Free-calcium and tension responses in single barnacle muscle fibres following the application of l-glutamate

Abstract

The effect of the putative transmitter, l-glutamate, on free intracellular Ca 2+, tension and membrane potential in single muscle fibres from the barnacle Balanus nubilus has been investigated. External application of l-glutamate (0.1–10 mM) resulted in a transient increase in free intracellular Ca 2+, monitored by the Ca 2+-activated protein aequorin. This increase in free intracellular Ca 2+ was associated with membrane depolarization and force development, and was followed by a period of ‘desensitization’ in which the preparation was unresponsive to l-glutamate. This could be reversed by removing l-glutamate from the external saline. External application of a number of closely related compounds, including d-glutamate and l-aspartate, were ineffective for initiating the transient light response. The l-glutamate response was virtually abolished in Na-free (Li) medium and completely abolished in Ca-free (Na) medium. The responses to l-glutamate were not reduced in Mg-free medium. The fibre's response to 1 mM l-glutamate was also inhibited by D-600 (10 μM) or by La 3+ (1 mM), suggesting that Ca was directly involved in the underlying ionic conductance changes brought about by this putative excitatory transmitter.