Quisqualate and carbachol-induced increases in intrasynaptosomal free calcium are mediated by different products of phospholipid hydrolysis

The mechanisms by which quisqualate and carbachol increase intrasynaptosomal free calcium ([Ca 2+] 1) were studied in rat cortical synaptosomes. Quisqualate (0.01–100 μM) and carbachol (100–1000μM) increased [Ca 2+] 1 in Fura-2 acetoxymethyl ester (Fura-2 AM)-loaded synaptosomes. The resting level of [Ca 2+] i was 118 nM. The maximum increase (55%) was produced by 10 μM quisqualate which had an EC 50 of 0.2 μM. The maximum increase (28%) elicited by carbachol occurred at 1000 μM and the EC 50 was 30 μM. The stimulatory effects of quisqualate on [Ca 2+] i were blocked by heparin (100 I.U.) but not by staurosporine (1 μM), nifedipine (1 μM) or ω-conotoxin fraction GVIA (ω-CgTx) (0.5 μM). On the other hand, the effects of carbachol on [Ca 2+], were abolished by staurosporine, nifedipine or ω-CgTx but not by heparin. Carbachol (100 μM) also significantly increased 45Ca accumulation into either resting or K + (30 mM)-depolarised synaptosomes and these effects were inhibited by staurosporine and nifedipine. Quisqualate (10 μM) had no effect on 45Ca accumulation under resting or depolarised conditions. When quisqualate and carbachol were used in combination, there were apparently additive effects on [Ca 2+] 1 but not on 45Ca accumulation. It is concluded that carbachol increases [Ca 2+] i by facilitating Ca 2+ entry through L-type Ca 2+ channels via a 1.2-diacylglycerol (DAG)-protein kinase C (PKC)-dependent pathway while quisqualate mobilises Ca 2+ from inositol 1,4,5-trisphosphate (IP 3)-sensitive stores.