Differential coupling of subtypes of the muscarinic receptor to adenylate cyclase and phosphoinositide hydrolysis in the longitudinal muscle of the rat ileum.

The binding affinities of selective muscarinic antagonists were compared with their ability to block receptor-mediated inhibition of adenylate cyclase and stimulation of phosphoinositide hydrolysis in the longitudinal muscle of the rat ileum. When measured by competitive inhibition of the binding of N-[3H]methylscopolamine, the binding properties of selective muscarinic antagonists were consistent with a two-site model. Approximately 84% of the binding sites (major sites) had high affinity for the M2-selective antagonists methoctramine and AF-DX 116 (11[[2-[(diethylamino)methyl]-1- piperidinyl]acetyl]-5,11-dihydro-6H-pyrido [2,3-b][1,4]benzodiazepine-6-one), whereas the remainder of the sites (minor sites) had high affinity for hexahydrosiladifenidol and its para-fluoro derivative. There was good agreement between the estimates of the dissociation constants of muscarinic antagonists for the major binding site and those measured by antagonism of the adenylate cyclase response. There was also good agreement between the dissociation constants of muscarinic antagonists for the minor binding site and those measured by antagonism of the phosphoinositide response and the contractile response. Our data indicate that there are at least two types of muscarinic receptors in the longitudinal muscle of the ileum, the more abundant being an M2 receptor, which mediates an inhibition of adenylate cyclase activity, and the less abundant being an M3 receptor, which triggers contraction and phosphoinositide hydrolysis.

Redistribution of neuronal lysosomes induced by colchicine: an electron microscopic quantitative study.

We have previously demonstrated that a single injection of the microtubule poison colchicine, into the lateral cerebral ventricle of the rat, induced a rapid redistribution of the lysosomal marker enzymes, dipeptidylaminopeptidase II (Dpp II) and acid phosphatase, from their normal location in neuronal cell bodies out into the dendrites. In the present study, we have quantitatively analyzed this phenomenon at the electron microscopic level by identifying and counting the number of lysosomes and mitochondria in neuronal cell bodies and dendrites of control and colchicine-treated rats. Areas examined included the anterior dorsal (AD) thalamus, pontine nucleus, and facial nucleus. The results show that the cytoplasm of these neurons contains significantly fewer large lysosomes after treatment with colchicine while the dendrites become abnormally enriched with large and small lysosomes after treatment. Lysosomes were rarely seen in the axons of either control or colchicine-treated animals. A significant increase in the density and the shape of mitochondria was also observed in the dendrites following colchicine treatment. The data presented support the hypothesis that neurons contain a transport system which selectively translocates lysosomes, and possibly other organelles, into dendrites. The size, shape, and number of these organelles may change as a result of this transport.