A rapid attenuation of muscarinic agonist stimulated phosphoinositide hydrolysis precedes receptor sequestration in human SH-SY-5Y neuroblastoma cells.

Agonist occupancy of muscarinic cholinergic receptors in human SH-SY-5Y neuroblastoma cells elicited two kinetically distinct phases of phosphoinositide hydrolysis when monitored by either an increased mass of inositol 1,4,5-trisphosphate, or the accumulation of a total inositol phosphate fraction. Within 5s of the addition of the muscarinic agonist, oxotremorine-M, the phosphoinositide pool was hydrolyzed at a maximal rate of 9.5%/min. This initial phase of phosphoinositide hydrolysis was short-lived (t1/2 = 14s) and after 60s of agonist exposure, the rate of inositol lipid breakdown had declined to a steady state level of 3.4%/min which was then maintained for at least 5-10 min. This rapid, but partial, attenuation of muscarinic receptor stimulated phosphoinositide hydrolysis occurred prior to the agonist-induced internalization of muscarinic receptors.