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Platelets via their secretions communicate and regulate endothelial cells (EC), which function as first level sensors in a biofeedback mgchanism to maintain blood and tissue fluid homeostasis. Platelets at a concentration of 105/mm3, serotonin (10-5M), thrombin (1 unit/ml), ADP (10-6M), catecholamines (10-6M) and histamine (10-6M) stimulate EC proliferation in culture from 150-1000 percent of controls. That agonists so diverse in form have a similar effect suggests the mobilization of a second messenger, such as calcium, as the common denominator. 45Calcium influx in cultured EC increased 5-24 times that of controls following incubation with platelets, platelet secretions, and substances known to circulate in blood following vascular injury. Increased adenine uptake was also observed in response to these agonists, which supports the premise of a regulating role for platelet secretions on EC.Lastly, the increased calcium flux was correlated with the induction of ornithine decarboxylase (ODC) activity. ODC is the rate limiting enzyme in the synthesis of polyamines that are theorized to regulate many cellular functions. ODC induction was 15 fold above baseline following stimulation with complete media, peaking at 3 hr. Preincubation with lanthanum chloride (LaCl3) inhibited this stimulation; the inhibition was reversed by the removal of the LaCl3. A similar response was obtained with 2 mM ethylene glycolbis. N,N'-tetraacetic acid in the media. These data support the validity of using EC in culture to simulate in vivo conditions, and the measurement of calcium flux, adenine uptake, and ODC activity, or combinations of the three, are significant parameters when cultured EC are used as an assay system.

Stimulation of Cultured Endothelial Cell Proliferation and Metabolism by Platelets and Vasoactive Agonists