Response of calcium-sensitive cells in the pars intermedia of the goldfish adapted to diluted sea water with different calcium and magnesium contents.

Goldfish kept in diluted Ca-free sea water (SW) (23 or 30%) or in Ca-Mg-deficient SW (23%) have a limited survival, release large amounts of mucus, and show spasmodic seizures. Plasma calcium decreases. The PAS-positive calcium-sensitive (Ca-s) cells of the pars intermedia show a low activity in diluted SW and in 23% Mg-free SW. In diluted Ca-free SW, Ca-s cells are stimulated, but cell hypertrophy is not uniform and often restricted to an area adjacent to the proximal pars distalis. Nuclear hypertrophy is significant in the reactive area, although less pronounced or even absent at the periphery of the lobe. Mitotic activity occurs in the Ca-s cells of goldfish gradually adapted to diluted Ca-free SW and Ca-Mg- deficient SW, and sacrificed after 19 and 28 days, respectively. A general stimulation of the Ca-s cells, which remains less intense than that in goldfish kept in deionized water (DW), appears unable to ensure the survival of the goldfish in an isosmotic Ca-free environment containing Mg2+ (0.1, 12 or 16.8 mM). These data are compared with those obtained in the eel kept in Ca-free SW.