Sodium pump molecular activity and membrane lipid composition in two disparate ectotherms, and comparison with endotherms

Abstract

Previous research has shown that the lower sodium pump molecular activity observed in tissues of ectotherms compared to endotherms, is largely related to the lower levels of polyunsaturates and higher levels of monounsaturates found in the cell membranes of ectotherms. Marine-based ectotherms, however, have very polyunsaturated membranes, and in the current study, we measured molecular activity and membrane lipid composition in tissues of two disparate ectothermic species, the octopus (Octopus vulgaris) and the bearded dragon lizard (Pogona vitticeps), to determine whether the high level of membrane polyunsaturation generally observed in marine-based ectotherms is associated with an increased sodium pump molecular activity relative to other ectotherms. Phospholipids from all tissues of the octopus were highly polyunsaturated and contained high concentrations of the omega-3 polyunsaturate, docosahexaenoic acid (22:6 (n-3)). In contrast, phospholipids from bearded dragon tissues contained higher proportions of monounsaturates and lower proportions of polyunsaturates. Sodium pump molecular activity was only moderately elevated in tissues of the octopus compared to the bearded dragon, despite the much greater level of polyunsaturation in octopus membranes. When the current data were combined with data for the ectothermic cane toad, a significant (P = 0.003) correlation was observed between sodium pump molecular activity and the content of 22:6 (n-3) in the surrounding membrane. These results are discussed in relation to recent work which shows a similar relationship in endotherms.

Physiological adaptation of an Antarctic Na+/K+-ATPase to the cold

“…However, a previous report on native Na + /K + -ATPases from Octopus vulgaris estimated turnover rates of 25-50s -1 at 27°C. These values, obtained using a biochemical approach, agree well with our value of 35s -1 at the same temperature (Turner et al, 2005). In a number of systems it has been shown that psychrophilic enzymes have higher catalytic rates than warm-adapted orthologs (Asgeirsson and Bjarnason, 1991;Asgeirsson and Bjarnason, 1993;Fields and Somero, 1998;Genicot et al, 1988;Graves and Somero, 1982;Hochachka and Somero, 2002;Kawall et al, 2002;Simpson and Haard, 1984;Yancey and Siebenaller, 1987).…”

Perspectives on the membrane fatty acid unsaturation/pacemaker hypotheses of metabolism and aging

“…The unsaturation index, or mean number of double bonds in the fatty acyl groups of phospholipids in a membrane, and the percentage of DHA were positively correlated with metabolic rate, proton leak, and Na+/K+ ATPase activity within birds, within mammals, and among several ectotherms in liver, kidney, heart, and skeletal muscle (Brand et al, 2003;Brookes et al, 1998;Hulbert et al, 2002c;Porter et al, 1996;Turner et al, 2005cTurner et al, , 2003. Differences in unsaturation index and DHA concentration in the muscle mitochondria of rats, toads, and lizards were also correlated with differences in their mitochondrial oxidative capacities (Guderley et al, 2005).…”

“…Whereas increases in the activity of specific membrane-bound proteins correlate with increasing unsaturation index, the underling mechanisms of this phenomenon remain unclear Else and Wu, 1999;Hulbert et al, 2006b;Hulbert, 2008b;Turner et al, 2006aTurner et al, , 2005cTurner et al, , 2003Wu et al, 2004). Although it could be assumed that differences in protein activity were a result of differences in membrane fluidity caused by the greater number of double bonds in fatty acids, overall membrane fluidity is only strongly increased by the addition of the first two double bonds to a saturated acyl chain; each subsequent double bond has less effect on membrane fluidity (Brenner, 1984;Pamplona et al, 2002a).…”

Ups and downs of intestinal function with prolonged fasting during aestivation in the burrowing frog, Cyclorana alboguttata

“…Non-linear regression analysis was used to determine maximum specific binding (i.e. sodium pump density; B max ) and was calculated assuming a 1:1 stoichiometry between sodium pump units and ouabain binding sites (Turner et al, 2005 ) by the average number of sodium pumps (B max , in pmolg -1 wet mass) per group (Else et al, 1996). The result was expressed as the number of ATP molecules hydrolysed by each sodium pump per min.…”