Increased platelet, but unaltered fibrinogen, accumulation in experimental thrombi in alloxan-induced diabetic rabbits

Does reduced membrane lipid fluidity underlie the altered thrombin-induced expression of integrin α(IIb)β(3) and PADGEM-140 in membranes of platelets from diabetic juveniles?

In diabetic patients, where the membrane lipid microviscosity of blood platelets is altered, the availability of platelet membrane receptors may change concomitantly. Platelet hypersensitivity in diabetic subjects was previously hypothesized to result from the nonenzymatic glycosylation-induced loss in platelet membrane fluidity. In our present study juvenile type 1 diabetic subjects were compared with their relevant controls with respect to thrombin-stimulated platelet activation in relation to glycation-induced impairments of platelet membrane dynamics. Our results indicate that: (a) the mean steady-state fluorescence polarization (p) of both 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-anilino-8-naphthalenesulphonate (ANS) in membranes from diabetic subjects were significantly greater than for control subjects, thus indicating reduced membrane lipid fluidity in diabetic platelets in various membrane regions; (b) the significantly higher [(3)H]NaBH(4) reduction, indicating the increased attachment of glucose to protein amino groups, was attributed to the proteins extracted from diabetic platelet membranes; (c) CD62-positive resting platelets were not significantly more abundant in diabetic patients; (d) basically, unaltered amounts of PADGEM-140 membrane antigen (CD62) copies were detected in resting diabetic platelets; (e) significantly higher numbers of membrane glycoprotein β(3) were found in diabetic platelets; (f) thrombin-induced elevations in the expression of CD61 (β(3)) and CD62 (PADGEM-140) occurred to much higher extent in platelets of diabetic patients, thus pointing to more profound activation of diabetic platelets by thrombin; (g) the total amounts of platelet membrane glycoprotein β(3) was significantly reduced in platelet lysates from diabetic subjects. We conclude that glycation-induced rigidization of platelet membranes might hypersensitize diabetic platelets to aggregating agents by rendering platelet membrane receptors more exposed to the external environment. Thus, thrombin may bind more efficiently to the exposed glycoprotein receptors (due to glycation) in diabetic platelets. Such excessive exposure and displacements toward the external environment might favour the accelerated shedding of some membrane proteins in diabetic platelets. We further suggest that their subsequent replacements would render platelet intrinsic storage pools exhausted and thus, might explain the diminished total amount of β(3) found in platelets of diabetic patients.

Platelet turnover in advanced diabetes

There is considerable information to indicate that when there is vessel injury, platelets can play a major role in the development of atherosclerosis and its thromboembolic complications in the non-diabetic population. Diabetes is a disease associated with an increased risk of vascular complications, but the nature of this association is unclear. Platelets from diabetic humans and animals are hypersensitive to agonists in vitro, but it is unclear if this hypersensitivity persists in vivo. Platelets survival and turnover in the circulation have been used as indicators of platelet behaviour in vivo, but platelet survival measurements do not necessarily correlate with platelet sensitivity in vitro and these measurements may be too insensitive to reflect platelet turnover on injured vessels or in thrombi, unless there is extensive vascular disease. Studies in rats and rabbits indicate that diabetes causes increased platelet interaction with experimentally-injured vessels and accumulation in thrombi. It is unclear, however, whether this altered behaviour of platelets at the injured vessel surface reflects changes in the platelets or in the vessel wall and whether it contributes to the increased risk for vascular complications in diabetic patients.