The interaction of the NK1 receptor antagonist CP‐96,345 with L‐type calcium channels and its functional consequences

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Abstract
1 We investigated the effects of the non-peptide NK1 receptor antagonist, CP-96,345, its inactive enantiomer CP-96,344, and the racemic mixture (±)-CP-96,345, on the binding of [3H]-nimodipine and [3H]-diltiazem to L-type calcium channels in rat cerebral cortex membranes. In isolated peripheral tissues containing tachykinin receptors, the effects of (±)-CP-96,345 have been compared with those of diltiazem. 2 In guinea-pig trachea, (±)-CP-96,345 produced antagonism of responses to the selective NK1 agonists [Sar9, Met(O2)11]SP and substance P-methyl ester that was apparently competitive in nature (pKB 7.0–7.5), while in guinea-pig ileum the antagonism was not surmountable. 3 The reduction of maximum responses by (±)-CP-96,345 in the guinea-pig ileum was not selective; it was obtained with muscarinic agonists and other agents, and was also observed in the portal vein of the rat where NK1 receptors are not present. 4 The tissue-specific reduction of maximum responses by (±)-CP-96,345 in ileum was reproduced by diltiazem. 5 (±)-CP-96,345 produced a concentration-dependent enhancement of [3H]-nimodipine binding to rat cerebral cortex membranes with a maximal stimulation of 186 ± 29% above control (EC50 83.2 nm). Scatchard analysis revealed that (±)-CP-96,345 increased the affinity of [3H]-nimodipine for its binding sites without affecting Bmax (control: KD = 0.32 nm; with 100 nm (±)-CP-96,345: KD = 0.074 nm). 6 CP-96,345, CP-96,344, and the racemate all inhibited [3H]-diltiazem binding in rat cerebral cortex membranes with Ki values of 22.5 nm, 34.5 nm and 29.9 nm respectively; a similar value was obtained for diltiazem itself (33.6 nm). In comparison, CP-96,345 and (±)-CP-96,345 inhibited the binding of [125I]-Bolton-Hunter-conjugated substance P in this tissue with Ki values of 59.6 nm and 82.0 nm respectively, while CP-96,344 had no measurable affinity (IC50 > 10 μm). 7 Substance P and a range of ligands selective for NK1, NK2, or NK3 receptors had no significant effect at 10 μm on either [3H]-diltiazem or [3H]-nimodipine binding. 8 The results indicate that in addition to possessing affinity for the NK1 receptor, the non-peptide antagonist, CP-96,345, displays high affinity for [3H]-diltiazem binding sites on L-type calcium channels. The functional effect that may be observed in integrated models will be a consequence of either property, or be a composite effect of NK1 receptor antagonism and L-channel blockade.