A low dose of angiotensin II increases inotropism through activation of reverse Na⁺/Ca²⁺ exchange by endothelin release

Objective: This work was aimed to prove that release/formation of endogenous endothelin acting in an autocrine/paracrine fashion contributes to the increase in contractility promoted by a low dose of angiotensin II. Methods: Isolated cat papillary muscles were used for force, pH_i, [Na ⁺]_i and [Ca²⁺]_i measurements and isolated cat myocytes for patch-clamp experiments. Results: In papillary muscles, 1.0 nmol/l angiotensin II increased force by 23±2% (n=4, P<0.05), [Na⁺]_i by 2.2±0.2 mmol/l (n=4, P<0.05), and peak (but not diastolic) Ca²⁺ from 0.674±0.11 to 0.768±0.13 μmol/l (n=4, P<0.05), without affecting pH _i. Force and [Na⁺]_i increase were abolished by inhibition of the Na⁺/H⁺ exchanger (NHE) with the inhibitor HOE642, blockade of endothelin receptors with the nonselective antagonist TAK044 and by inhibition of the endothelin-converting enzyme with phosphoramidon. Force but not [Na⁺]_i increase was abolished by inhibition of reverse Na⁺/Ca²⁺ exchange (NCX) with the inhibitor KB-R7943. Similar increase in force (21±2%, n=4, P<0.05) and in [Na⁺]_i (2.4±0.4 mmol/l, n=4, P<0.05) that were also suppressed by TAK044 and HOE642 were induced by exogenous 5.0 nmol/l endothelin-1. KB-R7943 reverted the endothelin-1 effect on force but not on [Na⁺]_i. In isolated myocytes, exogenous endothelin-1 dose-dependently increased the NCX current and shifted the NCX reversal potential (E_NCX) to a more negative value (ΔE _NCX: -10±3 and -17±5 mV, with 1 and 10 nmol/l endothelin-1, respectively, n=12). The latter effect was prevented by HOE642. Conclusion: Taken together, the results indicate that a low dose of angiotensin II induces release of endothelin, which, in autocrine/paracrine fashion activates the Na⁺/H⁺ exchanger, increases [Na ⁺]_i and changes E_NCX, promoting the influx of Ca²⁺ that leads to a positive inotropic effect (PIE).