Thyrotropin-releasing hormone stimulates GTP hydrolysis by membranes from GH4C1 rat pituitary tumor cells.

Abstract

TRH stimulates prolactin production by GH4C1 rat pituitary tumor cells, which possess high-affinity membrane receptors for the peptide. TRH caused up to a 50% increase in the activity of a low-Km GTPase in membranes from GH4C1 cells. The TRH stimulatory effect was maximal at GTP concentrations of .ltoreq. 1 .mu.M. TRH caused an increase in GTPase activity of between 0.2 and 20 pmol of GTP hydrolyzed per mg of protein per min, depending on GTP concentration, while TRH binding was 0.3 pmol/mg of protein. TRH did not stimulate GTPase activity in membranes from GH12C1, or GH-Y cells, 2 pituitary lines lacking TRH receptors. Stimulation of GTPase depended on occupancy of the TRH receptor; half-maximal increases in GTPase activity required 46 nM TRH and 25 nM [N3-methyl-His]TRH, but the TRH free acid was inactive. The apparent Kd of these peptides for receptors were similar when measured under the same conditions. Since TRH binding to receptor is regulated by guanyl nucleotides, since TRH stimulates of low-Km GTPase activity, then the TRH receptor is associated with a guanyl nucleotide regulatory protein in the lactotrope membrane.