Responses of human arterial muscle to stable vasoactive substances released from human platelets by collagen and heat aggregated IgG

Interaction between platelet-released serotonin and thromboxane A2 on human digital arteries

Synergism between the contractile effects of platelet-derived serotonin (5HT) and thromboxane A2 (TXA2) on a human blood vessel has been investigated by incubating strips of digital arteries in subcontractile concentrations of either 5HT or the TXA2-mimetic agent U46619. Either agonist U46619 or 5HT, in subcontractile concentrations, significantly potentiated the contractile response to the other. The 5HT antagonist ketanserin (10 mumol/l), the Ca2+ antagonist drugs verapamil (3 mumol/l), or nifedipine (10 nmol/l), or a Ca2+-free bathing medium, reduced the contractile responses to 5HT, but had no effect on the potentiation mediated by U46619. The interaction between TXA2 and 5HT derived from platelets was studied by measuring responses to platelets 1 min after aggregation (in the absence or the presence of ketanserin 10 mumol/l), and 20 min after aggregation. The results indicated that the response to platelets mediated by TXA2 and 5HT was greater than the sum of those mediated by TXA2 or 5HT separately. It is concluded that synergism between the contractile effects of 5HT and U46619 occurs in human blood vessels; that this is mediated by enhanced utilization of intracellular, rather than extracellular calcium; and that synergism can also occur when 5HT and TXA2 are released from stimulated human platelets.

Platelet releasing activity in sera of patients with rheumatoid vasculitis

Platelet releasing activity was found in sera fractionated by 30% ammonium sulphate in seven of 11 patients with systemic rheumatoid vasculitis, two of 16 patients with cutaneous rheumatoid vasculitis, and three of 18 patients with clinically uncomplicated rheumatoid arthritis. Serial studies in three patients suggest a key role of platelet activation by serum factors in the pathogenesis of systemic rheumatoid vasculitis. IgG rheumatoid factor or IgG containing immune complexes, or both, may be responsible for this platelet releasing activity.