Comparison of the α-adrenoceptor-mediated effects of β3-adrenoceptor ligands in rat pulmonary artery

Jul 14, 2005Naunyn-Schmiedeberg's archives of pharmacology

How beta3-adrenoceptor drugs affect alpha-adrenoceptor responses in rat lung arteries

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Abstract

Bupranolol and SR 59230A demonstrate competitive antagonist properties at alpha1-adrenoceptors in rat pulmonary arteries.

Bupranolol induces relaxation in arteries precontracted with phenylephrine but not with prostaglandin F2alpha.
SR 59230A causes relaxation in phenylephrine-precontracted arteries and a contractile response in PGF2alpha-precontracted arteries.
The contractile response induced by SR 59230A is not affected by the irreversible antagonist phenoxybenzamine.
BRL 37344 shows slight relaxation effects at high concentrations, while CL 316243 does not exhibit relaxation in either precontracted artery type.
Bupranolol and SR 59230A antagonize phenylephrine's effects in a competitive manner, with pA2 values of 6.38 and 7.08, respectively.
BRL 37344 shifts the concentration-response curve to phenylephrine to the right, indicating antagonist activity, while CL 316243 does not.

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We have recently shown that the beta-adrenoceptor ligands CGP 12177, bupranolol, and SR 59230A (aryloxypropanolamines), but not BRL 37344 and CL 316243 (phenylethanolamines), exhibit significant affinity for alpha1-adrenoceptors and that CGP 12177 displays partial agonist properties at alpha-adrenoceptors in rat pulmonary artery. In this study, bupranolol and SR 59230A were further evaluated for their potential alpha-adrenoceptor mediated effects (i.e., agonist and/or antagonist properties) in rat intralobar pulmonary artery and compared with BRL 37344 and CL 316243. Bupranolol induced a relaxation in phenylephrine-precontracted arteries, but had no effect in prostaglandin F2alpha(PGF2alpha) -precontracted ones. SR 59230A also elicited a relaxation in phenylephrine-precontracted arteries. In PGF2alpha -precontracted arteries, SR 59230A induced a contractile response that was insensitive to the irreversible alpha-adrenoceptor antagonist phenoxybenzamine. BRL 37344 at high concentrations, but not CL 316243, produced slight relaxation in both phenylephrine- and PGF2alpha -precontracted arteries. The contractile response to phenylephrine was antagonized by bupranolol and SR 59230A in a competitive manner (pA2: 6.38 and 7.08 respectively). The concentration-response curve to phenylephrine was also shifted to the right by BRL 37344 (mean pKb: 4.45), but not by CL 316243 (100 microM). This study indicates that the aryloxypropanolamine derivatives bupranolol and SR 59230A exhibit competitive antagonist, but no agonist properties on alpha1-adrenoceptors, SR 59230A also inducing alpha-adrenoceptor-independent contraction. Among the phenylethanolamines, BRL 37344 but not CL 316243, also exerts an antagonist effect on alpha1-adrenoceptors, with a much lower potency than the aryloxypropanolamines studied.

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Comparison of the α-adrenoceptor-mediated effects of β3-adrenoceptor ligands in rat pulmonary artery

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