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Vasodilator Effect of Terbutaline: In Vivo Evidence for the Existence of β2‐Adrenoceptors in the Microcirculation of Rats, Hamsters, and Guinea Pigs

The characteristics of β2-adrenoceptors were studied in vivo in the microcirculation of cremaster muscle and mesoappendix in rats, cheek pouch in hamsters, and mesentery in guinea pigs, using a video-microscope technique. Terbutaline (β2-selective agonist) caused vasodilatation in all four microcirculation preparations studied, whereas butoxamine (β2-selective antagonist) competitively inhibited the terbutaline-induced vasodilatation. The in vivo pA2 values of butoxamine inhibition of terbutaline vasodilator effect were estimated from Schild plots. The pA2 values (6.22–6.31) were statistically identical in all four microcirculations studied and similar to in vitro values reported in other tissues. In contrast, tazolol and prenalterol (both β1-selective agonists) did not produce any vasodilatation in the microcirculations studied. These results supported the existence of β2-adrenoceptors involved in vasodilatation in the microcirculation.