Figure 3.

Kinetics of cAMP decrease in MCF-7 cells enriched for membrane estrogen receptor-α (mER^high) cells. (a) Cells were treated with 1 pmol/l 17β-estradiol (E₂) for different time intervals at 37°C in the presence of 1 mmol/l 3-isobutyl-1-methylxanthine (IBMX). The intracellular cAMP (circles) and that in the medium (squares) from the same cells were assessed. (b) cAMP was produced by directly stimulating adenylyl cyclase with 10 μmol/l forskolin for 15 min at 37°C. The decrease in cAMP in the cytosol was tested at 37°C in the absence (open circles) and presence of 1 mmol/l IBMX (closed circles). The entire regression lines were compared by evaluating the differences between the sums of squares of the residuals of individual lines with the sum of squares of the residuals of the combined line using an F-test. The data are presented as means ± standard error. The regression lines were significantly different (P = 0.0001).