Differential Effect of Sodium and Magnesium on the Pituitary of Goldfish Adapted to Calcium‐Free Environments

Abstract

AbstractAdaptation to deionized water (DW) affects several cell types in the goldfish. The pars intermedia PAS‐positive cells are highly stimulated. Their low response or the absence of changes in goldfish kept in 1/3 Ca‐free seawater (SW) and in Ca‐free SW‐adapted eels, respectively, suggest that sodium and/or magnesium are interfering. To test this hypothesis, young goldfish were adapted to DW supplemented or not with sodium (50 and 140 mM) for 8 and 16 days or with magnesium (16.5 and 50 mM) for 16 and 30 days. Cytological and morphometric studies of the pituitary showed that prolactin (PRL) cell activity was reduced by sodium. Thyrotropic (TSH) cells were stimulated. The activity of melanocyte‐stimulating (MSH) cells increased in DW + Na+. Stimulation of the pars intermedia PAS+ cells in DW was partly inhibited by adding sodium; the cellular and nuclear areas increased only moderately, the endoplasmic reticulum (ER) was not conspicuous and mitotic activity disappeared.In DW + Mg2+ the activity of PRL, TSH and MSH cells tended to be lower after a long‐term adaptation. The response of the PAS+ cells was as high as that noted in DW; complete degranulation, enlargement of the ER and important mitotic activity. Differential responses to Na+ and Mg2+ are not due to pH differences in the solutions.External sodium is able then to reduce the response of the PAS+ cells in a Ca‐free environment, while magnesium is not inhibitory. Other cell types are also affected by high levels of Na+ and Mg2+.