Nitroxyl increases force development in rat cardiac muscle

Donors of nitroxyl (HNO), the reduced congener of nitric oxide (NO), exert positive cardiac inotropy/lusitropy in vivo and in vitro, due in part to their enhancement of Ca2+ cycling into and out of the sarcoplasmic reticulum. Here we tested whether the cardiac action of HNO further involves changes in myofilament-calcium interaction. Intact rat trabeculae from the right ventricle were mounted between a force transducer and a motor arm, superfused with Krebs-Henseleit (K-H) solution (pH 7.4, room temperature) and loaded iontophoretically with fura-2 to determine [Ca2+]i. Sarcomere length was set at 2.2-2.3 mu m. HNO donated by Angeli's salt (AS; Na2N2O3) dose-dependently increased both twitch force and [Ca2+]i transients (from 50 to 1000 mu M). Force increased more than [Ca2+], transients, especially at higher doses (332 +/- 33% versus 221 +/- 27%, P < 0.01 at 1000 mu M). AS/HNO (250 mu M) increased developed force without changing Ca2+ transients at any given [Ca2+](o) (0.5-2.0 mM). During steady-state activation, AS/HNO (250 mu M) increased maximal Ca2+- activated force (F-max, 106.8 +/- 4.3 versus 86.7 +/- 4.2 mN mm(-2), n = 7-8, P < 0.01) without affecting Ca2+ required for 50% activation (Ca-50, 0.44 +/- 0.04 versus 0.52 +/- 0.04 mu M, not significant) or the Hill coefficient (4.75 +/- 0.67 versus 5.02 +/- 1.1, not significant). AS/HNO did not alter myofibrillar Mg-ATPase activity, supporting an effect on the myofilaments themselves. The thiol reducing agent dithiothreitol (DTT, 5.0 mm) both prevented and reversed HNO action, confirming AS/HNO redox sensitivity. Lastly, NO (from DEA/NO) did not mimic AS/HNO cardiac effects. Thus, in addition to reported changes in Ca2+ cycling, HNO also acts as a cardiac Ca2+ sensitizer, augmenting maximal force without altering actomyosin ATPase activity. This is likely to be due to modulation of myofilament proteins that harbour reactive thiolate groups that are targets of HNO.