Comparison of β‐adrenoceptors mediating vasodilatation in canine subcutaneous adipose tissue and skeletal muscle

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Abstract
Blood flow changes in response to various drugs in simultaneously autoperfused canine subcutaneous adipose tissue and gracilis muscle were compared to study the vascular β‐adrenoceptors. Compared to isoprenaline the β2‐selective agonist salbutamol was 4–6 times more potent as a vasodilator in the muscle than in adipose tissue. Furthermore two β1‐selective agonists (Tazolol and H80/62) caused vasodilatation in adipose tissue but not in the gracilis muscle. When given by close i.a. injection after β‐adrenoceptor blockade, adrenaline was a more potent vasoconstrictor than noradrenaline in both tissues. Before β‐blockade, however, noradrenaline was the more potent vasoconstrictor in the gracilis muscle whereas adrenaline was more potent in adipose tissue. Intravenous infusion of adrenaline in doses causing vasodilatation in the muscle caused vasoconstriction in adipose tissue whereas intravenous infusion of noradrenaline caused vasoconstriction in both tissues. The present findings suggest that the β‐adrenoceptors mediating vasodilatation in skeletal muscle are mainly of the β2‐type, whereas β1‐adrenoceptors seem to predominate in subcutaneous adipose tissue. Since adrenaline is a much more potent β2‐ than β1‐agonist, these differences point to different roles of intravascular adrenaline in the two sites. In skeletal muscle circulating adrenaline is mainly a vasodilator whereas in subcutaneous adipose tissue it mainly acts as a vasoconstrictor.