Tracheal microvascular responses to beta-adrenergic stimulation in anesthetized rats

Previous study of adrenergic control of the tracheal vasculature in rats (1) demonstrated that beta-adrenergic blockade heightened arteriolar and large venular contractile responses to norepinephrine, a nonselective alpha-adrenergic agonist. The present study was undertaken to confirm the presence of functional beta-adrenergic receptors and to determine which beta-adrenergic receptor subtypes mediate vasodilation in this tissue. Tracheal adventitial arterioles (12.0 to 47.0 micro m initial diameter, n=39) and venules (48.0 to 98.5 micrometers initial diameter, n=44) were observed through a video microscope, and vessel diameters were measured. Vessels were preconstricted with 10(-4) M phenylephrine (PHE), a selective alpha 1-adrenergic agonist, to achieve sufficient tone for measurement of dilation responses. When vessels were treated only with PHE, arterioles and venules constricted to 55.9% and 67.6% of their initial diameter, respectively, after 15 min of suffusion. When preconstricted vessels were treated with the nonselective beta-adrenergic agonist isoproterenol (10(-5) M), both arterioles and venules significantly dilated from 63.4% to 82.9% and from 71.5% to 81.3% of their initial diameters. At high concentration (10(-5) M), the putative beta 2-adrenergic agonist terbutaline also caused preconstricted arterioles and venules to significantly dilate from 70.8% to 79.8% and from 71.5% to 83.4% of their initial diameters. The selective beta 1-adrenergic antagonist atenolol (10(-6) M) did not affect terbutaline-induced dilation in preconstricted arterioles, but greatly attenuated dilation in preconstricted venules. From these data, we conclude that beta 2-adrenergic receptors are present in and mediate dilation of tracheal arterioles, and also, that the dilation in large tracheal venules is mediated in large part through beta 1-adrenergic receptors.