Development of a Single-Fiber Preparation to Investigate the Contractile Properties of Obliquely Striated Muscles

Abstract

Obliquely striated muscles are ubiquitous in soft-bodied animals, and are so-named because the dense bodies that serve as anchors for the thin filaments are arranged at an oblique angle to the long axis of the cell. This important muscle type occurs in members of at least 14 phyla and may have evolved independently several times, yet the implications of oblique striation for muscle function are unknown. The goal of this project was to develop methods to measure the force-length properties of individual obliquely striated muscle cells. I adapted a pre- existing protocol for the isolation of neurons to enzymatically isolate individual obliquely striated muscle cells from the ventral longitudinal muscles of the polychaete worm Nereis virens. The isolated cells appeared healthy and many retained their ability to contract, either spontaneously or in response to electric field stimulation. The polychaete muscle fibers were ribbon-shaped and averaged 370 μm in length. I was also able to attach individual N. virens muscle fibers to 0.25 μm diameter tungsten pins connected to a force transducer and length controller, and then investigate the passive and active force-length relationships. In addition to validating the methodology, the mechanical measurements of the N. virens muscle fibers revealed a linear increase in passive force as fiber length increased. This deviation from the expected nonlinear increase in passive force merits further investigation. The isolated N. virens muscle cells were activated rapidly, generated force rapidly, but relaxed slowly. The data suggest that further research can be done to analyze active forces of obliquely striated muscles to aid in understanding the functional and evolutionary importance of obliquely striated muscle cells.