Differential effects of elevated calcium ion concentrations on inositol phospholipid responses in mouse and rat cerebral cortical slices

Abstract

Inositol phospholipid turnover in cerebral cortical slices from mouse and rat was assessed using a [ 3H]inositol pre-labelling technique followed by anion exchange chromatography to isolate [ 3H]inositol phosphates ([ 3H]InsP x ). In both mouse and rat cerebral cortical slices, elevating the CaCl 2 concentration of the Krebs medium from 1.3 to 4 mM did not significantly enhance the accumulation of [ 3H]InsP x in the absence of any stimulus, or in the presence of glutamate (3 mM), depolarizing concentrations of KCl (25 mM), 5-hydroxytryptamine (0.3 mM), the calcium ionophore A23187 (33 μM) or carbachol (1 mM). However, the accumulations of [ 3H]InsP x induced by histamine (1 mM) or noradrenaline (0.1 mM) were significantly increased by between 95 and 178% in cerebral cortical slices from both species by the elevation of extracellular calcium. Analysis of the individual inositol phosphates revealed that elevated ambient calcium enhanced the histamine-generated accumulations of [ 3H]InsP 2, [ 3H]InsP 3 and [ 3H]InsP 4 by up to two-fold, while only the [ 3H]InsP 3 response to carbachol was significantly increased. Under the same conditions, histamine, but not carbachol, selectively increased the accumulation of [ 3H]PtdInsP 2 by up to 50%. The [ 3H]InsP x responses to histamine and noradrenaline in combination with the calcium ionophore A23187 were greater-than-additive, inferring an enhancement of the receptor response by raised intracellular calcium. However, the combination of A23187 with glutamate or KCl resulted in significantly less-than-additive [ 3H]InsP x responses. The [ 3H]InsP x response to carbachol or 5-hydroxytryptamine was not significantly altered in the presence of A23187. Taken together, these results indicate heterogeneity between the mechanisms of inositol phospholipid turnover induced by these various stimuli in mammalian cerebral cortical slices.