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In order to clarify the mechanism of hypotensive activity of platelet activating factor (PAF), the effects of this drug on blood pressure in anaesthetized normotensive rats, on KCl- and noradrenaline-induced 45Ca uptake and contractile responses in rat aorta rings with and without endothelium were studied. PAF (3 micrograms kg-1, i.v.) showed long-lasting hypotensive effects in anaesthetized normotensive rats accompanied by a significant increase in heart rate. PAF (0.1-10 microM) did not relax the contractions induced by noradrenaline (10 microM) or K+ (60 mM) in rubbed or intact rat aorta. PAF did not affect the basal uptake of 45Ca2+ nor that induced by the two vasoconstrictor agents. In experiments in a calcium free medium, PAF (10 microM) had no effect on the noradrenaline- (10 microM) induced contractions. These results suggest that the hypotensive activity of PAF in normotensive anaesthetized rats is not due to a direct effect on rubbed and intact rat aorta rings (acting within the cell or blocking Ca2+ influx through L-type transmembrane calcium channels).