Effects of calcium ionophore A23187 and calcium antagonists on³²P_i incorporation into polyphosphoinositides of rat cortical synaptosomes

Abstract

• The role of Ca²⁺ on³²P_i incorporation into polyphosphoinositides (PPI) of rat cortical synaptosomes was studied.

• Stimulation of muscarinic receptor by carbachol (1 mM) resulted in a decrease in³²P_i incorporation into phosphatidylinositol-4,5-bisphophaphate (TPI) and phosphatidylinositol-4-phosphate (DPI), and an increase in³²P_i incorporation into phosphatidylinositol (PI) and phosphatidic acid (PA), whereas no significant effect on other membrane phospholipids was found. This response could be blocked by atropine (1 μM).

• The stimulatory effect of carbachol required Ca²⁺ in the medium; the presence of 0.5 mM EGTA blocked the effect of carbachol on PPI turnover completely.

• Calcium ionophore A23187, at 1 μM, had a similar effect on PPI turnover by carbachol (1 mM). At higher concentrations (10–100 μM) of A23187, the PPI turnover rate was much enhanced.

• Depolarization of the membrane by high potassium (60 mM) in the presence of calcium resulted in an enhanced PPI turnover, which was similar to the results of the carbachol (1 mM) effect but to a lesser extent.

• Calcium antagonists, diltiazem and trifluoperazine, at 10 μM could block the carbachol effect on³²P_i incorporation into PPI in this preparation.

• Our results suggest that the enhancement of PPI turnover in rat cortical synaptosomes by carbachol, calcium ionophore or high potassium requires Ca²⁺, and it can be blocked by compounds which interfere with the availability of this ion, such as EGTA or calcium antagonists.