Angiotensin II-induced inotropism requires an endocardial endothelium-nitric oxide mechanism in the in-vitro heart of Anguilla anguilla

Using an isolated working heart preparation we show that angiotensin II (ANG II), at concentrations of 10(-10)-10(-7) mol l(-1), elicits negative chronotropism and inotropism in the freshwater eel Anguilla anguilla. The negative inotropism was insensitive to losartan and CGP42112 (AT(1) and AT(2) ANG II receptor antagonists, respectively), and was abrogated by the AT(1) receptor antagonist CV11974, the G protein blocker pertussis toxin (PTx) and the muscarinic antagonist atropine. In contrast, it was not affected by the adrenoceptor antagonists propanolol, sotalol and phentolamine. Using donors (L-arginine) and inhibitors N(G)-monomethyl-(L)-arginine (L-NMMA), L-N(5)(1-iminoethyl)ornithine ((L)-NIO) of nitric oxide synthase (NOS), and haemoglobin as NO scavenger, we demonstrate that NO signalling is involved in ANG II-mediated inotropism. Pretreatment with Triton X-100, a detergent that damages the endocardial endothelium (EE), or with 1H-(1,2,4)oxadiazolo-(4,3-a)quinoxalin-1-one (ODQ), a specific inhibitor of soluble guanylate cyclase, or with the cGMP-activated protein kinase (PKG) inhibitor KT5328, abolished ANG II-mediated inotropism. Thus, ANG II-mediated inotropism occurs via an EE-NO-cGMP-PKG mechanism. ANG II did not affect the mechanical performance influenced by preload changes (i.e. the Frank-Starling response), which in the eel heart is modulated by NO. This EE-paracrine-mediated cardio-suppressive action of endoluminal ANG II suggests that the hormone plays an important intracardiac role in the fish heart.