Effects of platelet-activating-factor on contractions and 45Ca2+ influx induced by noradrenaline and potassium in rat rubbed and intact aorta - comparison with its hypotensive effect in anesthetized normotensive rats

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)