Phosphoinositide hydrolysis is correlated with agonist-induced calcium flux and contraction in the rabbit aorta

Abstract

In the present study changes in the extent of 32P labelling of membrane phospholipids were correlated with the alpha 1-adrenoceptor-induced events of increased 45Ca influx, 45Ca release and contraction in the rabbit aorta. Under basal conditions 32P incorporation into all phospholipids proceeded without saturation through 80 min of labelling. During a 5 min exposure to 10(-5) M norepinephrine (NE) after 25 min of prelabelling the incorporation of 32P into certain phospholipids was substantially increased. Phosphatidic acid (PA) labelling was increased above basal levels by 4.1 fold, phosphatidylinositol (PI) 2.5 fold and phosphatidylcholine (PC) 1.8 fold. Half maximal stimulation of 32P labelling of PA occurred at 2.0 microM, which was similar to the EC50 value for stimulation of 45Ca influx (2.5 microM) and 45Ca release (2.1 microM) but slightly higher than the value for contractile response (0.9 microM). Antagonist sensitivity studies reinforced the alpha 1 receptor subtype character of the rabbit aorta. Prazosin (10(-7) M) reduced agonist-induced events by 63-82% while yohimbine (10(-7) M) was without influence. Phenoxybenzamine (10(-8) M) reduced agonist-induced events by 56-76%. A temporal comparison showed that agonist stimulation of PA labelling was slower than 45Ca release, but similar to the time course of 45Ca influx. Hydrolysis of 32P-labelled phosphatidylinositol diphosphate (PIP2) was more rapid and paralleled 45Ca release. These findings suggest that PIP2 hydrolysis may account for the rapid phase of norepinephrine-induced contraction in rabbit aorta while PA or its immediate precursor diacylglycerol may account for receptor-induced Ca2+ influx.