Sarcoplasmic Reticulum of a Fast Lobster Muscle

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The sarcoplasmic reticulum (SR) is a flattened, convoluted sack within muscle cells that controls the movement of calcium ions, and thus regulates the contraction of muscle. It has been hard to study the properties of this membranous sack directly because it is so small and inaccessible within the muscle cell, but new techniques have removed both problems. Dr. Eisenberg has developed a mechanically skinned preparation of a specialized lobster muscle, the remotor of the second antenna, that contains a large amount of SR (70% of its volume) to control its very rapid contractions. This preparation can be studied with modern techniques of electrophysiology. Its membrane potential can be recorded directly, and the properties of the individual ion channels of its membrane can be measured. The objective of this research is a systematic study of the electrical properties of the SR compartment, emphasizing the properties likely to be involved in the control of the rapid contraction of the remotor muscle. The following features will be studied: the ions responsible for the resting potential of the SR and the effects of calcium ions, chloride ions, caffeine, and procaine which are known to affect excitation-contraction coupling. The two different types of calcium channel found in the SR will be studied, and agents that modulate their behavior will be sought. The resistance and capacitance of the SR compartment will be measured. This research is of great biological interest. Although it has been known for many years that the sarcoplasmic reticulum (SR) controls muscle contraction by regulating the levels of intracellular calcium, this will be the first time that direct measurements of the electrical properties of the SR membrane and its individual ion channels have been made in any preparation. This specialized, rapidly contracting lobster muscle should provide important clues to the general mechanisms of control of muscle contraction.