Tannin induces endothelium-dependent contraction and relaxation of rabbit pulmonary artery

The effects of tannin purified from cotton bracts were studied on pulmonary artery rings isolated from rabbit lungs. Tannin (2 to 50 micrograms/ml) caused concentration-dependent contractions of resting vessels, and maximal responses averaged 25% of the contraction induced by 10(-4) M exogenous norepinephrine. Tannin-induced contractions were blocked by removal of the endothelium and by pretreatment of intact tissues with either indomethacin or SQ 29,548, a thromboxane A2 receptor antagonist. In contrast, tissues contracted with 3 x 10(-7) M norepinephrine relaxed by 49 +/- 8% in response to 50 micrograms/ml tannin. Tannin-induced relaxations were unaltered by pretreatment of tissues with indomethacin but were significantly reduced by pretreatment with hemoglobin. We conclude that tannin causes the release of both smooth muscle contracting and relaxing factors from pulmonary artery endothelial cells. Thromboxane A2 is the contracting factor, whereas an endothelium-derived relaxing factor such as nitric oxide appears to be the primary relaxing factor that is released.