Endothelin-1 activates phospholipases and channels at similar concentrations in porcine coronary arteries

Sensitivity of endothelin-1 (ET-1)-ion channel interactions hasbeen proposed to exceed that of ET-1-phospholipase activation invascular smooth muscle. We wanted to determine whether short-circuiting ion channels with staphylococcal -toxin pores would shift the ET-1-force relation to the right as predicted from the above proposal. Medium size porcine coronary arteries (outer diameter 0.7-1.5 mm)were mounted on isometric force transducers. ET-1 concentration response curves were compared between intact rings and those subjected to -toxin treatment with Ca buffered at 0.1 µM. TheEC₅₀ for treated rings (1.5 ± 1.0 nM, n = 5 pigs) was similar tothat for intact rings (1.9 ± 0.4 nM). The Ca sensitivity of the-toxin-treated rings(EC₅₀ = 0.43 ± 0.08 µM) was similar to that reported by other laboratories for intact and-toxin-treated arteries and was shifted eightfold to the left by ahigh concentration of ET-1 (10 nM). Measurements of[³²P]phosphatidic acid([³²P]PA) levels wereused to evaluate phospholipase activity in intact arteries. The timecourses for [³²P]PAproduction and contraction were similar in response to high (100 nM)and to low (1 nM) ET-1. Significant increases in both steady-statecontraction and[³²P]PA occurred overa wide range of ET-1 concentrations tested (0.3-100 nM). Ourfindings support the concept that ET-1-phospholipase coupling isoperative over the whole concentration range that induces contractileresponses. It is suggested that both Ca entry and Ca sensitizationprocesses are activated by ET-1 at low concentrations (50) and that bothprocesses contribute significantly to the integrated response.