K+-p-nitrophenylphosphatase inhibition by neurotensin involves high affinity neurotensin receptor: influence of potassium concentration and enzyme phosphorylation

Neurotensin (NT), a 13-amino acid peptide. is widely distributed in the bra in and peripheral tissues of several mammalian species including man. In ad ult rat brain NT can bind to two distinct sites. one of high and the other of low affinity, corresponding to NT, and NT, receptor, respectively; struc turally unrelated to these two, a third NT receptor (NT3) has been describe d. We have previously shown that Na+, K+-ATPase is inhibited by NT when usi ng ATP as substrate. In order to determine whether K+-stimulated dephosphor ylation of this enzyme is involved, we tested NT effect by using p-nitrophe nylphosphate, a non-natural substrate. K+-p-nitrophenylphosphatase activity was inhibited 42% by NT at 8.6 x 10(-6) M Using an incubation medium conta ining 2 mM KCl but was unaffected in the presence of 5 or 20 mM KCl; howeve r, with such KCl concentrations, NT was enabled to inhibit enzyme activity (congruent to 35%) provided a suitable ATP:NaCl Mixture (0.6:45.0 mM) was a dded. Mg2+-p-nitrophenylphosphatase activity remained unaltered at all cond itions tested. Since SR 48692, a selective non-peptide NT, antagonist, abol ished NT effect, involvement of NT, receptor in enzyme inhibition is sugges ted. (C) 2001 Elsevier Science B.V. All rights reserved.