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We have previously characterized the neurotensin receptors on the circular smooth muscle (CM) of the canine small intestine (1). In the present studies, using radioligand binding technique, neurotensin receptors were localized on the membranes from deep muscular (DMP) and the submucous plexus while no binding was observed on either the longitudinal smooth muscle or myenteric plexus membranes. The high affinity binding sites (Kd 0.1-0.2 nM) on DMP membranes were similar to those on CM; the low affinity component was of much lower affinity (Kd ~40 nM). DMP had 4-6 times higher density of binding sites than the CM. The recognition properties of DMP receptors were similar to those on the CM and reduced sulfhydryl groups were required for the binding activity. The action of neurotensin on the contractility of the canine small intestine, therefore, appears to be through a direct action on the circular smooth muscle and through the prejunctional action on the DMP neurons through distinct receptors. Thiol groups in the neurotensin receptors may be important for the receptor function.
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