Abstract

Experiments were performed on single, isolated muscle fibers of the crabCallinectes danae to investigate the effects of neomycin and streptomycin on membrane Ca++ conductance in relation to excitation-contraction coupling (ECC). Neomycin (2–8 mg/ml) hyperpolarizes the fiber and increases the effective membrane resistance. The antibiotic increases the threshold for, and reduces the amplitude of the graded membrane responses which result from activation of Ca++ conductance on membrane depolarization. Treatment of the fiber with either procaine or tetraethylammonium (TEA) converts the graded responses into regenerative Ca-spikes. Neomycin (2–4 mg/ml) increases spike threshold, reduces the rate of rise of the upstroke and shortens the duration of the action potentials. With higher neomycin concentrations (up to 8 mg/ml) the spikes are abolished. Streptomycin (8 mg/ml) is also capable of blocking Ca-spikes. As a consequence of their effects on membrane Ca++ conductance the antibiotics reduce the mechanical output associated with both graded responses and Ca-spikes. The data support the postulate that membrane Ca++ activation is an essential step in ECC in skeletal muscle.

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