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

The effect of the putative transmitter, L-glutamate, on free intracellular Ca2+, 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 Ca2+, monitored by the Ca2+-activated protein aequorin. This increase in free intracellular Ca2+ 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 1mM L-glutamate was also inhibited by D-600 (10 micrometer) or by La3+ (1mM), suggesting that Ca was directly involved in the underlying ionic conductance changes brought about by this putative excitatory transmitter.