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Involvement of Calcium Influx in Muscarinic Cholinergic Regulation of Phospholipase C in Cerebellar Granule Cells

Abstract: Inositol phosphate accumulation on carbachol stimulation of rat cerebellar granule cells shows a marked dependence on factors affecting cytosolic Ca2+ concentration ([Ca2+]c). After 5 min, potassium depolarisation caused a modest accumulation of inositol phosphates but augmented the response to carbachol by a factor of 2–3. These effects of potassium 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 elevation of [Ca2+]c and agonist‐stimulated phospholipase C (PLC) activity. The maximal potentiation of carbachol‐stimulated inositol phosphate accumulation was achieved using 20 mM KCl, which increased [Ca2+]c from ∼20 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, increases in [Ca2+]c induced by the Ca2+ ionophore ionomycin were associated with more modest and less potent effects on agonist‐stimulated PLC. These 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 signalling mechanisms in cerebellar granule cells.