)-Nitro-L-arginine (L-NOARG), a nitric oxide (NO) synthase inhibitor, increased both nerve-mediated and NA-induced responses in endothelium-intact, but not in endothelium-denuded arteries. Moreover, endothelium-dependent responses to BHT-920 and isoproterenol were modified by L-NOARG. Tetraethylammonium (TEA) or 4-aminopyridine, the Ca2+-activated (K(Ca)) or voltage-dependent K+ (K(V)) channel blockers, respectively, enhanced the neurogenic contractions observed. TEA but not 4-aminopyridine increased NA-induced contractions. The ATP-sensitive K+ (K(ATP))-channel blocker glibenclamide failed to modify adrenergic contractions. Blockade of capsaicin-sensitive primary afferents increased EFS-induced contractions. In conclusion, adrenergic contractions are predominantly mediated by muscular alpha(1)-adrenoceptors, while endothelial alpha(2)- and beta-adrenoceptors play a minor role. Presynaptic alpha(2)- and beta-adrenoceptors cannot be precluded. Noradrenergic neurotransmission in rat radial artery seems to be modulated by both stimulation of endothelial NO, K(Ca), and K(V) channels and sensory C-fiber activation.

Acute dilation to alpha(2)-adrenoceptor antagonists uncovers dual constriction and dilation mediated by arterial alpha(2)-adrenoceptors

BACKGROUND AND PURPOSE

In mouse tail arteries, selective alpha(2)-adrenoceptor antagonism with rauwolscine caused powerful dilation during constriction to the alpha(1)-adrenoceptor agonist phenylephrine. This study therefore assessed phenylephrine's selectivity at vascular alpha-adrenoceptors and the mechanism(s) underlying dilation to rauwolscine.

EXPERIMENTAL APPROACH

Mouse isolated tail arteries were assessed using a pressure myograph.

KEY RESULTS

The alpha(2)-adrenoceptor agonist UK14,304 caused low-maximum constriction that was inhibited by rauwolscine (3 x 10(-8) M) but not by the selective alpha(1)-adrenoceptor antagonist prazosin (10(-7) M). Concentration-effect curves to phenylephrine, cirazoline or noradrenaline were unaffected by rauwolscine but were inhibited by prazosin, which was more effective at high compared with low levels of constriction. In the presence of prazosin, rauwolscine inhibited the curves and was more effective at low compared with high levels of constriction. Although rauwolscine alone did not affect concentration-effect curves to phenylephrine, noradrenaline or cirazoline, it caused marked transient dilation when administered during constriction to these agonists. Dilation was mimicked by another alpha(2)-adrenoceptor antagonist (RX821002, 3 x 10(-8) M), was dependent on agonist selectivity, and did not occur during adrenoceptor-independent constriction (U46619). During constriction to UK14,304 plus U46619, rauwolscine or rapid removal of UK14,304 caused transient dilation that virtually abolished the combined constriction. Endothelial denudation reduced these dilator responses.

CONCLUSIONS AND IMPLICATIONS

Inhibition of alpha(2)-adrenoceptors caused transient dilation that was substantially greater than the contribution of alpha(2)-adrenoceptors to the constriction. This reflects a slowly reversing alpha(2)-adrenoceptor-mediated endothelium-dependent dilation and provides a rapid, sensitive test of alpha(2)-adrenoceptor activity. This approach also clearly emphasizes the poor selectivity of phenylephrine at vascular alpha-adrenoceptors.

Alpha-1B adrenoceptors mediate neurogenic constriction in mesenteric arteries of normotensive and DOCA-salt hypertensive mice

The contribution of alpha-1 adrenergic receptor (alpha1-AR) subtypes to neurogenic constrictions of mesenteric resistance arteries from SHAM and deoxycorticosterone acetate-salt (DOCA-salt) hypertensive mice was assessed. Frequency-response curves (0.5-30 Hz) for transmural stimulation-evoked contractions were examined in SHAM and DOCA-salt arteries in vitro in the absence (control) and presence of prazosin (0.1 microM), PPADS (10 microM), yohimbine (1 microM), 5-methylurapidil (5-MU; 0.1 microM), L-765,314 (1 microM) and BMY-7378 (0.3 microM); selective antagonists at alpha1-, P2X, alpha2-, alpha1A-, alpha1B-, and alpha1D-AR, respectively. In SHAM arteries, prazosin but not PPADS inhibited neurogenic responses. L-765,314 substantially inhibited neurogenic responses while 5-MU had a small inhibitory effect. BMY-7378 did not alter contractile responses at all. In DOCA-salt arteries, prazosin reduced neurogenic responses with no further significant inhibition seen with PPADS. L-765,314 antagonized neurogenic constrictions to a level similar to that seen in SHAM arteries. Furthermore, 5-MU and BMY-7378 did not affect these responses. The density of noradrenergic nerves (assessed using glyoxylic acid-induced fluorescence) or norepinephrine (NE) content was not altered by DOCA-salt hypertension. These results indicate that NE is the primary mediator of neurogenic constriction of murine mesenteric arteries. Nerve-released NE acts primarily at alpha1B-and to a lesser extent at alpha1A-ARs in SHAM arteries whereas NE mediates neurogenic constrictions in DOCA-salt arteries by acting at alpha1B-ARs.