THE INTERACTION OF THE NK(1) RECEPTOR ANTAGONIST CP-96,345 WITH L-TYPE CALCIUM CHANNELS AND ITS FUNCTIONAL CONSEQUENCES

1 We investigated the effects of the non-peptide NK, receptor antagonist, CP-96,345, its inactive enantiomer CP-96,344, and the racemic mixture ( +/-)-CP-96,345, on the binding of [H-3]-nimodipine and [H-3]-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 NK, agonists [Sar9, Met(O2)11]SP and substance P-methyl ester that was apparently competitive in nature (pK(B) 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 NK, 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 [H-3]-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 [H-3]-nimodipine for its binding sites without affecting B(max) (control: K(D) = 0.32 nm; with 100 nm ( +/- )-CP-96,345: K(D) = 0.074 nm). 6 CP-96,345, CP-96,344, and the racemate all inhibited [H-3]-diltiazem binding in rat cerebral cortex membranes with K(i) 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 [I-125]-Bolton-Hunter-conjugated substance P in this tissue with K(i) values of 59.6 nm and 82.0 nm respectively, while CP-96,344 had no measurable affinity (IC50 > 10 muM). 7 Substance P and a range of ligands selective for NK1, NK2, or NK3 receptors had no significant effect at 10 muM on either [H-3]-diltiazem or [H-3]-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 [H-3]-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 rector antagonism and L-channel blockade.