Nitroxyl increases force development in rat cardiac muscle

摘要

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 Ca~(2+) 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 (pH7.4, room temperature) and loaded iontophoretically with fura-2 to determine [Ca~(2+)]i. Sarcomere length was set at 2.2-2.3 (mu)m. HNO donated by Angeli's salt (AS; Na_2N_2O_3) dose-dependently increased both twitch force and [Ca~(2+)] _i transients (from 50 to 1000 (mu)M. Force increased more than [Ca~(2+)]i 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 Ca~(2+) transients at any given [Ca~(2+)]_o (0.5-2.0 mM). During steady-state activation, AS/HNO (250 (mu)M) increased maximal Ca~(2+)- activated force (F_(max), 106.8 +- 4.3 versus 86.7 +- 4.2 mN mm~(-2), n = 7-8, P < 0.01) without affecting Ca~(2+) 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 Ca~(2+) cycling, HNO also acts as a cardiac Ca~(2+) 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.