Regulation of G protein mRNA levels by thyroliberin,vasoactive intestinal peptide and somatostatin in prolactin-producing rat pituitary adenoma cells

We have investigated the regulation of mRNA levels of alpha- and beta-subunits of guanine nucleotide-binding regulatory proteins (G proteins) by peptide hormones in prolactin producing rat pituitary adenoma cells (GH₃ cells) in culture. The cells were treated with thyroliberin (1 μm), vasoactive intestinal peptide (1 μm) or somatostatin (10 μM) for 6 to 48 hours. Thyroliberin and vasoactive intestinal peptide increased the levels of G_sα G_oα, G_i-2α, G_i-3α, G_xα, Gβ₃₆ and Gβ₃₅ mRNAs. The effect of vasoactive intestinal peptide was however earlier and more pronounced. G_i-2α mRNA levels showed the quantitatively largest alterations. Somatostatin upregulated G_sα and downregulated G_oα and G_i-2 mRNAs. G protein mRNAs for G_i-2α and G_oα were increased by exposure of the cells to a medium devoid of serum. We conclude that G protein mRNA levels are subjected to alterations by hormones that act through the corresponding G proteins in the regulation of prolactin synthesis and secretion.