ET-1 receptor gene expression and distribution in L1 and L2 cells from hypertensive sheep pulmonary artery

We examined gene and surface expression and activity of the endothelin (ET)-1 receptors (ETA and ETB) in subendothelial (L1) and inner medial (L2) cells from the main pulmonary artery of sheep with continuous air embolization (CAE)-induced chronic pulmonary hypertension (CPH). According to quantitative real-time RT-PCR, basal gene expression of both receptors was significantly higher in L2 than L1 cells, and hypertensive L2 cells showed significantly higher gene expression of ETB than controls. Expression of both genes in hypertensive L1 cells was similar to controls. Fluorescence-activated cell sorter analysis confirmed the increased distribution of ET(B) in hypertensive L2 cells. Although only the ETA receptors in control L2 cells showed significant binding of [125I]-labeled ET-1 at 1 h, both receptors bound ET-1 to hypertensive cells. Exposure to exogenous ET-1 for 18 h revealed that only the L2 cells internalized ET-1, and internalization by hypertensive L2 cells was significantly reduced when compared with controls. Treatment with ETA (BQ-610) and ETB (BQ-788) receptor antagonists demonstrated that both receptors contributed to internalization of ET-1 in control L2 cells, whereas in hypertensive cells only when both receptor antagonists were used in combination was significant suppression of ET-1 internalization found. We conclude that in sheep receiving CAE, alterations in ETB receptors in cells of the L2 layer may contribute to the maintenance of CPH via alterations in their expression, distribution, and activity.