Changes in extracellular calcium within the physiological range influence receptor-mediated inositol phosphate responses in brain and tracheal smooth muscle slices.
The effect of changes in extracellular calcium concentration ([Ca2+]e) on the incorporation of myo-[2-3H]-inositol into phosphoinositides and agonist-stimulated 3H-inositol phosphates (3H-InsPs) was examined in rat cerebral cortex and bovine tracheal smooth muscle slices. In brain slices, reduction in [Ca2+]e from 2.4 to 1.2 mmol/l resulted in an approximate doubling of the carbachol and noradrenaline-stimulated 3H-InsP response with no effect on the EC50 values. An identical effect of varying [Ca2+]e was observed for carbachol-stimulated 3H-InsP formation in tracheal smooth muscle with a further increase in 3H-InsPs evident at [Ca2+]e 0.6 mmol/l. In this tissue the effect of changes in [Ca2+]e on the incorporation of myo-[2-3H]-inositol into the total phosphoinositide pool directly paralleled the changes in 3H-InsPs except in conditions of no added calcium when 3H-InsP responses were markedly impaired. Additional studies in brain slices using buffer where the added calcium varied between 0 and 2.4 mmol/l, showed that both the carbachol stimulated formation of separate inositol phosphates during short incubation periods and incorporation of myo-[2-3H]-inositol into PtdInsP and PtdInsP2 under basal conditions was maximal at [Ca2+]e 0.3 mmol/l. Omitting Ca2+ from the buffer resulted in maximal labelling of PtdIns but a decrease in PtdInsP and PtdInsP2 labelling (compared with the level at [Ca2+]e 0.3 mmol/l) and a markedly impaired inositol polyphosphate response. Alterations in [Ca2+]e following 3H-inositol labelling but immediately prior to carbachol stimulation did not influence 3H-inositol polyphosphate responses. It is therefore clear that even relatively small changes in [Ca2+]e markedly influence agonist-stimulated 3H-InsP responses in brain and tracheal smooth muscle slices and that these reflect changes in the labelling of substrate inositol lipids.(ABSTRACT TRUNCATED AT 250 WORDS)
Baird
J G
JG
Department of Pharmacology, University of Leicester, United Kingdom.
Chilvers
E R
ER
Kennedy
E D
ED
Nahorski
S R
SR
D000818:Animals; D001923:Brain Chemistry / Q000187:drug effects*; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics; D002217:Carbachol / Q000494:pharmacology; D002417:Cattle; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000378:metabolism*; D008297:Male; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology; D051381:Rats; D013403:Sugar Phosphates / Q000378:metabolism*; D014132:Trachea / Q000187:drug effects
D007295:Inositol Phosphates; D013403:Sugar Phosphates; D002217:Carbachol; D002118:Calcium; D009638:Norepinephrine