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 plasm a membranes or also to neurons. We examined the distribution of VIP bi nding sites in different fractions and subfractions of smooth muscle m embranes prepared from the circular muscle layer of opossum internal a nal sphincter (IAS). The markers used for synaptosomal and smooth musc le plasma membranes were [H-3]saxitoxin binding and 5' nucleotidase ac tivity, respectively. Fractionation of the membranes was carried out b y differential centrifugation of the circular smooth muscle membranes. Further subfractionations were done by sucrose density gradient centr ifugation. VIP binding was high in the membrane fractions that were en riched with 5' nucleotidase activity. However, some VIP binding was al so 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 disp layed 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 b oth the smooth muscle plasma membranes and the synaptosomal membranes. The presence of VIP binding sites on the smooth muscle plasma membran e supports the role of VIP as an inhibitory neurotransmitter. The sign ificance of binding of the neuropeptide to the neuronal membranes rema ins to be determined.