EFFECTS OF CHANGES IN ENVIRONMENTAL CALCIUM ON PROLACTIN SECRETION INJAPANESE EEL, ANGUILLA-JAPONICA

Effects of changes in environmental Ca2+ on the secretion of prolactin , a possible hypercalcemic hormone, were examined both in vivo and in vitro in the Japanese eel, Anguilla japonica. Transfer of seawater- or freshwater-adapted fish to fresh water, fresh water containing 10 mmo l Ca2+.l-1 sea water, Ca2+-free sea water, or deionized water was acco mpanied by significant changes in plasma Ca2+ levels after 7 days, exc ept for the fish transferred from fresh water to fresh water and from sea water to sea water. Changes in external Ca2+ concentrations did no t affect plasma prolactin levels, although plasma prolactin levels as well as pituitary prolactin contents were significantly greater in fis h in a hypotonic environment than those in a hypertonic environment, r egardless of the external Ca2+ concentration. Hypercalcemia, induced b y removal of the corpuscles of Stannius, did not alter plasma prolacti n levles. Incubation of the pituitary in the medium with different Ca2 + concentrations (up to 2.9 mmol.l-1) did not affect the basal release of prolactin, except at an extremely low Ca2+ concentration (less tha n 0.1 mmol.l-1) where prolactin release was inhibited. Addition of Ca2 + ionophore (A23187) to the medium led to a marked and significant inc rease in prolactin release, indicating that an increase in intracellul ar Ca2+ stimulates prolactin release. However, the effect was not spec ific to prolactin cells; a similar increase was seen in growth hormone release. These results indicate that changes in environmental Ca2+ co ncentration may not be the primary factor influencing prolactin secret ion in the eel; changes in environmental osmolality or Na+ levels seem to be more critical for the regulation of prolactin secretion.