Response of PAS-positive cells of the pituitary pars intermedia in the teleost Carassius auratus to acid water

Summary The PAS-positive or PIPAS cells in the pars intermedia of goldfish are activated after reduction of the pH of the ambient freshwater from 7.5 to 3.5. The cells increase in number and exhibit a five-fold increase in cell volume. Granular endoplasmic reticulum occupies most of the cytoplasm. Goldfish PIPAS cells (also termed calciumsensitive cells) are thought to have a hypercalcemic function. Therefore, their activation in acid water may be caused by the severe drop in plasma calcium concentration following exposure of the fish to low water pH. However, activation of the PIPAS cells in response to acidification of the water is not prevented when the calcium concentration of the water is increased to levels that result in hypercalcemia instead of hypocalcemia. Activation of the PIPAS cells occurs also in fish exposed to acidified freshwater enriched with NaCl to an osmolarity similar to that of the blood. This prevents the reduction in plasma osmolarity and Na⁺ and Cl^- concentrations that follow exposure of goldfish to acidified normal freshwater. Our observations do not support the hypothesis that the PIPAS cells in goldfish produce a hypercalcemic hormone, or indeed any hormone involved in calcium metabolism or osmoregulation. The cells may be implicated in acid-base regulation (a characteristic of many types of fish when exposed to acidified water) but the evidence is indirect.