Effects of norepinephrine and 5-hydroxytryptamine on phosphoinositide-PO4 turnover in rabbit cornea

We have investigated: (a) Phospholipid composition and phosphoinositide-PO4 turnover in rabbit cornea tissues; and (b) the effects of adrenergic and serotonergic agonists on breakdown of phosphoinositides in the rabbit cornea. The data obtained from these studies can be summarized as follows: (1) in the cornea phosphatidylcholine and phosphatidylethanolamine constitute about 55%, phosphatidylinositol (PI) 10%, and phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) comprise about 1% each of the total phospholipids; (2) incubation of cornea in 32Pi-containing medium resulted in incorporation of radioactivity in tissue phospholipids. The radioactivity was highest in PIP2 (39%), followed by PI (19%), PIP (16%) and PA (5% of the total radioactivity). When compared with stroma and endothelium, the cornea epithelium was most active in phosphoinositide metabolism; (3) addition of norepinephrine (NE) or 5-hydroxytryptamine (5-HT), 200 microM each, to 32P-labeled cornea resulted in a loss of radioactivity in PIP and PIP2 by about 12- and 20%, respectively. Concomitantly, the radioactivity in PA and PI was increased by 44- and 66%, respectively. The effects of the neurotransmitters were time- and concentration-dependent. When added to the cornea labeled with myo [3H] inositol, NE and 5-HT increased the production of labeled myo-inositol phosphates; (4) prazosin (20 microM), but not yohimbine or propranolol, blocked the effects of NE. Similarly, the effects of 5-HT were antagonized by methysergide (20 microM) and ketanserin (10 microM) but not by prazosin. These data demonstrate that NE and 5-HT stimulate phospholipase C-mediated hydrolysis of PIP2 into diacylglycerol (DG) and myo-inositol trisphosphate (IP3). Furthermore, the effects of NE and 5-HT are mediated by alpha 1-adrenergic and 5-HT2 receptors, respectively. It is suggested that IP3, by releasing Ca2+ from ER, and DG, by activating protein kinase C, may function as second-messenger molecules which may participate in agonist-induced functional responses, including chloride transport, in the cornea epithelium.