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

Abstract

Sensitivity of endothelin-1 (ET-1)-ion channel interactions has been proposed to exceed that of ET-1-phospholipase activation in vascular smooth muscle. We wanted to determine whether short-circuiting ion channels with staphylococcal α-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 α-toxin treatment with Ca buffered at 0.1 μM. The EC50 for treated rings (1.5 ± 1.0 nM, n = 5 pigs) was similar to that for intact rings (1.9 ± 0.4 nM). The Ca sensitivity of the α-toxin-treated rings (EC50 = 0.43 ± 0.08 μM) was similar to that reported by other laboratories for intact and α-toxin-treated arteries and was shifted eightfold to the left by a high concentration of ET-1 (10 nM). Measurements of [32P]phosphatidic acid ([32P]PA) levels were used to evaluate phospholipase activity in intact arteries. The time courses for [32P]PA production and contraction were similar in response to high (100 nM) and to low (1 nM) ET-1. Significant increases in both steady-state contraction and [32P]PA occurred over a wide range of ET-1 concentrations tested (0.3–100 nM). Our findings support the concept that ET-1-phospholipase coupling is operative over the whole concentration range that induces contractile responses. It is suggested that both Ca entry and Ca sensitization processes are activated by ET-1 at low concentrations (<EC50) and that both processes contribute significantly to the integrated response.