The Effects of Platelet-Activating Factor and Platelet-Derived Compounds on Bovine Luteal Cell Progesterone Production1

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Abstract
This study was conducted to characterize bovine platelets with respect to serotonin (5-HT) concentration and platelet-activating factor (PAF)-activation and to examine the in vitro effects of PAF and platelet-derived compounds on bovine luteal progesterone (P4) production. The concentration of 5-HT in platelets, as determined by high-performance liquid chromatography, was 538.8 .+-. 40.83 ng/l .times. 108 platelets. Based on a circulating platelet concentration range of 2.3 .times. 108-5.8 .times. 108 platelets/ml, the circulating of 5-HT would be approximately 1239-3125 ng/ml of blood. Bovine platelets were found to aggregate in response to PAF (1-40 ng/0.5 ml), with maximal aggregation occurring at 20-40 ng/0.5 ml. Coincubation of luteal cells with platelets (1 .times. 107-4 .times. 108) enhanced luteal P4 production (p < 0.05). Addition of the 5-HT receptor antagonist mianserin blocked the platelet-induced increases in P4 (p < 0.05). Preincubation of platelets with indomethacin did not alter the production of P4 (p > 0.05), nor did the addition of propranolol (p < 0.05). Platelet-derived growth factor at 8 and 16 ng/ml enhanced basal P4 production (p < 0.05) but had no effect on the responsiveness of luteal cells to luteinizing hormone (LH) (p > 0.05). Low concentrations of PAF (0.001-1 .mu.g/ml) did not alter basal or LH-stimulated P4 (p > 0.05), whereas PAF at 10 .mu.g/ml decreased both basal and LH-stimulated P4 (p < 0.05). These results support a role for platelets and platelet-derived compounds in the enhancement of bovine luteal function, and together with other data lend support to a role for platelets and platelet-derived compounds in early maternal recognition of pregnancy.