Tamoxifen reduces calcium currents in a clonal pituitary cell line

The effect of the anti-estrogen Tamoxifen (Tx) on membrane electrical properties and the underlying calcium (Ca2+) conductances was examined in the clonal pituitary cell line GH3/B6 which exhibits calcium action potentials at rest. Electrophysiological recordings (109 cells) were made using either high resistance intracellular microelectrodes or the whole-cell recording (WCR) patch-clamp technique. Electrical activities of 39 spontaneously active GH3/B6 cells were recorded with sharp microelectrodes filled with 3 M KCl. The spikes were Ca2+-dependent since they were blocked by Cobalt ions (Co2+, 5 mM). When applied directly to the recorded cell, Tx (10(-7) M) inhibited action potential firing. This blockade was accompanied by a discrete hyperpolarization of the membrane potential (-2.8 +/- 2 m V) from rest (-39.5 +/- 5 m V) and a 10% increase in the input membrane resistance. The effect stopped soon after Tx removal (mean 11.4 +/- 6 sec), and Tx solvent was unable to elicit the response. Current clamp WCR with pipettes containing potassium gluconate (KGlu, 140 mM) confirmed these results (30 cells), but the addition of cell extract to KGlu was necessary to prevent rundown of the response and to obtain reproducible action potential blockade. Short (1-5 min) or long term (48 h) pretreatment with estradiol (10(-7) to 10(-5) M) did not change the response to Tx, thus indicating that its effect was not mediated through estrogen receptors. Voltage clamp WCR study of the effect of Tx (10(-7) M) using pipettes filled with cesium chloride (140 mM) showed that both fast and slow inactivating calcium conductances were inhibited (38 cells). The fast inactivating Ca2+ current was reduced by about 60-80% whereas slow inactivating Ca2+ current was completely inhibited. This action may represent one way by which the antitumoral effect of this antiestrogen is mediated.