Platelets from rats with diet-induced hypercholesterolemia are hypersensitive to thrombin through a pathway independent of released ADP or thromboxane A2 (TXA2) formation. We examined if platelets from rats with spontaneous hypercholesterolemia (HC) are similarly hypersensitive. HC rats (plasma cholesterol: 130±4 mg/dl, n=15) were compared with their normocholesterolemic genetic controls (NC) (87±4 mg/dl, p<0.001, n=16). Total cholesterol/109 platelets was not different between the groups (HC: 0.314±0.032 μmole, n=7; NC: 0.357±0.046 ymole, n=7). Washed platelets were prelabelled with 14c-serotonin. In the presence of aspirin (to inhibit TXA2 formation) and creatine phosphate/creatine phosphokinase (CP/CPK) (to remove released ADP), HC platelets aggregated more (22±2%, n=ll) than NC platelets (10±4%, n=12, p<0.01) in response to thrombin (0.065 U/ml); 14C release was not different. Thrombin causes inositol mono-, bis-, and trisphosphate (IP, IP2, IP3) formation from phosphoinositides (PI, PIP, PIP2 respectively) in rabbit platelets; IP3 may be involved in Ca++ mobilization and the release of granule contents. We examined if enhanced inositol phosphate formation was associated with hypersensitivity of HC platelets. Platelets were prelabelled with 3H-inositol and stimulated with thrombin (0.057 U/ml) for 30 sec in the presence of aspirin and CP/CPK. Li+ (20 mM) was used to prevent degradation of inositol phosphates to inositol. 3H-IP, IP2 and IP3 were isolated by ion-exchange chromatography. The increase in radioactivity (dpm/109 platelets) in IP2 and IP3 following thrombin stimulation was greater in HC platelets (IP2: 2210±160, n=4; IP3: 1430±180, n=4) than in NC platelets (IP2: 660±150, n=4, p<0.001; IP3: 490±100, n=4, p<0.01); IP was not different.
Thus platelets from spontaneously HC rats are hypersensitive to thrombin independently of released ADP or TXA2 formation. This hypersensitivity is associated with only moderate increases in plasma cholesterol and no detectable increase in total platelet cholesterol. Enhanced labelling of IP3 may indicate that enhanced activity of the pathways leading to IP3 formation is associated with this hypersensitivity.
