CAPACITATIVE CA2-RELEASING-HORMONE (TRH) IN GH(3) PITUITARY-CELLS( ENTRY CONTRIBUTES TO THE CA2+ INFLUX INDUCED BY THYROTROPIN)

Treatment of GH(3) cells with either hypothalamic peptide thyrotropin- releasing hormone (TRH), the endomembrane Ca2+-ATPase inhibitor thapsi gargin or the Ca2+ ionophore ionomycin mobilized, with different kinet ics, essentially all of the Ca2+ pool from the intracellular Ca2+ stor es. Any of the above-described treatments induced a sustained increase in intracellular Ca2+ concentration ([Ca2+](i)), which was dependent on extracellular Ca2+ and was prevented by Ni2+ but not by dihydropyri dines (DHPs), suggesting that it was due to capacitative Ca2+ entry vi a activation of a plasma membrane pathway which opened upon the emptyi ng of the intracellular Ca2+ stores. The increase of the plasma membra ne permeability to Ca2+ correlated negatively with the filling degree of the intracellular Ca2+ stores and was reversed by refilling of the stores. The mechanism of capacitative Ca2+ entry into GH(3) cells diff ered from similar mechanisms described in several types of blood cells in that the pathway was poorly permeable to Mn2+ and not sensitive to cytochrome P-450 inhibitors. In GH(3) cells, TRH induced a transient [Ca2+](i) increase due to Ca2+ release from the stores (phase 1) follo wed by a sustained [Ca2+](i) increase due to Ca2+ entry (phase 2). At the single-cell level, phase 2 was composed of a DHP-insensitive susta ined [Ca2+](i) increase, due to activation of capacitative Ca2+ entry, superimposed upon which DHP-sensitive [Ca2+](i) oscillations took pla ce. The two components of the TRH-induced Ca entry differed also in th at [Ca2+](i) oscillations remained for several minutes after TRH remov al, whereas the sustained [Ca2+](i) increase dropped quickly to presti mulatory levels, following the same time course as the refilling of th e stores. The drop was prevented when the refilling was inhibited by t hapsigargin. It is concluded that, even though the mechanisms of capac itative Ca2+ entry may show differences from cell to cell, it is also present and may contribute to the regulation of physiological function s in excitable cells such as GH(3). There, capacitative Ca2+ entry coo perates with voltage-gated Ca2+ channels to generate the [Ca2+]i incre ase seen during phase 2 of TRH action. This contribution of capacitati ve Ca2+ entry may be relevant to the enhancement of prolactin secretio n induced by TRH.