endothelin-1 and 1-3-dipropyl-8-cyclopentylxanthine

Myocardial hypoperfusion is accompanied by concomitant increases in adenosine and endothelin-1 (ET-1) production, but the vasodilatory effect of adenosine prevails over that of ET-1. Therefore, we hypothesized that adenosine-induced or ischemic preconditioning reduces the vasoconstrictive effect of ET-1. Coronary arteriolar diameter in vivo was measured using fluorescence microangiography in anesthetized open-thorax dogs. ET-1 (5 ng. kg(-1). min(-1) administered intracoronary, n = 10) induced progressive constriction over 45 min [25 +/- 6% (SE)]. The constriction was blocked by preconditioning with adenosine (25 microgram. kg(-1). min(-1) administered intracoronary) for 20 min and 10 min of washout (n = 10) or attenuated by ischemic preconditioning (four 5-min periods of ischemia, 9 +/- 5% at 45 min). To investigate the receptor involved in this process, coronary arterioles (50-150 micrometer) were isolated and pressurized at 60 mmHg in vitro. The ET-1 dose-response curve (1 pM-5 nM) was rightward shifted after preconditioning with adenosine (1 microM) for 20 min and 10 min of washout (n = 11). Blockade of A(2) receptors [8-(3-chlorostyryl)caffeine, 1 microM, n = 9] but not A(1) receptors (8-cyclopentyl-1,3-dipropylxanthine, 100 nM, n = 7) prevented this shift. These results suggest that adenosine confers a vascular preconditioning effect, mediated via the A(2) receptor, against endothelin-induced constriction. This effect may offer a new protective function of adenosine in preventing excessive coronary constriction.

Topics: Adenosine; Animals; Arterioles; Caffeine; Coronary Circulation; Coronary Vessels; Dogs; Dose-Response Relationship, Drug; Endothelin-1; In Vitro Techniques; Ischemic Preconditioning, Myocardial; Microcirculation; Purinergic P1 Receptor Antagonists; Vasoconstriction; Vasodilation; Vasodilator Agents; Xanthines

The aim of the present study was to investigate the mechanisms regulating endothelin-1 (ET-1) secretion in rat thyroid FRTL-5 cells. ET-1 was found to be secreted after stimulation with adenosine and ATP. The release of ET-1 was sensitive to pertussis toxin, indicating a role of G-proteins in the stimulus-secretion coupling. The stimulation evoked by ATP or adenosine was inhibited by the P1-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and in the presence of adenosine deaminase the adenosine- and ATP-mediated ET-1 secretion was abolished. These evidences suggest a role of a P1-adenosine receptor in the secretion of ET-1. Increasing cyclic AMP with forskolin decreased the adenosine-mediated secretion. In addition, the intracellular calcium chelator BAPTA or inhibition of calcium entry with Ni2+ prevented the response. Protein kinase C (PKC) is also partly involved in ET-1 secretion in FRTL-5 cells. Activation of PKC with the phorbol ester phorbol 12-myristate 13-acetate (PMA) stimulated the secretion of ET-1 in a time- and dose-dependent manner. Furthermore, downregulation of PKC decreased the secretion of ET-1 stimulated by adenosine. In conclusion, ET-1 secretion in FRTL-5 cells is stimulated via a pertussis toxin-sensitive P1-receptor pathway which is modulated by several signal transduction mechanisms including cAMP, Ca2+, and PKC.

Topics: Adenosine; Adenosine Deaminase; Adenosine Triphosphate; Animals; Calcium; Cell Line; Cyclic AMP; Cyclic GMP; Endothelin-1; Enzyme Activation; Nitric Oxide; Protein Kinase C; Purinergic P1 Receptor Agonists; Rats; Receptors, Purinergic P1; Secretory Rate; Signal Transduction; Thyroid Gland; Xanthines

This study aimed at characterizing the influence of endothelium on noradrenaline release from the canine pulmonary artery. Tritium overflow from intact or endothelium-free vessels preloaded with 0.2 mumol.l-1 3H-noradrenaline was evoked by electrical stimulation (1 Hz, during 5 min) or potassium (25-100 mmol.l-1). The fractional release of tritium evoked by electrical stimulation was increased by removing the endothelium [from 1.7 (1.2; 2.4) to 2.7(2.3; 3.2) x 10(-5).pulse-1, n = 10; P < 0.05]. Neither NG-nitro-L-arginine methyl ester (L-NAME) (up to 300 mumol.l-1) nor indomethacin (up to 30 mumol.l-1), nor endothelin-1 (up to 30 nmol.l-1), nor suramin (up to 300 mumol.l-1) changed tritium release evoked by electrical stimulation. In contrast, the selective A1-adenosine antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (3.3-33 nmol.l-1) concentration-dependently increased, and the selective A1-adenosine agonist N6-cyclopentyladenosine (CPA) (3.3-100 nmol.l-1) concentration-dependently decreased the evoked release of noradrenaline. Since the effects of DPCPX were observed in endothelium-intact tissues only, it may be concluded that adenosine secreted by the endothelium activates prejunctional release-inhibiting A1-receptors. Tetraethylammonium (TEA) (3.3-33 mmol.l-1) enhanced tritium overflow evoked by electrical stimulation more in endothelium-free than in endothelium-intact vessels, indicating that some K(+)-channel opener is involved in the inhibitory role of endothelium on noradrenaline release. Since it had been previously shown that A1-adenosine receptors are coupled to K(+)-channels, it is suggested that adenosine may inhibit noradrenaline release through the opening of K(+)-channels. In conclusion, the results show that in the canine pulmonary artery, adenosine is a good candidate for the endothelium-dependent inhibitory factor which is responsible for the reduction of noradrenaline release evoked by electrical stimulation.

Topics: Adenosine; Animals; Dogs; Electric Stimulation; Endothelin-1; Endothelium, Vascular; Female; Isotope Labeling; Male; Neuromuscular Junction; NG-Nitroarginine Methyl Ester; Nitric Oxide Synthase; Norepinephrine; Potassium; Potassium Channels; Pulmonary Artery; Purinergic P1 Receptor Antagonists; Suramin; Tetraethylammonium; Tetraethylammonium Compounds; Tritium; Xanthines