Mouse mesenteric lymph node cells were incubated with concanavalin A (Con A) with or without the rat form of calcitonin gene-related peptide (rCGRP) (0.1 fM-1 microM) +/- human (h)CGRP8-37 (1 microM) for 48 h. DNA synthesis was assessed by [3H]thymidine incorporation. Con A-stimulated DNA synthesis was suppressed by 13, 20, and 30% at 10 fM, 1 pM, and 100 pM of rCGRP, respectively. hCGRP8-37 (1 microM), a selective blocker of CGRP1 receptor, completely inhibited the suppression of DNA synthesis by rCGRP (10 fM-100 pM). rCGRP caused concentration-dependent elevations of cAMP levels, which were potentiated by pretreatment with 3-isobutyl-1-methylxanthine (0.3 mM, 10 min), an inhibitor of cAMP-phosphodiesterase. hCGRP8-37 (1 microM) significantly inhibited cAMP elevations induced by rCGRP at the lower concentrations, but not at the highest concentrations of rCGRP. These data suggest that rCGRP, at circulating levels (1-10 pM), appears to directly interact with receptors on mouse mesenteric lymph node cells that are coupled to cAMP generation, ultimately inhibiting lymphocyte proliferation. To test the involvement of CGRP in suppression of lymphocyte proliferation by serum from endotoxin-treated rats, mouse mesenteric lymph node cells were stimulated with Con A with or without dilutions of endotoxin treated rat serum. At a 1:20 dilution, DNA synthesis was suppressed 30%, at a 1:40 dilution, DNA synthesis was suppressed by 34%, and at a 1:80 dilution, DNA synthesis was suppressed 25%. At all serum dilutions, coincubation with hCGRP8-37 (1 microM) significantly inhibited the suppressive effect of the endotoxin treated rat serum. These data suggest that the immunosuppression observed during endotoxin shock may be due, at least in part, to CGRP in serum.