The effect of hydrocortisone on cAMP phosphodiesterase activity in human Iymphocytes and beef heart extract was investigated. At all substrate concentrations tested, inhibition of phosphodiesterase activity was demonstrated when both physiological and pharmacological concentrations of hydrocortisone were used (1.4 x 1O-3 M to 1.4 x 1O-7 M). Inhibition of phosphodiesterase activity by hydrocortisone was regularly observed in reaction mixtures wh ich contained only phosphodiesterase purified from beef heart and the reagents used for phosphodiesterase assay. Thus, it appears *This work was supported in part by USPHS Grants AM15853 and HD6779.
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