Specific effect of calcium ions on the calcium-sensitive cells of the pars intermedia in the goldfish

Summary Cytological changes in the calcium-sensitive (Ca-s) cells (formerly termed PAS-positive cells) of the pars intermedia were investigated in the goldfish after adaptation to deionized water (DW), with or without addition of sodium, potassium and magnesium. These ions were added as chloride salts at concentrations similar to those present in fresh water (FW). The marked stimulation of the Ca-s cells is not inhibited in DW supplemented with Na⁺ (0.35 mM/1), K⁺ (0.05 mM/1), and Mg²⁺ (0.2mM/1) for a period of 24 days. The inhibition of the response to DW with calcium chloride (2 mM/1) is reproduced with calcium formiate (2 mM/1). These data show that chloride ions are not responsible for the regression of the Ca-s cells observed in goldfish kept in DW supplemented with calcium chloride. The effect of calcium ions on the Ca-s cells appears to be specific. These results support the hypothesis that the Ca-s cells synthesize a factor (hypercalcin?) involved in calcium regulation, and that its release is influenced by the calcium content of the environment. The role of the pars intermedia in calcium metabolism is strengthened by the present results. Biochemical data suggest the presence of a hypercalcemic factor in the pituitary of fish (Parsons et al. 1978) and are in agreement with the present cytological findings.