Enhanced ET_A receptor-mediated contractile function is associated with reduced ET_A 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 ET_A 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 48h. 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<0.01) and ET_A receptor (51±2% compared with the initial value, n = 4–5, P<0.01) density in sheep bronchial explants after 48h. No ET_B receptors were detected. Thus, the culture of sheep bronchial explants was associated with an increase in ET_A receptor-mediated contractile function that was accompanied by a decrease in ET_A receptor density. In addition, the structural integrity of the ciliated epithelium was preserved using this culture protocol, a feature that is critical to successful respiratory viral infection. The significant changes in ET receptor density and function in these bronchial explants must be carefully considered when assessing any effects of respiratory viral infection in this model.