pituitrin and alpha-beta-methyleneadenosine-5--triphosphate

The objective of this study was to compare intracellular calcium concentration ([Ca(2+)](i)) and contractile responses in isolated rat middle cerebral artery (MCA) with those in basilar artery (BA) employing real-time confocal laser microscopy. KCl elicited transient [Ca(2+)](i) elevation and sustained contraction in both arteries; moreover, nearly equal responses were evident in both arteries. Application of 5-hydroxytryptamine (5-HT), vasopressin (VP), and alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-me ATP) also induced elevation of [Ca(2+)](i) and contraction in both arteries. The maximum response of 5-HT and VP necessary to increase [Ca(2+)](i) and to constrict the MCA was less in comparison to the BA; however, a linear relationship emerged between the maximum response of [Ca(2+)](i) and that of contraction. Additionally, the slope of the correlation regression line of MCA was nearly identical to that of BA. On the other hand, cyclopiazonic acid (CPA)-induced Ca(2+) release from store sites following contraction of MCA was distinct from that of BA. In MCA, velocity of [Ca(2+)](i) elevation in smooth muscle cells and Ca(2+)-wave propagation along smooth muscle cells induced by 5-HT were slower than those in BA. These observations revealed that different regions of arteries along the same cerebral tissue may display distinct [Ca(2+)](i) response; moreover, this difference may be one reason for the distinct contractile response.

Topics: Adenosine Triphosphate; Animals; Antineoplastic Agents; Arterioles; Basilar Artery; Calcium; Data Interpretation, Statistical; Immunohistochemistry; Indoles; Male; Microscopy, Confocal; Middle Cerebral Artery; Muscle Contraction; Muscle, Smooth, Vascular; Potassium Chloride; Rats; Rats, Wistar; Serotonin; Vasoconstrictor Agents; Vasodilator Agents; Vasopressins

Coexposure of hypothalamo-neurohypophyseal system explants to ATP and phenylephrine [PE; an alpha1-adrenergic receptor (alpha1-AR) agonist] induces an extended elevation in vasopressin and oxytocin (VP/OT) release. New evidence is presented that this extended response is mediated by recruitment of desensitization-resistant ionotropic purinergic receptor subtypes (P2X-Rs): 1) Antagonists of the P2X2/3 and P2X7-Rs truncated the sustained VP/OT release induced by ATP+PE but did not alter the transient response to ATP alone. 2) The P2X2/3 and P2X7-R antagonists did not alter either ATP or ATP+PE-induced increases in [Ca(2+)](i). 3) P2X2/3 and P2X7-R agonists failed to elevate [Ca(2+)](i), while ATP-gamma-S, an agonist for P2X2-Rs increased [Ca(2+)](i) and induced a transient increase in VP/OT release. 4) A P2Y1-R antagonist did not prevent initiation of the synergistic, sustained stimulation of VP/OT release by ATP+PE but did reduce its duration. Thus, the desensitization-resistant P2X2/3 and P2X7-R subtypes are required for the sustained, synergistic hormone response to ATP+PE, while P2X2-Rs are responsible for the initial activation of Ca(2+)-influx by ATP and ATP stimulation of VP/OT release. Immunohistochemistry, coimmunoprecipitation, and Western blot analysis confirmed the presence of P2X2 and P2X3, P2X2/3, and P2X7-R protein, respectively in SON. These findings support the hypothesis that concurrent activation of P2X2-R and alpha1-AR induces calcium-driven recruitment of P2X2/3 and 7-Rs, allowing sustained activation of a homeostatic circuit. Recruitment of these receptors may provide sustained release of VP during dehydration and may be important for preventing hemorrhagic and septic shock.

Topics: Adenosine Triphosphate; Adrenergic alpha-1 Receptor Agonists; Adrenergic alpha-Agonists; Animals; Blotting, Western; Calcium; Hypothalamo-Hypophyseal System; Immunohistochemistry; Immunoprecipitation; In Vitro Techniques; Male; Microscopy, Fluorescence; Oxytocin; Perfusion; Phenylephrine; Rats; Rats, Sprague-Dawley; Receptors, Adrenergic, alpha-1; Receptors, Purinergic P2; Receptors, Purinergic P2X2; Receptors, Purinergic P2X3; Receptors, Purinergic P2X7; Receptors, Purinergic P2Y1; Signal Transduction; Time Factors; Vasopressins

Previous studies have shown that basolateral ATP inhibits vasopressin action in the renal collecting tubule. Although there is evidence for an apical P2Y2 receptor in this tubule segment, it is not known whether apical ATP has similar effects. In the rat inner medullary collecting duct basolateral, but not apical, ATP (0.1-100 microM) reversibly inhibited vasopressin-induced increases in water permeability with an IC50 of 1.09 microM. Basolateral UTP, but not ADP, alpha,beta-methylene-ATP or 2-methylthio-ATP also inhibited vasopressin action. It is concluded that basolateral but not apical P2Y2 receptors inhibit vasopressin action in the collecting duct.

Topics: Adenosine Diphosphate; Adenosine Triphosphate; Animals; Cell Membrane Permeability; Dose-Response Relationship, Drug; In Vitro Techniques; Kidney Medulla; Kidney Tubules, Collecting; Male; Osmosis; Rats; Rats, Sprague-Dawley; Thionucleotides; Uridine Triphosphate; Vasopressins; Water

The present study investigated the effect of acidosis (reduction in pH of the Krebs' solution from 7.4 to 6.9) on responses to vasoconstrictors and vasodilators, with a focus on purines, in the rat isolated perfused mesenteric arterial bed. alpha,beta-Methylene ATP (alpha,beta-meATP) (10 microM), a selective P2X receptor agonist, elicited a desensitizing vasocontraction, which was not significantly affected by a reduction in pH to 6.9. Contractions to ATP were also not significantly different at pH 6.9 compared to pH 7.4. In contrast, contractile responses to noradrenaline, methoxamine, and vasopressin were greatly attenuated at pH 6.9 (by 48-83%; P<0.01). At raised tone, vasorelaxations to ADP at P2Y receptors, and to calcitonin gene-related peptide (CGRP), were not different at pH 7.4 and pH 6.9. These data indicate that a reduction in pH (to 6.9) differentially affects responses to vasoconstrictors in the rat mesenteric arterial bed. There is no effect on contractions mediated via P2X receptors, but contractions to noradrenaline, methoxamine and vasopressin are greatly attenuated.

Topics: Adenosine Diphosphate; Adenosine Triphosphate; Animals; Calcitonin Gene-Related Peptide; Dose-Response Relationship, Drug; Electric Stimulation; Hydrogen-Ion Concentration; In Vitro Techniques; Male; Mesenteric Arteries; Methoxamine; Norepinephrine; Potassium Chloride; Rats; Receptors, Purinergic P2; Vasoconstriction; Vasopressins

Although A1 noradrenaline (NA) neurons of the caudal medulla provide a direct, excitatory input to supraoptic vasopressin cells, they do not use NA as their primary transmitter. We have now tested the possibility that adenosine 5'-triphosphate (ATP) may fulfill this role. Extracellular recordings from rat supraoptic nucleus demonstrated that locally applied ATP excites neurosecretory vasopressin cells and that this effect is mimicked by the ATP receptor-agonist alpha,beta-methylene ATP and blocked by the ATP receptor-blocker suramin. Suramin did not block the excitatory effect of locally applied NA on vasopressin cells but did block excitations produced by vagus nerve stimulation, such excitations having previously been shown to involve a pathway in which the final relay is an input from the A1 cell group. These results indicate that certain central NA neurons use ATP as a transmitter and also provide the first demonstration of a specific physiological role for central purinergic neurons, i.e. regulation of secretion of the neurohormone vasopressin.

Topics: Adenosine Triphosphate; Animals; Male; Neurons; Norepinephrine; Purinergic Antagonists; Rats; Rats, Wistar; Supraoptic Nucleus; Suramin; Vagus Nerve; Vasopressins

1. The proposal that alpha,beta-methylene adenosine 5'-triphosphate (mATP) inhibits pressor responses in the pithed rat by selective desensitization of P2x-purinoceptors was examined by comparing the selectivity of its inhibitory effect on vascular responses in vitro and in vivo. 2. In isolated ring preparations of rat femoral and tail artery, which had been denuded of endothelium, mATP markedly reduced the contractile response to exogenous ATP but had no effect on the response of the arteries to exogenous noradrenaline (NA). 3. In the pithed rat a substantial proportion of the pressor response to sympathetic nerve stimulation was resistant to alpha-adrenoceptor blockade, suggesting a non-adrenergic component to the sympathetic vasoconstriction. 4. In the pithed rat, repeated administration of desensitizing doses of mATP attenuated the pressor response to sympathetic nerve stimulation by approximately 80%, suggesting that a component of the sympathetic vasoconstriction is mediated by ATP acting on vascular P2x-purinoceptors. However, the same mATP treatment also attenuated, to a similar degree, the pressor responses to intravenous NA, angiotensin II and vasopressin, indicating that the desensitization procedure was non-selective. 5. These results demonstrate that while mATP can be used to desensitize selectively P2x-purinoceptors in vitro, its attenuation of the sympathetic nerve-mediated pressor response in vivo is non-selective.

Topics: Adenosine Triphosphate; Adrenergic alpha-Antagonists; Angiotensin II; Animals; Arteries; Blood Pressure; Decerebrate State; Electric Stimulation; Endothelium, Vascular; In Vitro Techniques; Male; Muscle, Smooth, Vascular; Norepinephrine; Rats; Rats, Inbred Strains; Sympathetic Nervous System; Vasopressins