Calcium homeostasis in a clonal pituitary cell line of mouse corticotropes

Calcium homeostasis was studied following a depolarization-induced transien t increase in [Ca2+](i) in single cells of the clonal pituitary cell line o f corticotropes, AtT-20 cells. The KCl-induced increase in [Ca2+](i) was bl ocked in (i) extracellular calcium-deficient solutions, (ii) external cobal t (2.0 mM), (iii) cadmium (200 muM), and (iv) nifedipine (2.0 muM). The mea n increase in [Ca2+](i) in single cells in the presence of an uncoupler of mitochondrial function [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazon e, FCCP, 1 muM] was 54 +/- 13 nM (n = 9). The increase in [Ca2+](i) produce d by FCCP was greater either during or following a KCl-induced [Ca2+](i) lo ad. However, FCCP did not significantly alter the clearance of calcium duri ng a KCl-induced rise in [Ca2+](i). Fifty percent of the cells responded to caffeine (10 mM) with an increase in [Ca2+](i) (191 +/- 24 nM; n = 21) abo ve resting levels; this effect was blocked by ryanodine (10 muM). Thapsigar gin (2 muM) and 2,5 di(-t-butyl)-1,4 hydroquinone (BuBHQ, 10 muM) produced increases in [Ca2+](i) (47 +/- 11 nM, n = 6 and 22 +/- 4 nM, n = 8, respect ively) that increased cell excitability. These results support a role for m itochondria and sarco-endoplasmic reticulum calcium stores in cytosolic [Ca 2+](i) regulation; however, none of these organelles are primarily responsi ble for the return of [Ca2+](i) to resting levels following this KCl-induce d [Ca2+](i) load.