Physiological and cytochemical studies of Ca in the primary muscle of the trunk of Sagitta setosa (chaetognath)

The movements of Sagitta are conditioned by the presence of Ca(2+) in the external medium. When this ion is removed from artificial sea water, animals do not move. They swim again when Ca(2+) is present. Among the problems raised by this observation, we have studied the role of Ca(2+) in the contraction of the primary musculature. Physiological experiments show the central importance of the extracellular Ca(2+) and of its translocation through the membrane during the initiation of the contraction. Cytochemical data correlate these observations. They show that Ca(2+) is localized mainly at the level of the plasma membrane, its invaginations and in the poorly developed SR (less than 2% of cell). Like SR, mitochondria accumulate Ca(2+) but do not seem to participate in the regulation of these Ca movements except in abnormal situations. La(3+) blocks the entry of extracellular Ca(2+) and attaches to the membranes; this fixation is not the same on the plasma membrane and in its invaginations. The contractile apparatus of Sagitta primary musculature show remarkable specializations (Duvert and Savineau, 1986). It is composed of ribbon-shaped myofibrils of regular thickness surrounded by external membranes implicated in the fixation and the translocation of a pool of Ca(2+) necessary for initiating contraction. The poorly developed SR and the mitochondria probably modulate the functioning of the two types of fibres (A and B).