Meeting Abstract

50.6  Sunday, Jan. 5 11:30  Does Ca²⁺ diffusion limit myofibril size? KELLY, K.L.; GOLDING, C.; LOCKE, B.R.; DILLAMAN, R.M.; KINSEY, S.T.*; Univ. of North Carolina Wilmington, Wilmington; Florida State Univ., Tallahassee; Florida State Univ., Tallahassee; Univ. of North Carolina Wilmington, Wilmington; Univ. of North Carolina Wilmington, Wilmington kinseys@uncw.edu

Tail beat frequency in fishes generally decreases with increasing body length, and it follows that the frequency of Ca²⁺ fluxes during muscle contraction-relaxation cycles decreases during growth as well. Since Ca²⁺ cycling is a reaction-diffusion process, both the catalytic and diffusion demands are therefore relaxed during animal growth. We examined structure-function relationships in the Ca²⁺ cycling system during post-metamorphic growth in white muscle from the black sea bass (Centropristis striata) to test the hypothesis that Ca²⁺ diffusion limits myofibril size. Isolated white muscle from juvenile fish was capable of higher contractile frequencies and had higher SR volume density, smaller myofibril diameter, and a greater percentage of fast isoforms of parvalbumin, compared to muscle from adult fish. We developed a reaction-diffusion mathematical model of a myofibril to simulate Ca²⁺ cycling in the juvenile and adult fish muscle, and the model force production curves paralleled that of the experimental data over a broad range of contractile frequencies. Within the physiological range of contractile frequencies, gradients in [Ca²⁺] concentration were apparent, but [Ca²⁺] nonetheless was sufficiently high to saturate troponin C. Further, the model contractile profile output was unaffected when diffusion was infinitely fast (reaction control only), suggesting a limited role for diffusion in governing contraction-relaxation cycles. This notion is supported by simulations using hypothetically large myofibrils, which remained fully functional. These results suggest that myofibril size is governed by the need to have adequate sarcoplasmic reticulum membrane surface area and Ca²⁺ pump activity.