1. Endogenous noradrenaline release from washed platelets incubated under resting conditions and in the presence of thrombin was examined in 14 normal subjects and 10 subjects with type 1 (insulin-dependent) diabetes.

2. Irreversible aggregation of platelets from both normal and diabetic subjects was induced by thrombin (0.3 unit/ml). Platelets from diabetic subjects were more sensitive than platelets from normal subjects, extents of aggregation being 89% and 76%, respectively (P < 0.002).

3. Stimulation with thrombin (0.3 unit/ml) elicited marked platelet release of noradrenaline to the incubation medium in both normal and diabetic subjects. Supernatant noradrenaline concentrations obtained under thrombin-stimulated conditions did not significantly differ between normal and diabetic subjects. However, under resting conditions noradrenaline levels were significantly greater (+ 93%, P < 0.02) for diabetic than normal subjects.

4. Measurement of platelet noradrenaline contents after thrombin stimulation revealed no difference between normal and diabetic subjects. Under resting conditions, however, platelet noradrenaline levels were significantly lower (−46%, P < 0.02) for diabetic than normal subjects. Thus, in the diabetic subjects increased resting platelet efflux of noradrenaline is mirrored by a decreased platelet noradrenaline content.

5. A consequence of increases in resting catecholamine efflux may be enhanced platelet activity resulting in increased platelet aggregation.

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