Stimulation of neutral, magnesium-stimulated sphingomyelinase activity in the neurohypophysis of the rat by hypertonic saline ingestion.

Neutral, magnesium-stimulated sphingomyelinase and acid sphingomyelinase activities in the neurohypophysis, adenohypophysis and cerebrum of rats given 2.25% NaCl drinking water for 7 or 14 days were compared with the same enzyme activities in the tissues of control animals allowed free access to distilled drinking water. Neutral sphingomyelinase activity in the neurohypophysis was significantly increased in both experimental groups (7-day, 85.2 +/- 8.4 units/mg protein; 14-day, 110.1 +/- 14.8 units/mg; control, 61.4 +/- 5.5 units/mg). Acid sphingomyelinase activity was significantly but only transiently increased in the tissue (7-day, 73.2 +/- 3.4 units/mg; 14-day, 66.3 +/- 7.5 units/mg; control, 54.3 +/- 2.8 units). Enzyme activities in adenohypophysis and cerebrum of the experimental animals were not significantly different from those of controls. The results suggest a specific role for neutral, Mg2+-stimulated sphinomyelinase in neurosecretion.