Serotonin-induced changes in membrane potential and cytosolic free calcium in a clonal pituitary cell line (AtT-20) of cultured mouse corticotropes

Abstract

The effects of serotonin (5-HT) were examined on the cytosolic free calcium concentration, {Ca 2+} 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 {Ca 2+} i. The duration andthe amplitude of the 5-HT-induced increase in {Ca 2+} 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 {Ca 2+} i. Oscillations in {Ca 2+} i were initiated and maintained in previously quiescent corticotropes following brief exposures to 5-HT. Addition of cobalt (500 μM − 3.6 mM) to the extracellular solution abolished oscillations in {Ca 2+} i and produced transient but not sustained increases in {Ca 2+} 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 {Ca 2+} 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 {Ca 2+} i and cell depolarization.