Substance P-Induced Reduction in the Initial Accumulation of Cytosolic myo-[3H]Inositol in Rat Parotid Acinar Cells Mediated by the NK1 Tachykinin Receptor

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Abstract
Stimulation of rat parotid acinar cells by the tachykinin neurokinin (NK) 1 receptor agonist substance P (SP) resulted in a significant reduction in the initial accumulation of cytosolic myo-[3H]inositol. This effect was rapid, because a reduction of ∼15% could be seen already at 30 s, with the maximal effect (∼45%) being observed at 15 min. The response to SP stimulation Was temperature dependent, because at 4°C no reduction was found, jln addition, at 4°C, cytosolic myo-[3H]inositol represented only 10% of the labeled inositol accumulated at 37°C. The SP-induced reduct on in cytosolic ravo[3H]inositol accumulation was concentration dependent; the EC50 obtained for SP was 5.8 ± 2.5 nM. Spantide [N Arg1, D-Trp79, Leu]SP), a SP antagonist, used at a concentration oif 105 A/, gave a competitive shift of the dose-response curve to SP. Various tachykinins and their analogs were evaluated for their ability to reduce cytosolic mvo-[3H]inositol. [L-Pro9]SP and SP methyl ester, two highly selective agonists of NK1 receptors, reduced the initial accumulation of myo-H]inositol with EQo values of 2.3 and 67.0 nM, respectively. Long SP C-terminal fragments were more potent than shorter ones. SP N-terminal fragments and SP free acid were -without effect. [Pro7]NKB, a selective NKB analog, had no effect. The rank order of potency of mammalian tachykinins was SP > NKA > NKB. These findings and the close correlation between EC50 values and IC50 values obtained in binding studies implicate the NK 1 receptor. In addition, stimulation of muscarinic receptors by carbachol alscp resulted in a reduction in level of cytosolic mjw-[3H]inositol, with this effect being reversed by atropine. Moreover, atropine was unable tjo alter the SP-induced reduction in cytosolic myo-[3H]inositol accumulation. Other neurotransmitters, such as glutamic acid, serotonin, chplecystokinin, neurotensin, bradykinin, and neuropeptide Y, were without effect on initial cytosolic myo-[3H]inositol accumulation. In conclusion, NK1 and muscarinic receptors seem to regulate the membrane transport of inositol in acinar cells of the rat parotid gland. Measurement of the initial accumulation of cytosolic myo-[3H]inositol in this tissue could profitably be adopted as a very simple, rapid, [sensitive, and specific biochemical procedure for screening the activity of potential agonists and antagonists at NK1 receptors.