Voltage‐dependent calcium channels regulate GH4 pituitary cell proliferation at two stages of the cell cycle

Abstract

Calcium is an intracellular signal implicated in the regulation of cell proliferation. We have examined the growth regulatory role of voltage-dependent calcium channels (VDCC) in a rat pituitary cell line (GH₄C₁) that expresses two well-characterized VDCC subtypes (L and T) and is growth-inhibited by several agents known to enhance calcium entry. Thyrotropin-releasing hormone (TRH), tetrade-canoylphorbol acetate (TPA), and epidermal growth factor (EGF), each known to enhance calcium entry in GH₄ cells, decrease GH₄ cell number and incorporation of [³H]-thymidine. The growth inhibitory action of these agents is cytostatic with a predominant effect to block G₁ cells from entering S-phase. We next examined the growth regulatory action of pharmacologic agents that interact directly and specifically with type L VDCC. Activation of type L VDCC with the dihydropyridine BAY K8644 inhibits GH₄ proliferation as measured by cell number and [³H]-thymidine incorporation. This action of BAY K8644 is enhanced by a submaximal K⁺-maintained depolarization, and the growth inhibitory action of these agents is also cytostatic as evident by the block of G₁ cells from entering S-phase. Nimodipine, an antagonist specific for type L VDCC blocks (IC₅₀ = 30 nM) BAY K8644-inhibited cell proliferation by substantially reducing the S-phase block. Taken together these findings indicate that calcium entry through type L VDCC inhibits GH₄ cell proliferation by blocking entry into S-phase. By contrast, nimodipine caused only a small reversal of the TRH-induced S-phase block, suggesting that TRH inhibits proliferation by a mechanism that differs at least in part from L-channel activation. Unexpectedly, nimodipine, given alone, caused a substantial inhibition of GH₄ cell proliferation. This action of nimodipine was cytostatic, yet differed from calcium channel activators in that the percentage of S-phase cells was unchanged whereas G₂-M-phase cells increased with a parallel decrease in G₁-phase cells. Similar effects were also observed with other classes of calcium channel blockers. Taken together these results indicate that calcium entry through VDCC regulates GH₄ cell proliferation differently cepending on the stage of the cell cycle. In G₁-phase cells, sustained entry of calcium through type L VDCC blocks entry into S-phase. In G₂-M-phase cells entry of calcium promotes progression through mitosis.