Enhanced Pulmonary Alpha-2 Adrenoceptor Responsiveness Under Conditions of Elevated Pulmonary Vascular Tone

Alpha-1 and alpha-2 adrenoceptor-mediated vasoconstriction was studied in the in situ, autoperfused pulmonary circulation of the open-chest anesthetized dog under conditions of normal and elevated pulmonary vascular tone. Under conditions of normal pulmonary vascular tone (10 +/- 1 mm Hg), methoxamine, a selective alpha-1 adrenoceptor agonist, and B-HT 933, a selective alpha-2 adrenoceptor agonist, elicited maximal increases in lobar perfusion pressure of 5 and 2 mm Hg above resting pulmonary tone, respectively. When pulmonary vascular tone was elevated progressively with the thromboxane mimetic, U-46619, serotonin or PGF2 alpha, alpha-1 adrenoceptor-mediated pulmonary vasoconstrictor responses to methoxamine were unaffected, whereas alpha-2 adrenoceptor-mediated pulmonary pressor responses to B-HT 933 were enhanced. Overall the response to B-HT 933 was enhanced 4-fold when pulmonary perfusion pressure was elevated to 19.8 +/- 0.8 mm Hg with U-46619 and almost 5-fold when elevated to 27.0 +/- 1.2 mm Hg. Pulmonary vasoconstrictor responses to angiotensin II were unaffected by elevated pulmonary vascular tone. Enhanced responsiveness of B-HT 933 to elevated pulmonary vascular tone was antagonized by the selective alpha-2 adrenoceptor antagonist, rauwolscine (100 micrograms/kg i.v.), and unaffected by the selective alpha-1 adrenoceptor antagonist, prazosin (100 micrograms/kg i.v.). When canine intralobar pulmonary veins were studied in vitro they contracted to B-HT 933 whereas intralobar pulmonary arteries did not respond. These data indicate that alpha-2 adrenoceptor responsiveness is enhanced markedly and selectively under conditions in which pulmonary vascular tone is elevated.