1 We examined the contribution of each a 1 -adrenoceptor (AR) subtype in noradrenaline (NAd)-evoked contraction in the thoracic aortas and mesenteric arteries of mice. Compared with the concentration-response curves (CRCs) for NAd in the thoracic aortas of wild-type (WT) mice, the CRCs of mutant mice showed a significantly lower sensitivity. The pD 2 value in rank order is as follows: WT mice (8.21)4a 1B -adrenoceptor knockout (a 1B -KO) (7.77)4a 1D -AR knockout (a 1D -KO) (6.44)4a 1B -and a 1D -AR double knockout (a 1BD -KO) (5.15). In the mesenteric artery, CRCs for NAd did not differ significantly between either WT (6.52) and a 1B -KO mice (7.12) or a 1D -KO (6.19) and a 1BD -KO (6.29) mice. However, the CRC maximum responses to NAd in a 1D -and a 1BD -KO mice were significantly lower than those in WT and a 1B -KO mice. 2 Except in the thoracic aortas of a 1BD -KO mice, the competitive antagonist prazosin inhibited the contraction response to NAd with high affinity. However, prazosin produced shallow Schild slopes in the vessels of mice lacking the a 1D -AR gene. In the thoracic aorta, pA 2 values in WT mice for KMD-3213 and BMY7378 were 8.25 and 8.46, respectively, and in a 1B -KO mice they were 8.49 and 9.13, respectively. In the mesenteric artery, pA 2 values in WT mice for KMD-3213 and BMY7378 were 8.34 and 7.47, respectively, and in a 1B -KO mice they were 8.11 and 7.82, respectively. These pharmacological findings were in fairly good agreement with findings from comparison of CRCs, with the exception of the mesenteric arteries of WT and a 1B -KO mice, which showed low affinities to BMY7378.3 We performed a quantitative analysis of the mRNA expression of each a 1 -AR subtype in these vessels in order to examine the correlation between mRNA expression level and the predominance of each a 1 -AR subtype in mediating vascular contraction. 4 The rank order of each a 1 -AR subtype in terms of its vasoconstrictor role was in fairly good agreement with the level of expression of mRNA of each subtype, that is, a 1D -AR4a 1B -AR4a 1A -AR in the thoracic aorta and a 1D -AR4a 1A -AR4a 1B -AR in the mesenteric artery. No dramatic compensatory change of a 1 -AR subtype in mutant mice was observed in pharmacological or quantitative mRNA expression analysis.

Isoprenaline is known to produce vascular relaxation through activation of β-adrenoceptors. In recent years, β-adrenoceptor-activated vascular relaxation has been the focus of pharmacological study in terms of both the receptor subtypes and the intracellular signaling mechanisms which trigger smooth muscle mechanical functions. In addition, the possible contribution of the endothelium to β-adrenoceptor-activated relaxation of vascular beds has provoked considerable discussion, with consensus still to be established. In the present study, we examined the effects of isoprenaline on isolated mouse aortic smooth muscles to determine whether the presence of the endothelium plays a substantial role in the relaxation it produces. A possible role for nitric oxide (NO) as a primary endothelium-derived factor released in response to isoprenaline was also elucidated pharmaco-mechanically. In isolated thoracic and abdominal aortae precontracted with phenylephrine (3 × 10 -7 -10 -6 M), isoprenaline elicited relaxation in a concentration-dependent fashion (10 -9 -10 -5 M). In endothelium-denuded preparations, isoprenaline-elicited relaxation was reduced to 40~50% of the response obtained in endothelium-intact preparations. In the preparations treated with N G -nitro-L-arginine methyl ester (L-NAME, 3 × 10 -4 M; an NO synthase inhibitor) or 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxalin-1-one (ODQ, 10 -5 M; a soluble guanylyl cyclase inhibitor), isoprenalineelicited relaxation was attenuated almost to the same degree as the response in endothelium-denuded preparations. The degree of endothelium-dependency in isoprenaline-elicited relaxation was largely diminished when treated with propranolol (3 × 10 -6 M). The present findings indicate that isoprenaline substantially relaxes the mouse aorta with both endothelium-dependent and -independent mechanisms. The endothelium-dependent component seems to correspond to about 50% of the isoprenaline-elicited relaxation, and is almost entirely due to endothelium-derived NO. Activation of propranolol (3 × 10 -6 M)-inhibitable β-adrenoceptors seems to be primarily responsible for the NO-mediated endothelium-dependent pathway in isoprenaline-elicited Correspondence to:

The possible functional coupling between β1-adrenoceptor and MaxiK channels which results in smooth muscle relaxation was examined in the guinea-pig esophageal muscularis mucosae. Isoprenaline-elicited relaxation of esophageal smooth muscle was confirmed to be mediated through β1-adrenoceptors as the response was competitively antagonized by a β1-selective antagonist atenolol with a pA2 value of 7.01. Iberiotoxin (IbTx, 10 -7 M), a selective MaxiK channel inhibitor, substantially diminished the relaxant response to isoprenaline. The extent of the MaxiK channel contribution to the relaxant response was 15-40% of the control response when estimated as the E50%-Emax responses to isoprenaline. The relaxation to isoprenaline was also attenuated by high-KCl (80 mM) to the same degree as the relaxant response generated in the presence of IbTx, and thus the estimated extent of the K + channel contribution was 10-40%. These findings indicate that β1-adrenoceptors are substantially coupled with MaxiK channels to produce relaxation of esophageal smooth muscle in the guinea-pig. Although MaxiK channels account for the contribution of K + channels to the β1-adrenoceptor-mediated relaxation in this smooth muscle preparation, their contribution seems to be less when compared to the β2-adrenoceptor-mediated relaxation of tracheal smooth muscle.

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