Calcium-sensitive cells of the pars intermedia and osmotic balance in the eel. I. Responses to changes in the environmental calcium and magnesium.

The structure of the PAS-positive calcium-sensitive (Ca-s) cells of the pars intermedia was investigated in eels kept in deionized water (DW) or fresh water (FW) supplemented with Ca2+ of Mg2+ x Ca2+ (2mM) reduces considerably the response to DW; plasma osmolarity, Na+ and Ca2+ levels are not significantly affected. In eels adapted to DW for 21 or 28 days, showing highly stimulated Ca-s cells, an addition of CaCl2 for 2 days inhibits the release of granules, but does not immediately block their synthesis and the mitotic activity. The nuclear area is reduced, osmolarity and plasma sodium increase, but the rise in calcium is not always significant. Magnesium, at a 10-fold greater concentration than in FW (2mM), slightly inhibits the release of secretory granules without reducing other indicators of stimulation. In Ca-enriched FW, the Ca-s cells appear inactive. These data show that the PAS-positive cells in the pars intermedia of the eel are calcium-sensitive, similar to those of the goldfish; their role in calcium regulation is briefly discussed.