Properties of the Ryanodine Receptor Present in the Sarcoplasmic Reticulum from Lobster Skeletal Muscle

Abstract

Microsomal sarcoplasmic reticulum (SR) fractions from lobster skeletal muscle were found to bind [³H]-ryanodine. [³H]-ryanodine binding was enhanced by AMP, Ca²⁺ and caffeine, and significantly diminished by ATP, Ba²⁺ and Sr²⁺. Furthermore, dantrolene and ruthenium red, two classical inhibitors of Ca²⁺ release from the SR, blocked [³H]-ryanodine binding. Similarly, tetracaine, known to block the charge movement associated with excitation-contraction coupling in vertebrate muscle, inhibited the binding of the alkaloid. Our lobster SR preparation exhibited a single high-affinity ryanodine binding site (K_d=6.6 nM, B_max=10 pmol/mg protein). Since SDS-PAGE of the SR proteins revealed a major band c. 565 kDa which comigrated with the putative ryanodine receptor from both rat and chicken skeletal muscle, we concluded that lobster skeletal muscle is equipped with the 565 kDA ryanodine receptor. Finally, incorporation of the SR microsomal fraction from lobster into planar bilayer membranes revealed the presence of a ryanodine-sensitive Ca²⁺ channel activity (160 pS in symmetrical 200 mM CsCl solutions). We concluded that both the crustacean and vertebrate skeletal muscle ryanodine receptor share the relevant properties such as molecular weight and affinity for ryanodine and inositol 1,4,5 triphosphate. However, there are important differences between the two receptors including differential effects of the alkaloid on the Co²⁺ release channel and modulation of the receptor by nucleotides.

Key Words:

• E-C coupling
• calcium release channel
• crustacean muscle.