INVOLVEMENT OF CALCIUM INFLUX IN MUSCARINIC CHOLINERGIC REGULATION OFPHOSPHOLIPASE-C IN CEREBELLAR GRANULE CELLS

Inositol phosphate accumulation on carbachol stimulation of rat cerebe llar granule cells shows a marked dependence on factors affecting cyto solic Ca2+ concentration ([Ca2+](c)). After 5 min, potassium depolaris ation caused a modest accumulation of inositol phosphates but augmente d the response to carbachol by a factor of 2-3. These effects of potas sium were dependent on an extracellular source of calcium and could be partially blocked by specific (nifedipine) and nonspecific (verapamil ) calcium channel blockers. Measurements of [Ca2+](c) under a range of stimulatory conditions demonstrated a close correlation between the e levation of [Ca2+](c) and agonist-stimulated phospholipase C (PLC) act ivity. The maximal potentiation of carbachol-stimulated inositol phosp hate accumulation was achieved using 20 mM KCl, which increased [Ca2+] (c) from similar to 20 to similar to 75 nM, indicating the involvement of relatively low threshold Ca2+ channels and the high sensitivity of the relevant PLC to small changes in [Ca2+](c). By contrast, increase s in [Ca2+](c) induced by the Ca2+ ionophore ionomycin were associated with more modest and less potent effects on agonist-stimulated PLC. T hese results demonstrate a cooperative interaction between a receptor/ G protein-regulated PLC and voltage-stimulated elevations of [Ca2+](c) , which may function to integrate ionotropic and metabotropic signalli ng mechanisms in cerebellar granule cells.