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Testosterone-induced vasorelaxation in the rat mesenteric arterial bed is mediated predominantly via potassium channels.

ABSTRACT: We have investigated the involvement of nitric oxide and K(+) channels in the vasorelaxant responses to physiologically-relevant concentrations of testosterone in the rat isolated mesenteric arterial bed. Testosterone (100 pM - 10 microM) elicited concentration-dependent relaxations in the isolated mesenteric arterial bed (pEC(50)=9.47 (9.22 - 9.73, 95% CI), maximal relaxation, R(max)=62.8+/-2.0%, n=6). A nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME, 300 microM) or removal of the endothelium significantly inhibited maximal relaxations to testosterone (L-NAME: R(max)=51.4+/-1.1%, P<0.01, n=6; endothelium-denuded: R(max)=46.9+/-2.8%, P<0.001, n=5). Raising the extracellular K(+) concentration to 30 and 60 mM, or pre-treatment with 300 microM tetrabutylammonium chloride (TBA), a calcium-activated K(+) channel inhibitor, abolished vasorelaxations induced by testosterone. A selective inhibitor of ATP-sensitive K(+) (K(ATP)) channels, glibenclamide (10 microM) and an inhibitor of voltage-sensitive K(+) (K(V)) channels, 4-aminopyridine (4-AP, 1 mM) did not affect testosterone-induced responses. Vasorelaxation to 1 microM testosterone was significantly (P<0.05) inhibited by 100 nM charybdotoxin (ChTx), an inhibitor of large conductance calcium-activated K(+) (BK(Ca)) channels (control: 63.3+/-9.9%, n=6; ChTx: 11.9+/-12.7%, n=3). Neither the testosterone receptor antagonist, flutamide (10 microM) nor an aromatase inhibitor, aminoglutethimide (10 microM) inhibited testosterone-induced responses. In conclusion, the present findings demonstrate, in the rat isolated mesenteric arterial bed, that testosterone causes acute vasorelaxations at physiologically relevant concentrations which are, in part, mediated via NO- and endothelium-dependent pathways. However, the activation of BK(Ca) channels plays a substantial role in testosterone-induced vasorelaxation.

SUBMITTER: Tep-areenan P

PROVIDER: S-EPMC1573186 | biostudies-other | 2002 Feb

REPOSITORIES: biostudies-other

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Project description:Obesity plays an important role in the pathogenesis of hypertension. Renal dopamine D1-like receptor-mediated diuresis and natriuresis are impaired in the obese Zucker rat, an obesity-related hypertensive rat model. The role of arterial D1 receptors in the hypertension of obese Zucker rats is not clear.Plasma glucose and insulin concentrations and blood pressure were measured. The vasodilatory response of isolated mesenteric arteries was evaluated using a small vessel myograph. The expression and phosphorylation of D1 receptors were quantified by co-immunoprecipitation and immunoblotting To determine the effect of hyperinsulinemia and hyperglycemia on the function of the arterial D1 receptor, we studied obese Zucker rats (six to eight-weeks old) fed (6 weeks) vehicle or rosiglitazone, an insulin sensitizer (10 mg/kg per day) and lean Zucker rats (eight to ten-weeks old), fed high-fat diet to induce hyperinsulinemia or injected intraperitoneally with streptomycin (STZ) to induce hyperglycemia.In obese Zucker rats, the vasorelaxant effect of D1-like receptors was impaired that could be ascribed to decreased arterial D1 receptor expression and increased D1 receptor phosphorylation. In these obese rats, rosiglitazone normalized the arterial D1 receptor expression and phosphorylation and improved the D1-like receptor-mediated vasorelaxation. We also found that D1 receptor-dependent vasorelaxation was decreased in lean Zucker rats with hyperinsulinemia or hyperglycemia but the D1 receptor dysfunction was greater in the former than in the latter group. The ability of insulin and glucose to decrease D1 receptor expression and increase its phosphorylation were confirmed in studies of rat aortic smooth muscle cells.Both hyperinsulinemia and hyperglycemia caused D1 receptor dysfunction by decreasing arterial D1 receptor expression and increasing D1 receptor phosphorylation. Impaired D1 receptor-mediated vasorelaxation is involved in the pathogenesis of obesity-related hypertension.

Project description:1. In this study, intracellular Ca(2+) was measured as the Fura-2 ratio (R) of fluorescence excited at 340 and 380 nm (F(340)/F(380)) in nonpressurized rat mesenteric small arterioles ( (lumen diameter) 10-25 microm). 2. The response to depolarization using 75 mm KCl was an increase in R from a baseline of 0.96+/-0.01 ([Ca(2+)](i) approximately 74 nm) to 1.04+/-0.01 ( approximately 128 nm) (n=80). The response to 75 mm K(+) was reversibly abolished in Ca(2+)-free physiological saline solution, whereas phentolamine (10 microm) or tetrodotoxin (1 microm) had no effects. LaCl(3) (200 microm) inhibited 61+/-9% of the response. 3. A [K(+)]-response curve indicated that the Ca(2+) response was activated between 15 and 25 mm K(+). The data suggest that the Ca(2+) response was caused by the activation of voltage-dependent Ca(2+) channels. 4. Mibefradil use dependently inhibited the Ca(2+) response to 75 mm K(+) by 29+/-2% (100 nm), 73+/-7% (1 microm) or 89+/-7% (10 microm). Pimozide (500 nm) use dependently inhibited the Ca(2+) response by 85+/-1%. 5. Nifedipine (1 microm) inhibited the Ca(2+) response to 75 mm K(+) by 41+/-12%. The response was not inhibited by calciseptine (500 nm), omega-agatoxin IVA (100 nm), omega-conotoxin MVIIA (500 nm), or SNX-482 (100 nm). 6. Using reverse transcriptase-polymerase chain reaction, it was shown that neither Ca(V)2.1a (P-type) nor Ca(V)2.1b (Q-type) voltage-dependent Ca(2+) channels were expressed in mesenteric arterioles, whereas the Ca(V)3.1 (T-type) channel was expressed. Furthermore, no amplification products were detected when using specific primers for the beta(1b), beta(2), or beta(3) auxiliary subunits of high-voltage-activated Ca(2+) channels. 7. The results suggest that the voltage-dependent Ca(2+) channel activated by sustained depolarization in mesenteric arterioles does not classify as any of the high-voltage-activated channels (L-, P/Q-, N-, or R-type), but is likely to be a T-type channel. The possibility that the sustained Ca(2+) influx observed was the result of a T-type window current is discussed.

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Testosterone-induced vasorelaxation in the rat mesenteric arterial bed is mediated predominantly via potassium channels.

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