Abstract

Many marine dinoflagellates emit bright discrete flashes of light nearly instantaneously in response to either laminar or turbulent flows as well as to direct mechanical stimulation. The Hash involves a unique pH-dependent luciferase and a proton-mediated action potential across the vacuole membrane. The mechanotransduction process initiating this action potential is unknown. The present study investigated the role of Ca2+ in the mechanotransduction process regulating bioluminescence in the dinoflagellate Lingulodinium polyedrum. Calcium ionophores and digitonin stimulated luminescence in a Ca2+-dependent manner in the absence of mechanical stimulation. Mechanically sensitive luminescence was strongly inhibited by the intracellular Ca2+ chelator BAPTA-AM [1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester]; there was only a partial and irreversible dependence on extracellular Ca2+. Ruthenium Red, a blocker of intracellular Ca2+ release channels, inhibited mechanically sensitive luminescence. Luminescence was also stimulated by increasing K+, even in the absence of extracellular Ca2+; K+ stimulation was inhibited both by BAPTA-AM and Ruthenium Red. These results support the hypothesis that Ca2+ mediates stimulated bioluminescence and also indicate the involvement of intracellular Ca2+ stores. Rapid coupling between mechanical stimulation and mobilization of intracellular Ca2+ stores might occur through a mechanism similar to excitation-contraction coupling in skeletal muscle.