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pubmed:abstractText |
Although activation of muscarinic cholinergic receptors on 1321N1 human astrocytoma cells results in a linear accumulation of inositol phosphates for up to 60 min in the presence of LiCl [Masters, Quinn & Brown (1985) Mol. Pharmacol. 27, 325-332], activation of H1-histamine receptors resulted in an increase in total inositol phosphate formation that was maintained for less than 5 min. The effects of stimulation of these two receptors on accumulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] were also examined. Incubation of 1321N1 cells with carbachol resulted in a rapid accumulation of all three inositol phosphates, reaching a maximum within 30 s; this elevated value was maintained for up to 60 min. The rate of disappearance of Ins(1,3,4)P3 from carbachol-treated cells after the addition of atropine paralleled or exceeded the rate of disappearance of Ins(1,4,5)P3. Although the initial rates of accumulation of Ins(1,4,5)P3, Ins(1,3,4)P3 and Ins(1,3,4,5)P4 in the presence of histamine were similar to that observed with carbachol, the amounts of these inositol phosphates had returned to control values within 5 min after the addition of histamine. The results indicate that, although the acute effects of muscarinic receptor and H1-histamine receptor stimulation on phosphoinositide hydrolysis are very similar, the histamine receptor is desensitized rapidly, whereas the muscarinic receptor is not. This effect on histamine-receptor function is apparently homologous, since preincubation of 1321N1 cells with histamine did not decrease the subsequent response to carbachol.
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