Calcium entry through L-type Ca2+ channels has been shown to increase the number of Na+ channels in GH3 cells, a clonal line of rat pituitary cells. To test whether this Ca2+ influx affects the levels of Na+ channel mRNAs, we first examined which Na+ channel subunits are expressed in GH3 cells. By using RT-PCR with specific primers, we detected transcripts for four alpha subunits (Nav1.1, Nav1.2, Nav1.3 and Nav1.6) and two auxiliary subunits (beta1 and beta3) of Na+ channels in total RNA from control GH3 cells. Next, we optimized the RT-PCR conditions to allow detection of cDNAs in the linear range of the assay. These conditions were then used to assess the transcript levels of Na+ channels after chronic exposure (72 h) of GH3 cells to the L-type Ca2+ channel blocker nimodipine (1 microM) or the L-type channel agonist Bay K 8644 (0.5 microM). Nimodipine treatment caused a moderate reduction (approximately 30%) of the mRNA for Nav1.2 and a marked reduction (approximately 70%) of the mRNA for Nav1.3, whereas treatment with Bay K 8644 produced 90-130% increases in these same mRNAs. There were no concomitant changes in the mRNAs for Nav1.1 and Nav1.6. Moreover, beta1 and beta3 mRNA levels were also unchanged. Thus, GH3 cells express multiple Na+ channel subunits and L-type Ca2+ channel activity up-regulates in a specific way the mRNAs for Nav1.2 and Nav1.3. These findings improve our knowledge on the molecular diversity of Na+ channels in pituitary cells and extend the actual view about the regulation of Na+ channel gene expression by Ca2+ influx.