Enhanced ETA receptor-mediated contractile function is associated with reduced ETA receptor density in sheep bronchial explants maintained in culture

We are interested in developing an airway explant culture system using sheep bronchi in which to establish respiratory viral infection and from which tissue can be used for functional, biochemical and immunohistochemical studies involving the endothelins (ETs). Freshly harvested sheep bronchial airway smooth muscle contains a homogeneous population of the ETA receptor. However, the potency of ET-1 and maximum contractile response of sheep bronchial explants to ET-1 increased with time in culture, despite these parameters remaining constant for carbachol in explants maintained for up to 48 h. The possibility that this was caused by changes in ET receptor density was assessed using light microscopic quantitative autoradiography. In view of the increased responsiveness to ET-1 in cultured explants, it was surprising to demonstrate a significant decrease in total ET receptor (59+/-6% compared with the initial value, n=4-5; P