Apparent lack of involvement of camp as a mediator of lhrh stimulation of nuclear estrogen receptor activity in the rat anterior pituitary

We have previously shown that incubation of rat pituitary cells in the presence of LHRH results in specific enhancement of nuclear estrogen receptor (ER) binding which cannot be accounted for by simple intracellular translocation of cytoplasmic receptor. In the present study, the role of cAMP in this response has been examined. Suspended pituitary cells from adult ovariectomized rats primed with estradiol were incubated with varying concentrations of LHRH or a highly active LHRH analog (LHRH-A) for 30 min at 37°C, and levels of cAMP were determined. Total cAMP levels changed only in response to a concentration of 100 pmol/pituitary of either peptide; the stimulation by LHRH was twice that by LHRH-A. When the priming dose level of estradiol was reduced from 1.0 to 0.5 μg/day, stimulation of cells by 100 pmol of LHRH caused a much greater increase in cAMP levels. Separate incubation of subcellular fractions with a wide dose range of dibutyryl-cAMP (DBcAMP) resulted in a progressive loss of cytosol receptor binding capacity (which was also observed in whole cultured or suspended cells), but no significant concomitant change in nuclear receptor binding; however, whole cells in suspension or culture did show an increase in nuclear ER activity in the presence of 100 nM DBcAMP. This response was qualitatively, but not quantitatively, similar to that elicited by LHRH. When the effects of whole cell incubation with LHRH and LHRH-A on nuclear ER were compared with their effects on total cAMP levels, no correlation was observed; rising levels of cAMP did, however, coincide with falling levels of cytosol ER. Integration of these observations with our previous quantitative assessment of LHRH effects on the ER populations indicates that cAMP may mediate the diminution of cytosol ER levels, but it does not appear to be involved to any appreciable degree in the enhancement of the nuclear ER binding.