Effects of alpha,beta-methylene ATP on resistance and capacitance blood vessels of the cat intestinal circulation; a comparison with other vasoconstrictor agents and sympathetic nerve stimulation

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Comparative responses to alpha,beta-methylene-ATP in cat pulmonary, mesenteric, and hindquarter vascular beds

Responses to the P2X-purinoceptor agonist alpha,beta-methylene-ATP (alpha,beta-MeATP) were investigated in the pulmonary, hindquarter, and mesenteric vascular beds in the cat. Under constant-flow conditions, injections of alpha,beta-MeATP caused dose-related increases in perfusion pressure in the pulmonary and hindquarter beds and a biphasic response in the mesenteric circulation. In the pulmonary vascular bed, the order of potency was alpha,beta-MeATP > U-46619 > angiotensin II, whereas, in the hindquarters, the order of potency was angiotensin II > U-46619 > alpha,beta-MeATP. The order of potency was similar in the hindquarter and mesenteric beds when the pressor component of the response to alpha,beta-MeATP was compared with responses to angiotensin II and U-46619. The P2X-receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid attenuated the pressor response to alpha,beta-MeATP in the hindquarter circulation and the pressor component in the mesenteric vascular bed. Pressor responses to alpha,beta-MeATP were not altered by cyclooxygenase, alpha-adrenergic, or angiotensin AT(1) antagonists. These data show that alpha,beta-MeATP has potent pressor activity in the pulmonary circulation, where it was 100-fold more potent than angiotensin II. In contrast, alpha,beta-MeATP had modest pressor activity in the systemic bed, where it was 1,000-fold less potent than angiotensin II. These data suggest that responses to alpha,beta-MeATP are dependent on the vascular bed studied and may be dependent on the density of P2X receptors in the vascular bed.

Potentiation of responses to sympathetic nerve stimulation and vasoconstrictor agents by SK&F 103829 in the feline mesenteric circulation

1. The amplification of vasoconstrictor effects of several agonists and sympathetic nerve stimulation, caused by 5-HT2 receptor activation, was studied in the autoperfused mesenteric circulation of anaesthetized cats. To produce long lasting and selective 5-HT2 receptor stimulation we used SK&F 103829 (2,3,4,5 tetrahydro-8[methyl-sulphonyl]-1H3-benzazepin-7-ol methensulphonate). We assessed that SK&F 103829 was a strong contractile partial agonist in isolated preparations of rat tail artery and calf pulmonary artery. 2. The intrinsic activity of SK&F 103829 with respect to 5-hydroxytryptamine (5-HT) was 0.8 in rat tail artery and 0.6 in calf pulmonary artery. SK&F 103829-induced contractile responses were surmountably antagonized by ketanserin with a potency expected from its affinity for 5-HT2 receptors. SK&F 103829 surmountably antagonized the effects of 5-HT in rat tail artery with a pKp of 5.8. 3. Concentrations of SK&F 103829 causing greater than threshold constrictions enhanced vasoconstrictor responses of sympathetic nerve stimulation, noradrenaline, angiotensin II, methoxamine and alpha, beta-methylene ATP in the mesenteric arterial bed. Increases in mesenteric arterial pressure by noradrenaline, observed in the presence of prazosin, were also potentiated by SK&F 103829. 4. Ketanserin prevented both the constrictor effect of SK&F 103829 and the SK&F 103829-evoked potentiation of the responses to noradrenaline and angiotensin II in the mesenteric arterial bed. Ketanserin, however, failed to abolish (once established) the SK&F 103829-evoked potentiation of the constrictor effects caused by both noradrenaline and angiotensin II. 5. Short lasting constrictor effects of 5-HT were reversed to dilator effects by SK&F 103829 in both the mesenteric arterial and venous bed, thereby revealing the existence of vasodilator 5-HT receptors.6. The main finding is consistent with a sensitization of the mesenteric arterial bed to vasoconstrictor responses mediated through alpha 1- and alpha2-adrenoceptors, angiotensin II receptors and purinoceptors by SK&F 103829-evoked activation of 5-HT2 receptors. This property, together with the direct constrictor effect of the mesenteric arterial bed suggest that SK&F 103829 can reduce portal venous flow thereby being a useful therapeutic principle for the treatment of portal hypertension.