We have shown previously that FSH differentially regulated G-protein alpha i-subunit mRNA levels in primary cultures of rat Sertoli cells. The mechanism by which FSH regulated the steady state levels of these mRNAs was investigated by assessing the effect of FSH on G-protein alpha-subunit mRNA stability and the potential involvement of newly synthesized proteins. The half-life (t 1/2) of alpha-subunit mRNA was determined by inhibiting Sertoli cell transcription with actinomycin-D and quantifying alpha-subunit mRNA levels by Northern blot analyses. Transcripts for alpha i-2 and alpha 3 were extremely stable, possessing t1/2 of 44 and 51 h, respectively. In contrast, the t1/2 of alpha i-3 mRNA was only 3.6 h. Turnover of alpha i-1 mRNA occurred as a two-phase decay, with an initial t1/2, fast of 0.8 h, followed by a second phase with t1/2, slow of 11.1 h. Treatment of Sertoli cells with FSH in the presence of actinomycin-D destabilized the alpha i-1 mRNA to a single phase decay with a t1/2 of 5.7 h. FSH also decreased the stability of the alpha i-2 mRNA from a t1/2 of 51 h to a t1/2, fast of 16.5 h. These effects of FSH on alpha i-1 and alpha i-2 mRNA stability may contribute to the ability of FSH to decrease steady state alpha i-1 and alpha i-2 mRNA levels. The effect of FSH on alpha i-3 mRNA stability was not significant, suggesting an alternative mechanism regulating the FSH-mediated increase of alpha i-3 mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)