Abstract
The effects of extracellular nucleotides and agents which elevate intracellular cyclic adenosine 3′,5′-monophosphate (cyclic AMP) concentrations on human lymphocyte metabolism have been studied. Aminophylline, isoproterenol, and prostaglandins, all of which elevate lymphocyte cyclic AMP levels, inhibited incorporation of 3H-labeled thymidine, uridine, and leucine into the DNA, RNA, and protein of phytohemagglutinin (PHA)-stimulated lymphocytes. Aminophylline inhibition was maximal only when the inhibitor was added within 1 hr after exposure of cells to PHA, suggesting that a relatively early step in the lymphocyte transformation process may be affected.
The addition of various nucleotides to the culture medium also inhibited incorporation of labeled precursors. The best inhibitor, dibutyryl cyclic AMP (DU cyclic AMP), produced maximal inhibition only if present during the 1st hr after initial exposure to PHA. Among the various cyclic nucleotides derivatives of guanosine and adenine were the most effective inhibitors (substantial inhibition at 0.1 mM concentrations). However, the inhibition was not specific for nucleotides containing the cyclic phosphodiester moiety since the tri-, di-, and monophosphates of adenosine and guanosine were equally effective in diminishing thymidine uptake. The above inhibitions were not due to secondary effects of the inhibitors on the interaction of PHA with lymphocytes as judged by 125I-labeled PHA binding studies.
Low concentrations (1-10 μmoles/liter) of cyclic AMP produced slight stimulation of thymidine-3H uptake in resting lymphocytes (lymphocytes not stimulated with PHA). However, the effects were quite small as compared with those produced by PHA itself. Attempts to demonstrate increased thymidine uptake 48 hr after pulsing lymphocytes with aminophylline or isoproterenol were unsuccessful. The relationship of these observations to a possible regulatory role for cyclic AMP in PHA-stimulated lymphocytes is discussed.
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