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

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 approximately 15% could be seen already at 30 s. with the maximal effect (approximately 45%) being observed at 15 min. The response to SP stimulation was temperature dependent, because at 4 degrees C no reduction was found. In addition, at 4 degrees C, cytosolic myo-[3H]inositol represented only 10% of the labeled inositol accumulated at 37 degrees C. The SP-induced reduction in cytosolic myo-[3H]inositol accumulation was concentration dependent; the EC50 obtained for SP was 5.8 +/- 2.5 nM. Spantide [D-Arg1, D-Trp7.9, Leu11]SP), a SP antagonist, used at a concentration of 10(-5) M, gave a competitive shift of the dose-response curve to SP. Various tachykinins and their analogs were evaluated for their ability to reduce cytosolic myo-[3H]inositol. [L-Pro9]SP and SP methyl ester, two highly selective agonists of NK1 receptors, reduced the initial accumulation of myo-[3H]inositol with EC50 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 greater than NKA greater than NKB. These findings and the close correlation between EC50 values and IC50 values obtained in binding studies implicate the NK1 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)