Biochemical Basis for Inhibitory Control of Colon Function

Abstract

<p>Direct VIP-nerve and indirect ICC-mediated inhibitory control of colon motility were studied using biochemical techniques. In order to define a basis for VIP action in colon, longitudinal muscle, ICC-poor circular muscle and ICC-rich layers separated by dissection were characterized by electron microscopy and subcellular fractionation using markers for synaptosomes, smooth muscle membrane and mitochondria. Synaptosmal, mitochondrial and smooth muscle enriched membrane fractions were isolated but a putative ICC membrane enriched fractions has the highest 5'nucleotidase, Mg2+ATPASE activities and density of 125 I-VIP binding followed by synaptosome and smooth muscle membranes. High VIP binding density can be used as a marker for ICC membrane.</p> <p>The study of nitric oxide production in fractions from ICC-rich and-poor preparation by measuring nitrite levels showed that mitochondrial, synaptosome and ICC-rich membrane fractions produced nitric oxide but greater levels were seen in the ICC-membrane rich fraction. Nitric oxide synthase activity on ICC membrane was constitutive and calcium-dependant. VIP increased (2-fold) nitric oxide production in ICC-rich strips which was inhibited by L-NAME, reduced by EGTA and increased by exogenous calcium. Partial inhibition of VIP-induce nitric oxide production by ω-Conotoxin GVIA was recorded in the absence (22%) and presence (32%) of L-arginine suggesting that the source of nitric oxide was in large part non-neural, from ICC.</p> <p>I conclude that direct nerve-mediated control of longitudinal and circular muscle occurs through VIP binding on membrane receptors while indirect control of circular muscle occur through VIP-nerve induced production of nitric oxide from ICC to cause smooth muscle relaxation.</p>