Why big endothelin-1 lacks a vasodilator response.

The biphasic arterial blood pressure response to endothelin-1 (ET-1) results from a transient decrease, followed by a longer-lasting increase, in systemic vascular resistance. In contrast to ET-1, big endothelin-1 (bET-1) produces monophasic increases in systemic vascular resistance and arterial blood pressure. This is somewhat surprising, because bET-1 activity is reportedly due to ET-1, bET-1 being converted to ET-1 by a putative converting enzyme. In this study we tested two hypotheses that could explain the singular effect of bET-1 on the arterial vasculature: that bET-1 vasoconstriction, mediated by ET(A) receptors at the level of the smooth muscle, masks the release of endothelial derived vasodilators, and/or that the endothelium develops tachyphylaxis owing to prolonged activation of endothelial ET(B) receptors. In anesthetized rats, blockade of the vasoconstrictor activity of bET-1 with BQ-123, an ET(A)-receptor antagonist, did not reveal a masked bET-1 vasodilator component in the rat hindquarter. Furthermore, in the presence of bET-1 (after 3.0 nmol/kg bET-1 i.v.), low doses of ET-1 (0.03-0.3 nmol/kg) produced dose-dependent hindquarter vasodilation, indicating activation of endothelial ET(B) and therefore no tachyphylaxis. Collectively, these experiments suggest that i.v. administration of bET-1 results in little or no activation of endothelial ET(B) receptors and therefore lacks a vasodilator response.