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  • Title: Inhibition of phosphoinositide turnover by selective muscarinic antagonists in the rat striatum. Correlation with receptor occupancy.
    Author: Monsma FJ, Abood LG, Hoss W.
    Journal: Biochem Pharmacol; 1988 Jun 15; 37(12):2437-43. PubMed ID: 2839194.
    Abstract:
    In the rat corpus striatum, receptor occupancy and the inhibition of phosphoinositide turnover by muscarinic antagonists have been examined under very similar conditions with respect to tissue preparation and buffer composition. The results suggest a good correlation between receptor occupancy and inhibition by muscarinic antagonists, of the carbachol-stimulated turnover of inositol phospholipids, measured by the accumulation of [3H]inositol phosphates in the presence of 5 mM LiCl. In the presence of 10 mM carbachol (CCh), the accumulation of labeled inositol phosphates was increased 8-fold above basal levels (EC50 = 95 microM). Inclusion of antagonists resulted in a dose-dependent inhibition of the 0.1 mM CCh-stimulated inositol phosphate accumulation, with a rank order of potency of atropine greater than trihexyphenidyl greater than pirenzepine greater than or equal to gallamine. Radioligand binding studies with [3H]-l-quinuclidinyl benzilate [( 3H]QNB) in a cell aggregate preparation revealed a single class of saturable, high affinity [3H]QNB binding sites exhibiting a Kd of 74 pM and a Bmax of 2.85 pmol/mg protein. The antagonists examined were able to inhibit the binding of [3H]QNB with the same rank order of potency as for the inhibition of carbachol-stimulated phosphoinositide turnover (atropine greater than trihexyphenidyl greater than pirenzepine greater than or equal to gallamine). Although the inhibition of phosphoinositide turnover and [3H]QNB binding by the nonselective antagonist atropine was best described by interaction at a single site, inhibition of phosphoinositide turnover and [3H]QNB binding by both pirenzepine, which is selective for M1 receptors, and gallamine, which is selective for M2 receptors, is complex. Pirenzepine was much more potent than gallamine for both binding to receptors and inhibiting phosphoinositide turnover. Nonlinear curve-fitting analysis indicated that slope factors for inhibition of phosphoinositide turnover (analogous to Hill coefficient for binding) by only subtype selective antagonists were significantly less than unity. The above-mentioned antagonist interactions together with the apparently multicomponent stimulation of phosphoinositide turnover by carbachol suggest that phosphoinositide turnover may be coupled to more than one muscarinic receptor subtype in the corpus striatum.
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