Two types of calcium channels and hormone release in human pituitary tumor cells

Abstract

Voltage-gated Ca2+ channel currents were examined in human growth hormone-producing cells using the whole cell variation of patch electrode voltage clamp. In 20 mM Ba2+, Na+-free medium, the inward current was composed of a fast-inactivating component (transient type) and a steady component (long-lasting type). These two types had different properties. The transient type inactivated with the depolarizing prepulse but the long-lasting type did not. The transient type was activated at more negative potential levels than the long-lasting type. Deactivation kinetics of the transient type were slower than that of the long-lasting type. Nitrendipine inhibited and Bay K 8644 enhanced Ca2+ channel currents, and the effects of these agents were more prominent on the long-lasting type than on the transient type. Hormone release was inhibited by nitrendipine and enhanced by Bay K 8644, indicating involvement of Ca2+ influx through voltage-gated channels in the release process.

Endogenous pacemaker activity of rat tumour somatotrophs

“…The delayed rectifier K¤ current of GC cells would also play a role in spike repolarization since it activates around −10 mV but would not participate in the AHP as it deactivates rapidly, in less than 10 ms (authors' unpublished observation). The L-and TÏR-type currents described here have similar properties to those reported in GH-related cell lines including the mammosomatotroph GH× cells (Hagiwara & Ohmori, 1982;Armstrong & Matteson, 1985;Simasko, Weiland & Oswald, 1988;Scher ubl & Hescheler, 1991;Herrington & Lingle, 1992;Zong, Yassin & Tanabe, 1995), GHÚ cells (Cohen & McCarthy, 1987) and human growthhormone producing pituitary adenoma cells (Yamashita, Matsunaga, Shibuya, Teramoto, Takakura & Ogata, 1988). The major difference concerns the inactivation of the TÏR-type current, which is faster in GC cells.…”

Requirement of GTP on somatostatin-induced K+ current in human pituitary tumor cells.

“…According to immunohistochemical and hormone-release studies (5,6), most cells in adenomas 1 and 2 were considered to be GH-producing cells and a small amount of PRL-secreting cells may have existed in adenoma 3 in addition to GH-secreting cells. A more positive identification of GH-producing cells has been described elsewhere by using the reverse-hemolytic plaque assay (7). Electrophysiological Analysis.…”

Simultaneous Measurement of Changes in the Membrane Potential and the Intracellular Ca2+ Concentration Caused by Somatostatin in Human GH-Producing Pituitary Tumor Cells.

“…This effect is mediated through G protein and is independent of the reduction in intracellular cAMP or Ca2+. In human GH-producing cells , the influx through the voltage-gated Ca2+ channel current was closely correlated to hormone secretion [21]. Therefore membrane hyperpolarization should be involved in the inhibition of hormone secretion by SRIF.…”