Involvement of alpha noradrenergic receptors in mediation of brain polyphosphoinositides metabolism in vivo.

The acute effects of norepinephrine (NE) on polyphosphoinositide (PPI) metabolism were examined in regions of rat brain in vivo with the techniques of intracisternal injection and microwave irradiation fixation. NE (80 nmol/g of brain) increased PPI incorporation of both [32P] orthophosphate and [3H] inositol by 50 to 100% within 5 min of the simultaneous intracisternal administration of both drug and radiolabel. These effects of NE were more pronounced in the brainstem and cerebellum than in the cerebral cortex, and were greater for triphosphoinositide than for diphosphoinositide. The increases were blocked by phentolamine and phenoxybenzamine but not by propranolol or atropine, suggesting that alpha noradrenergic receptors mediated the effect. Similar metabolic responses were observed with PPI precursors, phosphatidic acid and monophosphoinositide. However, no other phospholipid was altered in response to the agents administered in these studies (data not shown). Increased incorporation of radiolabels into PPI was never accompanied by detectable increases in endogenous PPI concentrations, suggesting that head group turnover was stimulated rather than whole molecule synthesis. Also, the increase in PPI phosphate radiolabel incorporation was not due to NE effects on the specific activity of phosphate donor precursors (nucleotides). The present data suggest that the PPI may be involved in the membrane processes that result from stimulation of alpha noradrenergic receptors in brain.