REGULATION OF ENDOTHELIN RECEPTOR EXPRESSION IN VASCULAR SMOOTH-MUSCLE CELLS

Endothelin-1 (ET-1) has been implicated in atherosclerosis, hypertensi on, and restenosis, all of which involve abnormal vascular smooth musc le cell function and/or proliferation. We have previously established that human umbilical vein smooth-muscle cells (HUVSMCs) can secrete ET , (1) whereas A10 cells do not. Therefore, we investigated the effect of exogenously added ET-1 on receptor density (B-max) of A10 cells. Co mpared to controls, A10s exposed to ET-1 for 48 h showed a significant decrease (79%) in receptor density, with no change in affinity. In co ntrast, incubation of A10s with the ET(A)-selective antagonist FR13931 7 (at a concentration blocking 99% of ET receptors) for 48 h caused a significant increase (245%) in B-max and a significant decrease in aff inity. These changes persisted after coincubation with both ET-1 and F R139317, indicating that the antagonist was able to block the effects of exogenous ET-1. In concordance, incubation of HUVSMCs with FR139317 for 24 h caused a significant increase in receptor density (>1,000%), although there was no change in levels of immunoreactive (IR) ET or b ig ET-1. However, after a shorter incubation (1 h), there was no chang e in B-max but IR ET was significantly elevated by 878%, whereas IR bi g ET-1 was depressed by 86% compared to controls. Incubation of HUVSMC s with FR139317 did not affect receptor affinity. Our observations sug gest that whereas the application of exogenous ET to A10s causes downr egulation of ET receptor expression, the ET(A) antagonist FR139317 cau ses upregulation of ET receptor expression in VSMCs regardless of thei r ability to secrete ET. In VSMCs capable of secreting ET, acute antag onism of the ET(A) receptor causes a significant increase in IR-ET and a decrease of IR-big ET-1 levels.