Meeting Abstract

P2.92  Wednesday, Jan. 5  The influence of muscle fiber size on the cost of ion transport JIMENEZ, Ana G.*; DILLAMAN, Richard M.; KINSEY, Stephen T.; Univ. of North Carolina Wilmington agj6818@uncw.edu

Muscle fibers are often among the largest cells in the animal kingdom. Since excessive fiber size imposes diffusion constraints on muscles, it is difficult to explain why selection may favor large fibers. Johnston et al. (2003; 2004; 2006) have proposed the optimal fiber size hypothesis, which states that some fishes may balance the need for small fibers that promote rapid diffusive flux against potential metabolic cost savings associated with large fibers. Since the fiber surface area to volume ratio (SA:V) decreases with increasing fiber size, there will be less membrane surface in large fibers over which ions must be pumped to maintain the membrane potential. To test this hypothesis, we measured the cost of Na+-K+-ATPase (NAK) function with respect to fiber SA:V. We examined resting white muscle from juveniles and adults of 19 species of fishes and crustaceans. Muscle fiber size was measured microscopically, basal ATP demand and the cost of the NAK were determined using 31P-NMR coupled with selective inhibitors, and maximal NAK activity was measured using spectrofluorometry. Mean fiber size ranged from 12.1 to 961.9 µm and was positively correlated with mean body mass, which ranged from 0.085 to 1007 g. NAK activity was positively correlated with fiber SA:V, which is consistent with the hypothesis, and the NAK ATP demand accounted for up to 70% of the total energy expenditure in resting muscle. We also found that in the vast majority of species the lower SA:V in adults compared to juveniles was associated with a lower cost of ion transport. These results suggest a potential advantage of large muscle fibers.