Neuromedin-N is not released with neurotensin from rat ileum

Neuromedin-N (NN) and neurotensin (NT) were shown recently to be encoded in the same precursor molecule. Colocalization and corelease of ileal NT and NN have not yet been demonstrated and were investigated in the rat using antisera that separately recognized intact NT and NN in ileal extracts. Immunofluorescence labeling of full thickness ileal wall revealed that NN-positive fluorescence was only found in the N-cells. However, only 50% of the N-cells also contained NN-like immunoreactivity (NN-LI). This was associated with a level of extractable NN that was 5-fold lower than that of NT. Corelease of NN- and NT-LI was investigated with the isolated, vascularly perfused jejunoileum model by using various substances that were described as potent stimulants of NT release in vivo. Luminal infusion of mixed nutrients, oleic acid (100 mM), glucose (5%), and taurocholic acid (1%) induced a well sustained release of NT, with plateau secretion of about 200%, 120%, 300%, and 700% above basal, respectively. Vascular bombesin (10(-7) M) and carbachol (10(-5) M) provoked a biphasic release of NT, consisting of a transient rise (approximately 600% above basal) followed by a less pronounced but sustained response. HPLC analysis of portal effluent revealed that 70-80% of NT-LI was intact NT. NN-LI was not coreleased with NT even upon vascular coinfusion of phenanthroline, which markedly protected exogenously infused NN. The coexistence but lack of corelease to any significant degree of NN with NT suggests different fates of these two precursor-related peptides within the ileal mucosa.