Bruce Lages

Studies of the Release from Human Platelets of the Growth Factor for Cultured Human Arterial Smooth Muscle Cells

Platelets contain a growth-promoting factor for arterial smooth muscle cells (SMC) that may play a major role in atberogenesis. We have studied some of the effects of the platelet-derived growth factor (PDGF) on human arterial SMC in culture and the release of PDGF from human platelets in relationship to other released substances. Material released from platelets was highly potent in stimulating human SMC to proliferate. A substantial portion of the growth-promoting activity of human serum could be attributed to a factor(s) released from platelets. Similar dose-response patterns to PDGF were observed with human SMC and with mouse 3T3 cells. The time-course of release of PDGF and its concentration dependence on human thrombin were determined in comparison with serotonin, ADP, ATP, an acid bydrolase, platelet factor 4 (PF4), and β-thromboglobulln (βJTG). PDGF activity was assayed by stimulation of the incorporation of +-H-thymidine into DNA of 3T3 cells; PF4 and βJTG were measured by newly developed radioimmunoassays. PDGF, PF4, and βJTG were released from platelets by lower concentrations of thrombin than those required for release of the other components. The results suggest that PDGF, PF4, and βTG are localized in the platelet in granules different from either the dense bodies (that contain serotonin, ADP, ATP) or the acid hydrolase-containing granules, possibly in α-granules. The contents of these PDGF- containlng a-granules are actively released during the release reaction and are particularly sensitive to release by low doses of thrombin.

Isolated deficiency of platelet procoagulant activity

This is a study of a 34 year old woman with a moderate to severe bleeding disorder in whom impaired platelet procoagulant activity (PPA) was found by several methods, including tests of factor 3 availability (PF-3a), prothrombin consumption and contact activation. No deficiencies of platelet adhesion, aggregation, secretion, metabolism or granule-bound substances were detectable. Under adequate platelet coverage, this woman underwent two surgical procedures without difficulty. These findings demonstrate the role of PPA in hemostasis and indicate that a defect in PPA can be an isolated occurrence. The abnormalities in PF-3a found in this patient could be due to the diminished number of factor V binding sites, resulting in impaired factor Xa binding, found in separate studies by Majerus et al.

Acquired storage pool deficiency with increased platelet-associated IgG: Report of five cases

Acquired abnormalities of platelet aggregation have been reported with increasing frequency. We studied five patients (including two with systemic lupus erythematosus and one with compensated chronic idiopathic thrombocytopenic purpura) in whom platelet aggregation responses to collagen, epinephrine and ADP are impaired; in all cases, we found that levels of platelet-associated immunoglobulin G (IgG) were increased. In all five patients substances stored in platelet-dense granules (ATP, ADP, serotonin and calcium) were diminished. The content of the alpha-granule substance, beta-thromboglobulin, was also decreased in most cases, whereas the levels of two secretable acid hydrolase enzymes (beta-glucuronidase and beta-N-acetyl glucosaminidase) were within normal limits. These findings are similar to those observed in subtypes of congenital storage pool deficiency. However, in contrast to the congenital disorder, a membrane-bound (nonsecretable) acid phosphatase was also decreased in the patients with acquired storage pool deficiency. These findings suggest that impaired platelet aggregation on an acquired basis may, in some patients, be due to immune platelet damage resulting in a distinctive type of platelet storage pool deficiency.

Inability of divalent cation complexes of A23187 to induce platelet secretion

Extracellular calcium produces a dose-dependent inhibition of the early time course of A23187-induced 14C-5HT secretion in human platelets which is independent of the occurrence of aggregation. Pre-incubation of A23187 with calcium to form the A23187-Ca complex results in markedly prolonged lag periods prior to the onset of secretion at Ca concentrations of 0.25 - 1.0 mM and total abolition of the secretion response at 2.0 mM Ca, indicating that these effects of extracellular Ca are due to specific interactions of Ca and A23187. Under conditions which favor dissociation, the complexes of A23187 with Sr, Ca, and Mg, but not Mn, all induce platelet secretion but produce an inhibition of the early secretion response which generally parallels their relative complex stabilities, i.e., Sr less than Ca approximately equal to Mg less than Mn. Under conditions which prevent dissociation, none of these complexes induce secretion. These results indicate that the complexes of A23187 with divalent cations, including Ca, are unable to induce platelet secretion, most likely because of an inability to penetrate the platelet plasma membrane.