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 μm. HNO donated by Angeli's salt (AS; Na2 N3O3) dose-dependently increased both twitch force and [Ca2+]i transients (from 50 to 1000 μM). Force increased more than [Ca2+]i transients, especially at higher doses (332 ± 33% versus 221 ± 27%, P < 0.01 at 1000 μM). AS/HNO (250 μM) increased developed force without changing Ca2+ transients at any given [Ca2+]o (0.5-2.0 mM,). During steady-state activation, AS/HNO (250 μM) increased maximal Ca2+- activated force (Fmax, 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 (Ca50, 0.44 ± 0.04 versus 0.52 ± 0.04 μ 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.
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