Desensitization of the vascular contractile response to cumulative doses of α2-adrenoceptor agonists

1986 ◽  
Vol 64 (10) ◽  
pp. 1343-1345 ◽  
Author(s):  
D. W. Cheung
Keyword(s):  
Dose Response ◽  
Saphenous Vein ◽  
Contractile Response ◽  
Rat Tail Artery ◽  
Contractile Responses ◽  
Tail Artery ◽  
Α2 Adrenoceptor ◽  
Adrenoceptor Agonists ◽  
Rat Tail

Contractile responses to single or cumulative doses of α-adrenoceptor agonists were compared in the tail artery and the saphenous vein of the rat. In the rat tail artery, there were no differences in the dose–response relationships to noradrenaline, methoxamine, and KCl whether the agonists were applied as single or cumulative doses. However, the responses to single doses of clonidine and B-HT 920 were significantly larger than similar doses applied cumulatively. In the rat saphenous vein, responses to single doses of noradrenaline, clonidine, and B-HT 920 were also significantly larger than the corresponding cumulative doses. However, there was no difference in the responses to KCl. It was suggested that desensitization of α2-adrenoceptors in these vessels may result in the diminished responses to cumulative doses of the agonists. Desensitization appeared to be specific to α2-adrenoceptors, since the effect was not observed in responses mediated by the α1-adrenoceptors and KCl.

Calcium channel activation in vascular smooth muscle by BAY K 8644

1984 ◽  
Vol 62 (11) ◽  
pp. 1401-1410 ◽  
Author(s):  
C. M. Su ◽  
V. C. Swamy ◽  
D. J. Triggle
Keyword(s):  
Smooth Muscle ◽  
Dose Response ◽  
Response Curve ◽  
Bay K 8644 ◽  
Rat Tail Artery ◽  
Channel Activation ◽  
Tail Artery ◽  
Voltage Dependent ◽  
Adrenoceptor Agonists ◽  
Rat Tail

BAY K 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)pyridine-5-carboxylate) and CGP 28 392 (ethyl-4(2-difluoromethoxyphenyl)-1,4,5,7-tetrahydro-2-methyl-5-oxofuro-[3,4-b]pyridine-3-carboxylate) are closely related in structure to nifedipine and other 1,4-dihydropyridine Ca2+ channel antagonists. However, both BAY K 8644 and CGP 28 392 serve as activators of Ca2+ channels. In the rat tail artery, responses to BAY K 8644 are dependent upon [Formula: see text] and prior stimulation by K+ or by the α-adrenoceptor agonists, phenylephrine and BHT 920 (6-allyl-2-amino-5,6,7,8,-tetrahydro-4H-thiazolo[4,5-d]azepin dihydrochloride). Responses are blocked noncompetitively by the Ca2+ channel antagonists D-600 ((−)-D-600 > (+)-D-600) and diltiazem, but competitively by nifedipine (pA2 = 8.27). This suggests that activator and inhibitor 1,4-dihydropyridines interact at the same site. BAY K 8644 potentiates K+ responses and Ca2+ responses in K+-depolarizing media. The leftward shift of the K+ dose–response curve produced by BAY K 8644 suggests that this ligand facilitates the voltage-dependent activation of the Ca2+ channel. The pA2 value for nifedipine antagonism of BAY K 8644 responses is significantly lower than that for nifedipine antagonism of Ca2+ responses in K+ (25–80 mM) depolarizing media (9.4–9.6), suggesting that the state of the channel may differ according to the activating stimulus.

Effects of pertussis and cholera toxins on α-adrenoceptor function in rat tail artery: differences in hypertension

1993 ◽  
Vol 71 (10-11) ◽  
pp. 791-799 ◽  
Author(s):  
Xiao-Fang Li ◽  
Christopher R. Triggle
Keyword(s):  
Cholera Toxin ◽  
Pertussis Toxin ◽  
Rat Tail Artery ◽  
Response Curves ◽  
Contractile Responses ◽  
Tail Artery ◽  
Adrenoceptor Antagonist ◽  
Adrenoceptor Agonist ◽  
The Right ◽  
Rat Tail

The α1- and α2-adrenoceptor-stimulated contractile responses of rat tail artery rings were compared in Sprague–Dawley (SD), spontaneously hypertensive (SHR), and Wistar–Kyoto (WKY) rats that were untreated, treated with pertussis toxin, or treated with cholera toxin. The maximal responses, expressed as milligrams of tension, induced by clonidine (an α2-adrenoceptor agonist) and cirazoline (a selective (α1-adrenoceptor agonist) were significantly greater in SHR than in SD or WKY, and the tissues were more sensitive to the agonists in SHR or SD than in WKY. Yohimbine (0.1 μM), a selective α2-adrenoceptor antagonist, shifted the dose–response curves for clonidine to the right. The effects of yohimbine were greater in SD than in WKY or SHR, but not different between WKY and SHR. Prazosin (0.05 μM), a selective α1-adrenoceptor antagonist, shifted the dose–response curves of cirazoline to the right, but the effects of prazosin were not different among these three strains of rats. Nifedipine (0.05 μM) completely blocked the response to clonidine in SD and WKY; however, in SHR, approximately one-third of the response to clonidine was resistant to nifedipine. Nifedipine, at 0.05 μM, only partially inhibited responses to cirazoline in SD, SHR, and WKY, and no differences were noted between the strains. Pertussis toxin pretreatment (50 μg/kg, 3 days before experiment) almost completely blocked the responses to clonidine, but only partially inhibited those to cirazoline. After pertussis toxin pretreatment, the responses (maximal effects and EC50s) to clonidine and cirazoline were not significantly different in arteries from the three strains of rats. A combination of pertussis toxin and nifedipine resulted in an additive inhibition of the responses induced by cirazoline. cholera toxin pretreatment (0.3 mg/kg, 3 days before experiment), however, had no effects on the contractile responses induced by either clonidine or cirazoline, or on the inhibitory effects of nifedipine in SHR, SD, and WKY. These results indicate that (i) the maximal responses to α1- and α2-adrenoceptor agonists are enhanced in rat tail artery rings from SHR; (ii) tissues from SH and SD rats are also more sensitive to cirazoline and clonidine than are tissues from WKY; (iii) responses to clonidine, but not cirazoline, in tissues from the SHR are less sensitive to nifedipine than tissues from SD and WKY; (iv) a G-protein sensitive to pertussis but not cholera toxin is involved in the regulation of both α1 and α2-adrenoceptor signal transduction processes in rat tail artery smooth muscle; and (v) pretreatment with pertussis toxin reduces the enhanced response levels of SHR tissues so that the maximal contractile responses to both α1- and α2-adrenoceptor agonists are equivalent in arteries from the three strains of rats.Key words: pertussis toxin, cholera toxin, α1-adrenoceptor, α2-adrenoceptor, rat tail artery ring.

Amiloride acts as an α-adrenergic antagonist in the isolated rat tail artery

1986 ◽  
Vol 64 (7) ◽  
pp. 931-933 ◽  
Author(s):  
Vladimír Palatý
Keyword(s):  
Contractile Response ◽  
Krebs Solution ◽  
Rat Tail Artery ◽  
Tail Artery ◽  
Adrenergic Antagonist ◽  
Adrenergic Transmission ◽  
Endogenous Noradrenaline ◽  
Site Of Action ◽  
Postsynaptic Site ◽  
Rat Tail

In the final concentration of 100 μM, amiloride increased substantially the overflow of endogenous noradrenaline and decreased that of 3,4-dihydroxyphenylethylene glycol from the rat tail artery into Krebs solution supplemented with 10 μM veratridine. The overflow of the amine into a 120 mM-K version of Krebs solution was unaffected by amiloride, while that of the glycol was reduced. Abolition of the contractile response to 10 μM veratridine by 2 μM phentolamine indicated that the response was due to release of endogenous noradrenaline. Addition of amiloride in the final concentrations of 10 and 100 μM caused relaxation of strips contracted by the alkaloid. The dose–response relations for exogenous noradrenaline measured in the absence or presence of 50 μM amiloride indicated that the drug acted as a reversible competitive α-adrenergic antagonist. The phentolamine-resistant component of the contractile response to the 120 mM-K solution was unaffected by 100 μM amiloride. Although the exact site of action of amiloride remains to be determined, it can be concluded that amiloride inhibits adrenergic transmission at a postsynaptic site at a step preceding elevation of myoplasmic Ca2+.

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