Activation of the contractile system in crustacean muscle: Ultrastructural evidence for the role of the T system

Abstract

Freeze-fracture and thin sections of lobster abdominal fast flexor muscle were used to study the morphology of the sarcoplasmic reticulum (SR) and T system of crustacean muscle. Tannic acid mordanting, which can result in a dense black deposit in the T system lumen, was used to distinguish T system from SR membranes. Ferritin was also used as an extracellular tracer to confirm the tannic acid method. The T system consists of an extensive network of flattened sacs which fills most of the space between the mycfibrils and is in close contact with them. The SR also appears as flattened sacs, sometimes with fenestrations. There is extensive junctional contact between the SR and T system. Quantitative estimates of the volume and surface area of the membranes show that the T system has about 50 % more surface area than the SR. The intramembrane particle (IMP) density of the PF face of the T system is about 1100/ μm 2 membrane, while the IMP density of the PF face of the SR is about 4800/ μm 2 membrane. In morphology, extent, and IMP density, the T system of lobster abdominal fast flexor muscle appears (AFF) adapted to provide at least part of the Ca 2+ for muscle activation and the transport system for relaxation.