
Meeting Abstract
The bulk of ATP demand in resting skeletal muscle is devoted to maintenance of the sarcolemmal membrane potential, the sarcoplasmic reticulum (SR) Ca+ gradients, and protein turnover. We have previously found that skeletal muscle fibers often grow as large as possible, to the brink of diffusion limitation, which minimizes sarcolemmal surface area:volume (SA:V) and reduces the costs associated with the Na+-K+ ATPase (NKA). The diamondback water snake (Nerodia rhombifer) devotes much of its body mass to skeletal muscle, eats infrequently, and is often sessile, making it advantageous to minimize muscle maintenance costs. We tested the hypothesis that fiber size and myofibril size would increase with animal growth, and that the NKA and SR/ER Ca2+ ATPase (SERCA) would be proportional to fiber and myofibril SA:V, respectively. Muscle fiber size increased with body mass, however, the NKA activity did not decrease in proportion to fiber SA:V as hypothesized. Myofibril size also increased with body mass, but the SERCA activity increased, rather than decreasing in proportion to SA:V as expected. Myonuclear density decreased with increasing body and fiber size, suggesting a decrease in the capacity for protein turnover in larger snakes. Further, the nuclei shifted toward a more intermyofibrillar distribution during fiber growth, which is indicative of fibers growing to sizes that induce diffusion limitation. These results suggest that muscle fibers are growing as large as possible, but the lack of a relationship between fiber size and NKA activity does not support the notion that this is a cost saving strategy. The elevated SERCA activity in larger animals may indicate that more SERCA activity is needed to support Ca2+ fluxes over longer diffusion distances.