E. Arakawa et al., EFFECTS OF CHANGES IN ENVIRONMENTAL CALCIUM ON PROLACTIN SECRETION INJAPANESE EEL, ANGUILLA-JAPONICA, Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology, 163(2), 1993, pp. 99-106
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.