Effects of rabbit platelets on vascular smooth muscle cell DNA synthesis

During platelet aggregation a factor is released which stimulates DNA synthesis in vascular smooth muscle cells. We have examined the capacity of plasma free suspensions of intact rabbit platelets to stimulate DNA synthesis in vascular smooth muscle cells in tissue culture. Platelet-rich plasma was obtained by low-speed centrifugation of citrated blood and passed through sterile columns of sepharose 2B beads. The resultant plasma-free platelet suspension was adjusted to 400,000 platelets/mm3. An aliquot was completely aggregated with thrombin, centrifuged and the supernatant examined for ability to stimulate incorporation of [3H]thyrmidine into DNA by cultured vascular smooth muscle cells. Sonication of an aliquot of platelet suspension released an equivalent amount of this stimulating factor. Another aliquot of platelet suspension of identical volume was added directly to the test smooth muscle cells. Stimulation by intact platelets co-cultured with smooth muscle cells was more than twice as great as that of the thrombin-aggregated supernatant. The latter was not increased by increasing the thrombin concentration nor by increasing the time of exposure. The potency of intact platelets was decreased only slightly by segregating them in Millipore chambers indicating the direct contact was not responsible for its enhancement.