Distribution of functional adrenergic receptor subtypes in the microcirculation of rat trachea

Although blood flow to the pulmonary airways is known to be largely under sympathetic control, virtually nothing is known about adrenergic regulation of vascular segments within the airway microcirculation. To evaluate the distribution of functional adrenergic receptor subtypes in the microvessels of the large airways, the change in diameter of adventitial vessels in rat trachea was measured following suffusion with selective and nonselective receptor agonists and antagonists. Microvessels were viewed with a video microscope, and vessel diameters were measured using video calipers. Arterioles (11.0 to 40.0 microns, n = 32), small postcapillary venules (11.0 to 26.0 microns, n = 16), medium venules (28.0 to 59.5 microns, n = 40), and large collecting venules (61.0 to 99.0 microns, n = 42) were distinguished. Similar sensitivities to norepinephrine (NE), a mixed alpha 1 and alpha 2 agonist, were observed in arterioles and medium venules with EC50 (agonist concentration needed to produce 50% of the maximal response) for contraction of 2.4 x 10(-7) and 3.3 x 10(-7) M, respectively. Large venules (EC50 of 1.6 x 10(-6) M) were significantly (p < 0.05) less sensitive than arterioles to NE. In the presence of propranolol, a beta receptor antagonist, the EC50 values for NE were not different between the three vessel groups, although the response to low doses of NE was significantly increased in arterioles. When vessels were treated with propranolol and phenylephrine, a selective alpha 1 agonist, arterioles (EC50 of 4.1 x 10(-7) M) were significantly more sensitive than large venules (EC50 of 4.9 x 10(-6) M).(ABSTRACT TRUNCATED AT 250 WORDS)