The development of glio-interstitial tissue in Mytilus retractor muscle depends on Na+-Ca2+ antagonism

The development of glio-interstitial cell processes has been studied by quantitative electron microscopy in the anterior byssal retractor muscle of mussels kept in various artificial sea-waters. After 20 days, the number of glio-interstitial processes per unit area of muscle section from animals adapted to diluted sea-water (700 mosM) is not significantly different from the control (1100 mosM) but it is almost doubled in mussels adapted to concentrated sea-water (1400 mosM). The diluted sea-water has a high [Ca2+]/[Na+]2 molar ratio (6.81 X 10(-5)) and the concentrated sea-water a relatively low one (3.34 X 10(-5)); all the ions are present in the same proportions as in the control. In a second experiment, diluted sea-water (700 mosM) with a low [Ca2+]/[Na+]2 (3.34 X 10(-5)) and concentrated sea-water (1400 mosM) with a high ratio (6.81 X 10(-5)) are tested: the results agree with the prediction that the development of glio-interstitial processes depends on the relative concentrations of Na+ and Ca2+ rather than on osmotic pressure or ionic strength. In the third experiment, five artificial sea-waters are employed with decreasing [Ca2+]/[Na+]2 ratios, all at the same osmotic pressure of 1100 mosM: the results suggest that the salinity-induced proliferation of glio-interstitial processes is directly dependent on the [Ca2+]/[Na+]2 ratio. Glial proliferation thus occurs in reaction to the relative lack of Ca2+, or excess of Na+, in the environment; it is proposed that the glio-interstitial tissue plays a role in regulating the concentration of Ca2+ in the vicinity of the muscle and/or the nerve cells.