Evidence suggests that magnesium (Mg) deficiency may play a key role in cardiovascular disease. In particular, Mg deficiency may lead to a potentiation of platelet aggregation. However, the factor(s) regulating intracellular-free Mg concentration ([Mg2+]i) in platelets is not known. We studied the effects of insulin on the changes of [Mg2+]i in human platelets. Preincubation of hirudinized platelet-rich plasma with insulin had a dose- and time-dependent effect on the increase of [Mg2+]i measured in Mag-fura-2-loaded cells with a fluorescence spectrophotometer. The maximal effect was achieved by incubation with 200 microU/mL insulin for 30 min. [Mg2+]i increased from the basal value of 266 +/- 23 mumol to 355 +/- 46 (SD, P < 0.001). In the presence of an antiinsulin receptor monoclonal antibody the effect of insulin was abolished suggesting that the Mg transport mechanism was an insulin-receptor mediated process. Furthermore, the insulin-stimulated Mg transport was inhibited by the addition of chelating agent ethylenediaminete-traacetate while the receptor binding was not altered. These findings suggest that insulin can translocate Mg from the extracellular space. Insulin alone had no effect on the changes of intracellular calcium (Ca) concentration using Ca-Fura-2 as a probe. In addition, glucose (5 mg/mL) was not effective in altering either the Mg or Ca concentration. Insulin (100 microU/mL) decreased thrombin-induced platelet aggregation (washed platelets resuspended in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-Tyrode buffer). Similarly, the production of the proaggregatory eicosanoids thromboxane B2 was inhibited by insulin from 16 +/- 1 ng/10(8) platelets to 13 +/- 2 (P < 0.05). The results suggest that insulin, through the interactions with its receptors may be a key factor regulating, Mg transport in human platelets.