SEROTONIN-INDUCED CHANGES IN MEMBRANE-POTENTIAL AND CYTOSOLIC-FREE CALCIUM IN A CLONAL PITUITARY CELL-LINE (ATT-20) OF CULTURED MOUSE CORTICOTROPES

The effects of serotonin (5-HT) were examined on the cytosolic free calcium concentration, (Ca2+)(i), in fura-2-loaded cells from a clonal cell line derived from the mouse anterior pituitary gland (AtT-20/D16v). Brief applications of 5-HT produced transient increases in (Ca2+)(i). The duration and the amplitude of the 5-HT-induced increase in (Ca2+)(i) depended on the duration of 5-HT application. Longer duration (>150 msec) applications of 5-HT produced a transient increase followed by a prolonged plateau phase of (Ca2+)(i). Oscillations in (Ca2+)(i) were initiated and maintained in previously quiescent corticotropes following brief exposures to 5-HT. Addition of cobalt (500 mu M - 3.6 mM) to the extracellular solution abolished oscillations in (Ca2+)(i) and produced transient but not sustained increases in (Ca2+)(i) in response to 5-HT. Electrophysiological responses to 5-HT were recorded in separate experiments using the whole-cell patch clamp recording technique. Application of 5-HT to single cells produced a depolarization which was accompanied by a decrease in membrane conductance when measured under voltage clamp. The duration of the depolarizing response also increased with the duration of the 5-HT application. With longer duration 5-HT applications, the 5-HT-induced depolarizing response resembled the spontaneous action potential responses recorded in the same cell. However, the 5-HT-induced depolarizations were unaltered in the presence of extracellular cobalt. This suggests that the transient increase in (Ca2+)(i) recorded when 5-HT was applied in the presence of cobalt may represent intracellular release of calcium. These results demonstrate that 5-HT activates single cultured corticotropes directly by producing an increase in (Ca2+)(i) and cell depolarization.