Isolation of desensitization activity by reverse-phase chromatography. Inhibition of CNP-induced elevations of cGMP levels in GC-B/3T3 cells by fractions from the second analytic C18 column. Fifty-μl aliquots of the 0.5 ml fractions were dried and dissolved in 50 μl PBS and 5 μl added to the cells in a final volume of 0.5 ml. IBMX (0.25 mM) and CNP (20 nM) were added and cGMP levels were determined after 5 min incubation with CNP as described under Methods. Error bars indicate the range of duplicate determinations.