Effects of neurotensin on electrical and mechanical properties of smooth muscles in longitudinal and circular layers of the guinea-pig ileum

Abstract

Effects of neurotensin (NT) on the electrical and mechanical activities of longitudinal and circular muscles of the guinea-pig ileum were investigated using the microelectrode and isometric tension recording methods. In longitudinal muscles, the resting membrane potential was not affected by NT (0.1-30 nmol/l), but NT did provoke the contraction when applied in concentrations over 1 nmol/l. TTX (0.1 mumol/l) neither modified the resting membrane potential nor the contraction evoked by NT, under condition of pretreatment with alpha- and beta-adrenoceptor blockers. In circular muscles, NT (over 0.1 nmol/l) consistently hyperpolarized the membrane and increased the ionic conductance. The hyperpolarization appeared with a transient hyperpolarization, which gradually declined with a long time course. Using apamin and various concentrations of Ca, the NT-induced hyperpolarization was classified into two subtypes; fast and slow. The former was composed of maximum hyperpolarization due to activations of the Ca independent K channel, and the latter was composed of late hyperpolarization, due to activations of the Ca dependent K channel. During the NT-induced hyperpolarization in circular muscles, the amplitude of non-adrenergic, non-cholinergic inhibitory junction potential (i.j.p.) evoked by field stimulation was reduced. This reduction induced by 0.5 nmol/l NT was mainly due to hyperpolarization of the membrane, and that observed in a high concentration of NT (3 nmol/l) was directly involved in ionic mechanisms contributing to the generation of i.j.p. In circular muscles, NT (over 3 nmol/l) did relax the tissue pre-contracted with 17.8 mmol/l K, but NT (below 30 nmol/l) did not relax the tissue pre-contracted by 39.6 mmol/l K.(ABSTRACT TRUNCATED AT 250 WORDS)