Exogenous ATP enhances calcium influx in intact thymocytes.
Recent observations have indicated that exogenous adenosine triphosphate (ATP) may influence lymphocyte functions such as proliferation and cytoxicity. Here we report a novel activity of extracellular ATP--it specifically increases Ca2+ uptake in murine lymphocytes. ATP added to thymocytes increases the rate of [45Ca2+] uptake by up to 20-fold. The increased rate is seen with ATP concentrations as low as 500 microM and is half-maximal at approximately 2 mM ATP. The magnitude of stimulation by ATP is dependent on Mg2+ concentration, and ATP-Mg2+ complex is probably the true activator. Of the high-energy phosphate-containing compounds tested, including deoxy-ATP, only GTP showed a modest stimulation of calcium uptake. ADP, AMP, cyclic AMP, and adenosine did not significantly increase calcium uptake. Cellular integrity as indicated by trypan blue exclusion and ethidium bromide/acridine orange staining was unaffected by ATP. Ca2+ influx is the major mode of action of ATP in raising intrathymocyte Ca2+ levels, because neither the Ca2+ efflux nor the [45Ca2+]-Ca2+ exchange was significantly altered in the presence of ATP. Verapamil, a Ca2+ channel blocking agent, could not prevent the ATP effect, suggesting that ATP may be acting by a mechanism other than the voltage-dependent Ca2+ channel. An analysis of intracellular and extracellular ATP levels by chemiluminescence assay indicated no significant ATP entry into intact lymphocytes. Also, ATP added to the medium containing thymocytes was destroyed (approximately 50% by 20 min). The nonhydrolyzable ATP analogs, AMPPCP and AMPPNP, were unable to stimulate a significant amount of Ca2+ uptake, suggesting the involvement of a cell surface phosphotransferase activity. This was supported by the demonstration of a threefold to fivefold increase in the labeling of protein and phospholipid fractions obtained from intact thymocytes exposed to [gamma 32P]ATP for 30 min. Ca2+ is believed to play an important role in a variety of lymphocyte functions, including mitogenesis and natural killer cell activity. The data herein thus provide a potential mechanism for the action of exogenous ATP on these lymphocyte functions.
Lin
J
J
Krishnaraj
R
R
Kemp
R G
RG
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002470:Cell Survival / Q000187:drug effects; D005110:Extracellular Space / Q000378:metabolism; D006868:Hydrolysis; D007424:Intracellular Fluid / Q000378:metabolism; D007473:Ion Channels / Q000378:metabolism*; D008213:Lymphocyte Activation / Q000187:drug effects; D008274:Magnesium / Q000378:metabolism; D008297:Male; D051379:Mice; D008808:Mice, Inbred CBA; D011685:Purine Nucleotides / Q000494:pharmacology; D013601:T-Lymphocytes / Q000378:metabolism*
D007473:Ion Channels; D011685:Purine Nucleotides; D000255:Adenosine Triphosphate; D008274:Magnesium; D002118:Calcium