B-thromboglobulin content of human blood platelets

Study of in vivo platelet activation in uncomplicated essential hypertension

Beta-thromboglobulin (BTG) and platelet factor 4 (PF4), platelet alpha-granule specific proteins, and serotonin (5-HT) which is stored in dense granules are released when platelets are activated. To investigate in vivo platelet activation in uncomplicated essential hypertension, platelet 5-HT and PF4 contents, plasma BTG and PF4 concentrations, as well as urinary BTG levels were assessed in normotensive and hypertensive subjects. Plasma BTG and PF4 concentrations and urinary BTG levels were comparable in both groups. Mean platelet 5-HT content was significantly decreased in hypertensive subjects without modification of the intraplatelet PF4 content. These data suggest first of all that the decrease in platelet 5-HT content is due mainly to the inhibition of platelet 5-HT uptake previously described, and second of all that no significant in vivo platelet activation occurs in essential hypertensive subjects devoid of cardiovascular complications.

Effect of storage on platelet release and aggregation responses

The effect of storage conditions on the aggregation and release responses of platelets have been investigated, using ADP as activating agent and beta-thromboglobulin (BTG) as a measure of the release reaction. Spontaneous release of BTG during storage was also investigated. Platelets held at room temperature maintained their ability to aggregate and release BTG in response to ADP after 3 days storage. After storage for 1 day at 4 degrees C platelets showed normal aggregation in response to ADP but did not release BTG. After 4-6 days, platelets stored at room temperature showed a smaller spontaneous loss of BTG than those stored at 4 degrees C. Spontaneous release of BTG from platelets in whole blood stored at 4 degrees C was significantly lower than from platelets stored as concentrates, although platelets prepared from whole blood stored for 1 day showed neither release nor aggregation in response to ADP stimulation. We suggest that ADP-stimulated BTG release may be a sensitive tests of platelet function during storage.

Effects of thrombin, acetyl salicylic acid and prostaglandins on spontaneous efflux of norepinephrine and beta-thromboglobulin from platelets in calcium/magnesium deficient environment

The effect of different substances on the spontaneous efflux of 14C-norepinephrine (14C-NE) and beta-thromboglobulin (beta TG) from platelets in Ca2+ - and Mg2+ - free buffer was tested. Addition of increasing concentrations of thrombin (0.015-0.625 U/ml) resulted in an immediate increase of 14C-NE detectable in the supernatants, but the slopes of the efflux curves from 10 min after thrombin addition were almost identical and parallel with the slope before thrombin addition. Also beta-thromboglobulin was steadily released after thrombin addition. Prostaglandin E1 and prostacyclin in concentrations that gave a 30-fold increase in cAMP accumulation resulted in a moderate decrease in efflux of 14C-NE from 10 to 30 min but did not influence the efflux thereafter up to 80 min. Acetyl salicylic acid in vivo and in vitro (200-500 mumol/l) did not influence the efflux of 14C-NE or beta TG. The conclusions are that factors known to exert considerable effects on aggregation, desaggregation and acute release produce only transitory effects in a Ca2+ - and Mg2+ - free environment and that the basal efflux of alpha-granule components seems to be a stable process.

Spontaneous efflux of platelet alpha-granule components in relation to 14C-norepinephrine in buffers

In an investigation of the spontaneous efflux of beta-thromboglobulin (beta-TG), 14C-norepinephrine (14C-NE) and growth stimulating activity (GSA), measured as incorporation of 3H-thymidine in rabbit aortic smooth cells, from platelets resuspended in Ca++- and Mg++-free buffer a continuous increase in 3H-thymidine incorporation was seen during 120 min. There was good stoichiometric parallelism between beta-TG and 14C-NE during the 80 minutes the experiment lasted, indicating that norepinephrine is incorporated in alpha-granules and release by exocytosis.