DISTRIBUTION OF VIP BINDING-SITES IN OPOSSUM INTERNAL ANAL-SPHINCTER CIRCULAR SMOOTH-MUSCLE

The main goal of the present study was to determine whether vasoactive intestinal polypeptide (VIP) binds exclusively to smooth muscle plasma membranes or also to neurons. We examined the distribution of VIP binding sites in different fractions and subfractions of smooth muscle membranes prepared from the circular muscle layer of opossum internal anal sphincter (IAS). The markers used for synaptosomal and smooth muscle plasma membranes were [H-3]saxitoxin binding and 5' nucleotidase activity, respectively. Fractionation of the membranes was carried out by differential centrifugation of the circular smooth muscle membranes. Further subfractionations were done by sucrose density gradient centrifugation. VIP binding was high in the membrane fractions that were enriched with 5' nucleotidase activity. However, some VIP binding was also found in fractions with high saxitoxin binding. Membranes prepared from isolated smooth muscle cells and myenteric neurons both had high VIP binding. Membranes prepared from isolated smooth muscle cells displayed higher 5' nucleotidase activity, whereas membranes prepared from isolated myenteric plexi had higher saxitoxin binding. In conclusion, in the opossum IAS circular smooth muscle, binding of VIP occurs on both the smooth muscle plasma membranes and the synaptosomal membranes. The presence of VIP binding sites on the smooth muscle plasma membrane supports the role of VIP as an inhibitory neurotransmitter. The significance of binding of the neuropeptide to the neuronal membranes remains to be determined.