Calcium homeostasis in growth hormone (GH)-secreting adenoma cells: effect of GH-releasing factor

Human GH-secreting tumors are heterogenous regarding their basal secretory activity and response to GH-releasing factor (GRF). We have investigated whether such different secretory properties could be accounted for by alterations of intracellular mechanisms occurring at the calcium level. Basal free intracellular calcium concentrations ([Ca2+]i) and Ca2+ responses to GRF were studied in single cells cultured from fragments of five GH-secreting pituitary adenomas. We used the microspectrofluorimetric method and indo-1 as the fluorescent probe. The cell populations cultured from the tumors of patients A and C showed increased hormone secretion in response to GRF in vitro, whereas cultures from patients B, D, and E were unresponsive to the peptide. Basal [Ca2+]i measured in the five cell populations ranged from 82 +/- 18 to 118 +/- 27 nM. A 10-sec application of 10 nM GRF induced an increase in [Ca2+]i in 60% and 54% of A and C cells, respectively. In the nonresponsive cell populations, the number of calcium responses to GRF was lower, 26% (B cells), 5% (D cells), and 10% (E cells). Two principal responses types were observed: 1) an initial increase in [Ca2+]i, followed by a sustained plateau phase lasting for more than 200 sec; and 2) a monophasic peak of increased [Ca2+]i lasting approximately 1 min before returning to baseline levels. GRF responses were totally suppressed in the absence of Ca2+ ions in the external medium. Sixteen to 30% of the cells cultured from four of the five tumors showed spontaneous fluctuations of [Ca2+]i. These spontaneous Ca2+ transients were suppressed in Ca(2+)-free medium. The number of cells exhibiting such Ca2+ transients decreased with time in culture. Basal hormone secretion was higher in cultures from patient D, in which no spontaneous Ca2+ transients were observed in any of the 72 studied cells, and in cultures from patients E, in which only 16% of cells were spontaneously active. We conclude that 1) in human responsive somatotrophs, the involvement of Ca2+ in GRF stimulus-secretion coupling mechanisms is apparently similar to that described in somatotrophs of other species; 2) the lack of a secretory response to GRF observed in some tumors may result from impairment of Ca2+ responsiveness in either cell recruitment or response amplitude and/or duration; and 3) spontaneous rhythmic Ca2+ activity is apparently dissociated from basal hormone secretion in some of these tumor cells.