Potassium chloride- and norepinephrine-induced contractile responses in rabbit pulmonary blood vessels

Responses to KCl and norepinephrine (NE) were examined with ring segments of right extrapulmonary artery, intrapulmonary artery and intrapulmonary vein isolated from rabbit lung. From concentration-response curves to KCl, optimal applied loads were determined to be 5 g for right extrapulmonary artery and intrapulmonary artery and 500 mg for intrapulmonary vein. Both individual and cumulative concentration-response curves to KCI and NE were performed. All gave reproducible results; however, tissues were more sensitive to cumulative addition of either contractile agent. Of three successive cumulative NE concentration-response curves, second and third curves were most reproducible in relation to both sensitivity to NE and maximal force development. Cumulative NE concentration-response curves also exhibited a triphasic pattern, consisting of an initial contractile response (10(-10) to 10(-6) M NE) followed by relaxation or reversal of contractile force development (10(-6) to 10(-4) M NE), then a second contractile response at concentrations of NE greater than 10(-4) M. This response pattern was most prominent in intrapulmonary vessels and was absent in rabbit aorta.