Significant Qualitative Differences Exist between Thyrotropin and Prolactin Secretory Dynamics Induced by Pituitary Cell Swelling

Abstract

Cell swelling produced by a variety of techniques is a potent stimulus intensity-related inducer of an immediate secretory burst of thyroid-stimulating hormone (TSH) and prolaction (PRL) secretion from anterior pituitary cells. A 2-min "square wave" exposure to either hyposmolarity or isotonic urea induced stimulus intensity-correlated TSH and PRL secretory bursts peaking within 3 min, but the PRL zenith occurred 1 min later than that of TSH. With continuous exposure to these stimuli, TSH secretion rapidly decreased and remained only slightly above the unstimulated rate after 5 min. PRL secretion fell to and remained below the unstimulated level after 10 min. After stopping the stimulus, another secretory burst ("off" response) occurred with PRL, but not with TSH. A progressive "ramp" increase in stimulus intensity over 18 min induced a corresponding gradual increase in TSH secretion; there was a progressive depression, rather than increase, in PRL secretion during the stimulus ramp, with an off response secretory burst when the stimulus was discontinued. Removal of extracellular Ca2+ or addition of verapamil to the medium did not alter the dynamics of hyposmolarity-induced TSH secretion, but markedly altered those of PRL secretion; there was no off response PRL secretion and a hyposmolar ramp induced a corresponding gradual increase in PRL secretion, with a return to baseline after removing the stimulus. The dramatic qualitative differences in the response of the thyrotroph and lactotroph may reflect differences between the cell types in the size of secretory vesicles, membrane potential, the mechanism of exocytosis, and/or the role of Ca2+ influx across the plasmalemma.

Cell volume increase and extracellular Ca2+ are needed for hyposmotically induced prolactin release in tilapia

“…In many cells placed in hyposmotic media, the initial swelling due to water influx is followed rapidly by losses of ions and/or organic solutes that lead to RVD (19,20). In many cell types, RVD is observed within 20 min of hyposmotic stimulation (2,20,21,32,37,39). The absence of a rapid RVD in the PRL cell may reflect the smaller shift in osmolality used in this study (15% reduction in osmolality) compared with those typically used by others, i.e., 25-50% (2, 20, 21, 31, 32, 39).…”

Hypotonicity and peptide discharge from a single vesicle

“…Previous studies demonstrated prolactin release from isolated tissue (18,34,41,42) or cells in response to hyposmotic stimulation (41,50), which required extracellular calcium (23,40,41). While these findings suggest that prolactin is likely released by exocytosis in response to hyposmotic stress, secretory activity at the single vesicle level in response to hypotonicity was not monitored directly yet.…”

Comparison of ramp and square-wave exposure to thyrotrophin-releasing hormone or depolarizing K+ on cytosolic Ca2+ concentration and prolactin secretion in GH4C1 cells

“…For ramp expo¬ sure, a gradient-maker originally designed to make sucrose concentration gradients for ultracentrifugation was used to gradually increase the concentration of stimulator. The final concentration was the same as that achieved with square-wave exposure (Wang et al 1991). Comparison of the dynamics of concentrations achieved in the perifusion chamber effluent using the square-wave or ramp system is shown in Fig.…”

“…We have found a marked difference in the dynamic secretory pattern of PRL secretion induced by squarewave compared with ramp application of an osmotic stimulus (Wang et al 1991). In this study we have compared the dynamics of changes in [Ca ]¡ and PRL secretion in response to square-wave or ramp exposure to TRH or depolarizing .…”