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The plant lectin, concanavalin A (Con-A), and the ionophore, A-23187 (specific for divalent cations), stimulated glucose transport in rat thymocytes. Con-A stimulation developed more slowly and was somewhat less extensive than that of A-23187. Both responses showed saturation dose dependencies. The two responses were poorly additive, suggesting that A-23187 may saturate regulatory processes shared by the two stimulatory mechanisms.Doses of methylisobutylxanthine (MIX) and prostaglandin Ez which raised adenosine 3' :5'-monophosphate (cAMP) levels in these cells also antagonized the Con-A stimulation of glucose transport but did not inhibit basal glucose transport or the A-23187 stimulation. Dibutyryl-cAMP and 8-bromo-cAMP also antagonized Con-A stimulation without inhibiting basal glucose transport. MIX antagonized high Con-A doses about as strongly as it did low Con-A doses, suggesting that MIX did not compete in the Con-A binding step or other process saturable by Con-A. [3H]Con-A binding was not affected by MIX.The stimulatory effects of Con-A and A-23187 were reduced by reduction of Ca ++ in the medium. Both Con-A and A-23187 enhanced 45Ca § influx and cellular Ca § content. The A-23187 dose, which was saturating for glucose transport stimulation, enhanced Ca ++ influx and cellular Ca ++ content more than did the Con-A dose which was saturating for glucose transport stimulation. The dose of MIX which specifically antagonized Con-A stimulation of glucose transport proved also to reduce Ca ++ influx and cellular Ca ++ in the presence of Con-A but not in the presence of A-23187. Thus, glucose transport correlates rather well with cellular Ca ++ .These results are compatible with the view that Ca ++ in a cellular compartment can promote glucose transport, that Con-A's enhancement of Ca ++ entry contributes to its stimulation of glucose transport, and that MIX antagonizes Con-A action at least partly by reducing Ca ++ entry. The action of MIX is apparently mediated by cAMP. 456

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