Nitric Oxide Metabolites in Hypoxia Freezing and Hibernation in wood frogs

Abstract

Nitric oxide (NO) is a gaseous free radical that in diverse organisms performs many signaling and protective functions, such as vasoregulation, inhibition of apoptosis, antioxidation, and metabolic suppression. Increased availability of NO may be especially important during life history periods when organisms must contend with multiple stresses. We investigated dynamics of the NO metabolites, nitrite (NO2-) and nitrate (NO3-), in the blood plasma, heart, liver, and skeletal muscle of the wood frog (Rana sylvatica), an amphibian that endures chronic cold, freezing, hypoxia, dehydration, and extended aphagia during hibernation. We found elevated concentrations of NO2- and/or NO3- in the plasma (up to 4.1-fold), heart (3.1-fold), and liver (up to 4.1-fold) of frogs subjected to experimental hypoxia (24 h, 4°C), and in the liver (up to 3.8-fold) of frogs exposed to freezing (48 h, -2.5 °C), suggesting that increased NO availability aids in survival of these stresses. During a 38-week period of simulated hibernation, NO2- and/or NO3- increased in the plasma (up to 10.4-fold), heart (up to 3.3-fold), and liver (5.0-fold) during an initial 5-week winter-conditioning regimen and generally remained elevated thereafter. In hibernation, plasma NO2- was higher in frogs indigenous to Interior Alaska than in conspecifics from a more temperate locale (southern Ohio), suggesting that NO availability is matched to the severity of environmental conditions prevailing in winter. Our results, together with published values for other species, suggest that the NO protection system is of general importance in the stress adaptation of vertebrates.