Thyrotropin-releasing hormone modulation of uridine uptake in rat pituitary cells. Characterization of the responses.

Thyrotropin-releasing hormone (TRH), L-pyroglutamyl-Lhistidyl-L-prolineamide, modulates hormone synthesis and secretion by certain clonal strains of rat pituitary cells.An additional response of such cells to TRH reported here is the modulation of uridine uptake.Within 40 min of TRH addition to the culture medium, uridine uptake in GH,C, cells was stimulated approximately Z-fold.This acute enhancement was followed by inhibition of uridine uptake to 60% of control values by 24 h following TRH addition (chronic response).Acute enhancement of uridine uptake by TRH was evident only in cultures 30 h or longer after medium replenishment.Acute stimulation and chronic inhibition of uridine uptake were observed only with cell strains which possess specific receptors for TRH.Halfmaximal acute enhancement of uridine uptake occurred at 0.5 nM TRH whereas chronic inhibition was half-maximal at 2.8 nu TRH.Five TRH analogs varying in affinity for the TRH receptor over a 5 log range were tested for acute and chronic effects on uridine uptake.The rank ordering of potencies of these analogs for acute and chronic effects were similar and resembled the rank ordering of affinities of the peptides for the TRH receptor.Uptake of pyrimidine ribonucleosides was stimulated acutely and inhibited chronically by TRH treatment, whereas uptake of purine ribonucleosides was largely unaltered.TRH acutely stimulated the uptake of uridine, cytidine, phosphate, and hexose with similar time courses; uptake was maximally stimulated to 150 to 200% of control by 40 min of TRH treatment following an S-to IO-min lag.Slow binding of TRH to the cells was excluded as the cause for the lag in experiments demonstrating that TRH receptor occupancy was complete within 2 min at peptide concentrations used in the uptake studies.Acute enhancement of substrate uptake by TRH did not require ongoing protein synthesis.Enhancement of uptake by TRH is an early cellular response to the tripeptide and