thromboxane-a2 and iberiotoxin

The present study was undertaken to determine whether treatment with genistein, the plant-derived estrogen-like compound influences agonist-induced vascular smooth muscle contraction and, if so, to investigate related mechanisms. The measurement of isometric contractions using a computerized data acquisition system was combined with molecular experiments. Genistein completely inhibited KCl-, phorbol ester-, phenylephrine-, fluoride- and thromboxane A(2)-induced contractions. An inactive analogue, daidzein, completely inhibited only fluoride-induced contraction regardless of endothelial function, suggesting some difference between the mechanisms of RhoA/Rho-kinase activators such as fluoride and thromboxane A(2). Furthermore, genistein and daidzein each significantly decreased phosphorylation of MYPT1 at Thr855 had been induced by a thromboxane A(2) mimetic. Interestingly, iberiotoxin, a blocker of large-conductance calcium-activated potassium channels, did not inhibit the relaxation response to genistein or daidzein in denuded aortic rings precontracted with fluoride. In conclusion, genistein or daidzein elicit similar relaxing responses in fluoride-induced contractions, regardless of tyrosine kinase inhibition or endothelial function, and the relaxation caused by genistein or daidzein was not antagonized by large conductance K(Ca)-channel inhibitors in the denuded muscle. This suggests that the RhoA/Rho-kinase pathway rather than K(+)-channels are involved in the genistein-induced vasodilation. In addition, based on molecular and physiological results, only one vasoconstrictor fluoride seems to be a full RhoA/Rho-kinase activator; the others are partial activators.

Topics: Animals; Aorta; Blotting, Western; Carcinogens; Cardiotonic Agents; Genistein; Isoflavones; Male; Muscle Contraction; Muscle Relaxation; Peptides; Phenylephrine; Phorbol Esters; Phosphorylation; Phytoestrogens; Potassium Channel Blockers; Potassium Channels, Calcium-Activated; Potassium Chloride; Protein Phosphatase 1; Rats; Rats, Sprague-Dawley; rhoA GTP-Binding Protein; Sodium Fluoride; Thromboxane A2; Toxins, Biological

We examined the hypothesis that activity of Ca2+-dependent K+ channels is increased in the basilar artery during chronic hypertension. Diameter of the basilar artery was measured using a cranial window in anesthetized normotensive Wistar-Kyoto rats (WKY, arterial pressure = 109 +/- 3 mmHg, mean +/- SE) and stroke-prone spontaneously hypertensive rats (SHRSP, arterial pressure = 179 +/- 4 mmHg). Responses of the basilar artery to topical application of tetraethylammonium ion (TEA), an inhibitor of Ca2+-dependent K+ channels, were examined in WKY and SHRSP. Vessel diameter decreased by 2 +/- 1 and 4 +/- 0.1% in WKY and by 7 +/- 2 and 18 +/- 1% in SHRSP (P < 0.05 vs. WKY) in response to 10(-4) and 10(-3) M TEA, respectively. Similar results were obtained using iberiotoxin (10(-8) and 10(-7) M), a highly selective inhibitor of Ca2+-dependent K+ channels. In contrast to constrictor responses to TEA and iberiotoxin, constrictor responses of the basilar artery in response to serotonin and U-46619 were similar in WKY and SHRSP. In WKY rats that were made chronically hypertensive (arterial pressure = 172 +/- 6 mmHg) after treatment for 4 wk with N(G)-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase, constriction of the basilar artery in response to TEA was also enhanced. These findings suggest that activity of Ca2+-dependent K+ channels is enhanced in the basilar artery in vivo in two models of chronic hypertension. Thus Ca2+-dependent K+ channels in the basilar artery may be activated during chronic hypertension, perhaps as a response to elevation of intracellular concentration of Ca2+.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Animals; Basilar Artery; Calcium; Hypertension; Muscle Contraction; Peptides; Potassium Channel Blockers; Potassium Channels; Prostaglandin Endoperoxides, Synthetic; Rats; Rats, Inbred SHR; Rats, Inbred WKY; Serotonin; Tetraethylammonium; Tetraethylammonium Compounds; Thromboxane A2; Vasoconstriction; Vasoconstrictor Agents; Wasp Venoms

The functional importance of Ca++ activated K+ (K(Ca)) channels in cGMP mediated relaxation of pressurized septal arteries (internal basal diameter 213 +/- 4 microm) was investigated. Vascular tone was increased by the thromboxane A2 analogue, U-46619 and internal pressure was maintained at 60 mmHg. Vessels were tested with an endothelium independent agonist (nitroprusside) and endothelium dependent agonist (acetylcholine) of nitric oxide which activates soluble guanylate cyclase. Receptor activation of particulate guanylate cyclase was tested by atrial natriuretic peptide. Direct changes in intracellular cGMP concentration were done with the cell permeable analog, 8-Bromo-cGMP. Tetraethylammonium ion (TEA+), 1 mM, significantly inhibited relaxation to nitroprusside from 10(-7) to 10(-3) M with a maximal inhibition of 53 +/- 8% at 10(-3) M. Relaxation to acetylcholine from 10(-9) M to 10(-5) M was significantly inhibited by TEA+ with a maximal inhibition of 52 +/- 13% at 10(-7) M. TEA+ significantly inhibited relaxation to 8-Bromo-cGMP from 10(-6) M to 10(-3) M with a maximal inhibition of 59 +/- 14% at 10(-4) M. The relaxation response to atrial natriuretic peptide from 10(-12) M to 10(-7) M was significantly inhibited by TEA+ with a maximal inhibition of 84 +/- 5% at 10(-11) M. The large conductance K(Ca) channel blocker, iberiotoxin, eliminated the relaxation response to 8-Bromo-cGMP (10(-3) M). The results suggest that a large portion of the dilator action of cGMP is mediated by effects on K+ membrane channels.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Acetylcholine; Animals; Atrial Natriuretic Factor; Calcium; Coronary Vessels; Cyclic GMP; In Vitro Techniques; Muscle Relaxation; Muscle, Smooth, Vascular; Nitroprusside; Peptides; Potassium Channels; Prostaglandin Endoperoxides, Synthetic; Rats; Scorpion Venoms; Tetraethylammonium; Tetraethylammonium Compounds; Thromboxane A2; Vasoconstrictor Agents

The aim of the present study was to investigate the contribution of large-conductance calcium-activated potassium (large-conductance KCa) channels to adenosine (Ado)- and nitroprusside-mediated relaxation in small coronary arteries. Canine subepicardial arteries (170 +/- 23 microns at 120 mmHg) were studied as in vitro pressurized vessels. Pressure-diameter experiments showed myogenic tone over a physiological range of pressures. Tone was increased with the thromboxane A2 analogue 9,11-dideoxy-11 alpha,9 alpha-epoxy-methanoprostaglandin F2 alpha (U-46619). Tetraethylammonium (TEA+; 1 mM) significantly inhibited Ado-induced [and by implication, adenosine 3',5'-cyclic monophosphate (cAMP)-induced] relaxations at Ado concentrations ranging from 0.1 to 10 microM with maximal inhibition (61 +/- 8%) at 1 microM Ado. The large-conductance KCa-channel blocker iberiotoxin (IbTX; 0.01-0.1 microM) inhibited Ado-mediated relaxation in a concentration-dependent manner. Inhibition by IbTX increased with increasing vessel pressure (i.e., 45 +/- 12% at 40 mmHg and 83 +/- 20% at 120 mmHg). TEA+ had a minimal effect (8 +/- 3%) on relaxation induced by nitroprusside. Similar results were found with acetylcholine and bradykinin. These results suggest that (in dog coronary arteries with diameter < 200 microns) large-conductance KCa-channel modulation may play a major role in cAMP-mediated relaxation but is not significant in guanosine 3',5'-cyclic monophosphate-mediated relaxation.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Acetylcholine; Adenosine; Animals; Bradykinin; Calcium; Coronary Vessels; Dogs; Dose-Response Relationship, Drug; In Vitro Techniques; Muscle, Smooth, Vascular; Nitroprusside; Peptides; Potassium Channel Blockers; Potassium Channels; Prostaglandin Endoperoxides, Synthetic; Scorpion Venoms; Tetraethylammonium; Tetraethylammonium Compounds; Thromboxane A2; Vasoconstrictor Agents; Vasodilation