Background: Ca2+ is without question the most versatile intracellular second messenger in cellular processes initiated by growth factors and hormones and involved in diverse cellular and organ functions such as cell cycle regulation, apoptosis and gene expression. Among the plethora of Ca2+ binding proteins in cells, the most ubiquitous and abundant is Calmodulin (CaM). One action of CaM is to activate members of a family of Ser/Thr protein kinases called Ca2+/CaM-dependent protein kinases or CaM kinases (CaMK’s). There is a significant “crosstalk” between different intracellular pathways.We wanted to study the expression and regulation of a newly discovered CaM kinase Iâ isoform (Løseth et al., 2000) in closely related pituitary cells wich produce growth hormone (GH) and prolactin (PRL). GH and PRL synthesis is known to be affected by a number of different hormones, hypothalamic releasing factors and chemicals wich modulate intracellular signaling systems. 17â estradiol and TRH are major regulators of PRL production while the synthetic glucocorticoid dexamethason stimulates GH production. Bromocriptin a dopamin agonist, inhibits both PRL and GH production.Methods: The cells were grown in plastic bottles for 10 days and thereafter harvested. RNA was isolated by the Trizol-method, and applied on RNA gels for electrophoresis. RNA was then transferred to a filter (Northern blotting) and hybridized to radioactive DNA probes to quantify the amount of CaMK Iâ mRNA in the two GH cell lines. Moreover the amount of CaMK Iâ mRNA was compared between treated and untreated cell cultures using GAPDH mRNA as internal standard.Results and conclusions: The differences in the relative amount of CaM kinase Iâ mRNA/GAPDH mRNA in the controls and treated cell cultures were not significantly different. This indicates that, according to the house-keeper-gene GAPDH, CaM kinase has not been regulated by the substances added. The steady level production of CaM kinase Iâ mRNA is higher in GH12C1 than in GH4C1 cells. This could be due to the fact that GH12C1 cells excrete growth hormone, whilst the GH4C1 cells excrete prolactin. One hypothesis is therefore that CaM kinase could be involved in signaling pathways active in growth hormone producing phenotype. The cellular functions in wich the Cam kinase Iâ is involved are still elusive.