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Characterization of the effects of lithium on phosphatidylinositol (PI) cycle activity in human muscarinic ml receptor‐transfected CHO cells
Author(s) -
Atack John R.,
Prior Angela M.,
Griffith Douglas,
Ragan C. Ian
Publication year - 1993
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1993.tb13884.x
Subject(s) - carbachol , inositol , muscarinic acetylcholine receptor , chinese hamster ovary cell , phosphatidylinositol , endocrinology , inositol phosphate , medicine , chemistry , lithium (medication) , receptor , inositol trisphosphate , biology , biochemistry , signal transduction
1 The effects of lithium on [ 3 H]‐inositol and [ 3 H]‐cytidine incorporation into [ 3 H]‐inositol monophosphates ([ 3 H]‐InsP 1 ) and [ 3 H]‐cytidine monophosphorylphosphatidate ([ 3 H]‐CMP‐PA), respectively, and inositol 1,4,5‐trisphosphate (InsP 3 ) and inositol 1,3,4,5‐tetrakisphosphate (InsP 4 ) mass were studied in carbachol‐stimulated human ml muscarinic receptor‐transfected Chinese hamster ovary cells (ml CHO cells). 2 Lithium alone (10 m m ) had no appreciable effects on any of the four parameters measured; it was only in carbachol‐stimulated cells that the effects of lithium became apparent. 3 In the presence of carbachol (1 m m ), lithium (10 m m ) caused a relatively rapid (within 5 min) accumulation of [ 3 H]‐InsP 1 and [ 3 H]‐CMP‐PA which continued up to about 20–30 min, after which accumulation slowed down. On the other hand, the elevation in InsP 3 and InsP 4 levels produced by carbachol was not altered by lithium in the short‐term and only at later times (> 20–30 min) was the response attenuated, with InsP 3 and InsP 4 levels approaching basal. 4 The effects of lithium on carbachol‐stimulated [ 3 H]‐InsP 1 and [ 3 H]‐CMP‐PA accumulation and the attenuation of the carbachol‐induced elevation of InsP 3 and InsP 4 were all dose‐dependent, with EC 50 s in the region of 1 m m . 5 The lithium‐induced effects on [ 3 H]‐CMP‐PA and InsP 3 and InsP 4 in carbachol‐stimulated cells could be reversed, in a dose‐dependent manner, by preincubation with exogenous myo ‐inositol (EC 50 = 2–3 m m ) but not by the inactive analogue scyllo ‐inositol, indicating that these effects occur as a consequence of depletion of inositol. 6 The temporal effects of lithium are consistent with lithium inhibiting inositol monophosphatase, causing accumulation of InsP 1 , resulting in lower free inositol levels. This leads to accumulation of CMP‐PA and reduced PI synthesis which, once agonist‐linked membrane inositol phospholipids are depleted, produces attenuated InsP 3 and InsP 4 responses. 7 These results in ml CHO cells support the hypothesis that lithium affects the PI cycle cell signalling pathway by depletion of inositol due to inhibition of inositol monophosphatase.