Thyroxine treatment of aged or young rats demonstrates that vascular responses mediated by β‐adrenoceptor subtypes can be differentially regulated

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Abstract
1 Responses to vascular relaxant drugs were obtained on KCl (15 mM)-contracted isolated ring preparations of pulmonary artery and aorta from young (1–2 months old) and aged (> 16 months old) rats. These vessels contain both β1- and β2-adrenoceptors. 2 Relaxant responses (i.e. relaxation expressed as a % of the KCl-induced contraction) to isoprenaline, procaterol (β2-seleotive partial agonist), fenoterol (β2-selective) and noradrenaline (β1-selective) but not those to forskolin, 3-isobutyl-1-methylxanthine, enprofylline or sodium nitrite, were smaller on preparations from aged rats than on those from young rats. 3 Thyroxine (T4)-treatment (1 mg kg−1 s.c. thrice weekly for 3–5 weeks) of aged or young rats enhanced responses to isoprenaline and noradrenaline but reduced those to procaterol, when compared with preparations from age-matched saline-treated control rats. 4 The agonist order of potency, determined in young rats, was isoprenaline>noradrenaline>adrenaline in preparations from T4-treated rats compared with isoprenaline > adrenaline > noradrenaline in saline-treated control rats. 5 It is concluded (a) that the age-related decline in vascular responses to β-adrenoceptor agonists involves β-adrenoceptor mechanisms specifically and possibly β2-adrenoceptors more than β1-adrenoceptors; and (b) that T4-treatment of rats enhances β1-adrenoceptor-mediated and reduces, or does not change, β2-adrenoceptor-mediated responses of preparations of rat pulmonary artery and aorta. In preparations from control rats β2-adrenoceptors were functionally predominant but in preparations from T4-treated rats β1-adrenoceptors appeared to become functionally predominant.