Excitation-contraction coupling in cardiac muscle of lobster (Homarus americanus); the role of the sarcolemma and sarcoplasmic reticulum

The T-tubules and sarcoplasmic reticulum (SR) serving excitation-contraction (EC) coupling in lobster (Homarus americanus) cardiac muscle are similar to those in mammalian myocardium. Tetanic contraction is elicited by a burst of action potentials from the cardiac ganglion. In this study we evaluated the roles of the sarcolemma and SR in EC coupling of the ostial valve muscle (orbicularis ostii m. or OOM) of lobster heart. The OOM was mounted in a bath with saline on a microscope stage; force was measured by strain gauge. [Ca²⁺]_i was measured using iontophoretically micro-injected fura-2 salt. Peak [Ca²⁺]_i, peak tetanic force and time to peak [Ca²⁺]_i increased with that of stimulus train duration (TD), to a maximum at a TD of 500 ms. Force increased with [Ca²⁺]_o. Cd²⁺ reduced force by 90%; ryanodine and caffeine reduced tetanic [Ca²⁺]_i transients by 80% and 70%, and force by 90% and 80%, respectively. Ryanodine, caffeine and cyclopiazonic acid slowed the decline of [Ca²⁺]_i and force during relaxation. Relaxation required [Na⁺]_o. The rate of decline of [Ca²⁺]_i appeared to be a sigmoidal function of the [Ca²⁺]_i and increased for any [Ca²⁺]_i with TD. Inactivity slowed relaxation of force; stimulation accelerated relaxation. These data suggest important contributions of Ca²⁺ transport both across the sarcolemma and across the SR membrane during EC-coupling of lobster cardiac muscle, while average cytosolic [Ca²⁺]_i regulates the rate of [Ca²⁺]_i elimination during relaxation.