CELL-VOLUME AND SHAPE OSCILLATIONS IN RAT TYPE-II SOMATOTROPHS AT HYPOTONIC CONDITIONS

The size and shape of growth hormone (GH)-producing rat type-II somato trophs was studied during osmotic manipulation, When somatotrophs were exposed to large osmotic stress (200 and 225 mOsm), the peak projecte d cell area (PCA) was 132.9% +/- 12.6% and 116.8% +/- 2.8% (P < 0.01) and triggered a regulatory volume decrease (RVD) to avoid lysis, At lo wer osmotic stress (250 mOsm), the rate of swelling was slower, and th e volume reached a steady state at 109.4% +/- 2.4% (P < 0.05) and was without RVD. At 275 and 287 mOsm, the swelling was delayed [PCA peak a t 3-4 min; 105.8% +/- 1.5% (P < 0.05) and 104.2% +/- 1.7%] and then sh owed repeated synchronized cycles of swelling and shrinkage (P < 0.05) . The data suggest that somatotrophs may have more than one mechanism for volume regulation. One mechanism is for large swelling (classic RV D response), whereas the other represents more physiological mechanism s for regulating the cell volume within a more limited geometry range. For low osmotic stress (250-287 mOsm), the somatotrophs became less s pherical during swelling and, thus, were without membrane dilation. Th erefore, this type of volume regulation must work independently from m embrane stress. Related volume regulation mechanisms may underlie the previously observed volume fluctuations in somatotrophs seen during se cretory stimulation with GH-releasing hormone. (C) 1995 Wiley-Liss, In c.