Capacitance increases of dissociated tilapia prolactin cells in response to hyposmotic and depolarizing stimuli.

Prolactin (PRL) is the major hormonal mediator of adaptation to hyposmotic conditions. In tilapia (Oreochromis mossambicus), PRL cells are segregated to the rostral pars distalis of the anterior pituitary facilitating the nearly pure culture of dissociated PRL cells. Membrane capacitance (C(m)) was recorded at 1Hz or higher for tens of minutes as a surrogate monitor of PRL secretion by exocytosis from cells under perforated patch clamp. The study compares secretory responses to trains of depolarizing clamps (100 at 2.5 Hz, from -70 to +10 mV for 100 ms) to the physiological stimulus, exposure to hyposmotic medium, here a switch from 350 to 300 mOsm saline ([Ca²⁺] 15 mM). Two-thirds of cells tested with each stimulus responded. In response to depolarizing clamps, C(m) increased linearly at an average rate of 7.2 fF/s. The increase was also linear in response to hyposmotic perfusion, but the average rate was 0.68 fF/s. Response to depolarization was reversibly blocked in Ca²⁺-omitted saline, or in saline with 30 μM Cd²⁺. It was unaffected by 0.1 μM tetrodotoxin. By contrast, responses were reduced but not absent during perfusion of hyposmotic saline with Ca²⁺-omitted; 30 μM Cd²⁺ appeared to enhance the hyposmotic response. BAPTA-AM eliminated responses to both stimuli, confirming that secretion was dependent on increases of intracellular [Ca²⁺]. Together with previous observations from this laboratory of [Ca²⁺](i) with simultaneous collection and immunoassay of perfusate for PRL, we conclude that depolarization and hyposmotic stimuli initiate secretion by independent mechanisms.