Effects of pO 2 on the activation of skeletal muscle ryanodine receptors by NO: A cautionary note

Abstract

Eu et al., reported that O 2 dynamically controls the redox state of 6–8 out of 50 thiols per skeletal ryanodine receptor (RyR1) subunit and thereby tunes the response of Ca 2+-release channels to authentic nitric oxide (NO) [J.P. Eu, J. Sun, L. Xu, J.S. Stamler, G. Meissner, The skeletal muscle calcium release channel: coupled O 2 sensor and NO signaling functions, Cell 102 (2000) 499–509]. A role for O 2 was based on the observation that RyR1 can be activated by submicromolar NO at physiological (∼10 mmHg) but not ambient (∼150 mmHg) pO 2. At ambient pO 2, these critical thiols were oxidized but incubation at low pO 2 reset the redox state of these thiols, closed RyR1 channels and made these thiols available for nitrosation by low NO concentrations. Eu et al., postulated the existence of a redox/O 2sensor that couples channel activity to NO and pO 2 and explained that “the nature of the ‘redox/O 2 sensor’ that couples channel activity to intracellular redox chemistry is a mystery”. Here, we re-examined the effect of pO 2 on RyR1 and find that incubation of RyR1 at low pO 2 did not alter channel activity and NO (0.5–50 μM) failed to activate RyR1 despite a wide range of pO 2 pre-incubation conditions. We show that low levels of NO do not activate RyR1, do not reverse the inhibition of RyR1 by calmodulin (CaM) even at physiological pO 2. Similarly, the pre-incubation of SR vesicles in low pO 2 (for 10–80 min) did not inhibit channel activity or sensitization of RyR1 to NO. We discuss the significance of these findings and propose that caution should be taken when considering a role for pO 2 and nitrosation by NO as mechanisms that tune RyRs in striated muscles.