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 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 (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 (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.