Dietary Nitrate Enhances the Contractile Properties of Human Skeletal Muscle

Abstract

Dietary nitrate, a source of nitric oxide (NO), improves the contractile properties of human muscle. We present the hypothesis that this is due to nitrosylation of the ryanodine receptor and increased NO signaling via the soluble guanyl cyclase-cyclic guanosine monophosphate-protein kinase G pathway, which together increase the free intracellular Ca concentration along with the Ca sensitivity of the myofilaments themselves.

Figure

Proposed mechanisms by which dietary NO₃⁻ influences muscle contractile function in humans. After ingestion, NO₃⁻ is converted to NO₂⁻ by bacterial nitroreductases in the oral cavity and endogenous nitroreductases (e.g., xanthine oxidoreductase) in muscle itself. This increase in NO₂⁻ in turn leads to enhanced production of the free radical NO. Elevated NO bioavailability then results in multiple effects, as shown in the figure. These include nitrosylation of the sarcroendoplasmic reticululm RyR, which increases Ca²⁺ release by “locking” this channel in the open configuration. The subsequent increase in free intracellular [Ca²⁺]_i contributes to the improvements in twitch force (F_tw), rate of force development (dF/dt), estimated maximal shortening velocity (V_max), and maximal power (P_max) of muscle that have been observed after dietary NO₃⁻ intake. Simultaneously, however, the increase in [Ca²⁺]_i also results in activation of skeletal muscle myosin light chain kinase (skMLCK) via the Ca²⁺-CaM pathway and hence an increase in myosin regulatory light chain phosphorylation (pRLC). This results in greater Ca²⁺ sensitivity of the contractile apparatus, thereby also contributing to the increases in F_tw, dF/dt, V_max, and P_max. Paralleling these events, the increase in NO also results in activation of sGC and hence an increase in cGMP production. This increase in cGMP stimulates PKG activity, which in turn enhances regulatory light chain phosphorylation and hence Ca²⁺ sensitivity, thus improving muscle contractile function. Finally, also shown are the effects of increased NO on TropI, on the myosin heavy chain, and on the SERCA. An increase in NO can enhance nitrosylation of TropI in fast-twitch (but not slow-twitch) fibers, but this inhibits (lines) Ca²⁺ sensitivity, which is the opposite of what has been observed after dietary NO₃⁻ supplementation. Similarly, elevation in NO could increase nitrosylation of myosin, but this would diminish, not enhance, muscle contractile function. Elevated NO also can inhibit SERCA directly, but this would also tend to diminish muscle function, by slowing the rate of relaxation and resulting in eventual depletion of sarcoplasmic reticulum Ca²⁺ stores. Thus, these other potential effects of NO cannot explain the dietary NO₃⁻-induced improvements in contractility that have been found repeatedly. cGMP, cyclic guanosine monophosphate; NO, nitric oxide; NO₂⁻, nitrite; NO₃⁻, nitrate; PKG, protein kinase G; RyR, ryanodine receptor; SERCA, sarcoendoplasmic reticulum Ca²⁺ ATPase; sGC, soluble guanyl cyclase.