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Distribution of VIP binding sites in opossum internal anal sphincter circular smooth muscle
Abstract:
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 [3H]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.
Insights
Vasoactive intestinal polypeptide (VIP) binds to both smooth muscle and neuronal membranes in the opossum internal anal sphincter. This finding supports VIP's role as an inhibitory neurotransmitter in smooth muscle function.
Area of Science:
- Neurogastroenterology
- Smooth Muscle Physiology
- Neuropeptide Signaling
Background:
- Vasoactive intestinal polypeptide (VIP) is a neuropeptide implicated in smooth muscle relaxation.
- Its precise localization and binding sites within gastrointestinal tissues, specifically the internal anal sphincter (IAS), require further elucidation.
Purpose of the Study:
- To investigate the distribution of VIP binding sites in the opossum internal anal sphincter (IAS).
- To determine if VIP binds exclusively to smooth muscle plasma membranes or also to neuronal elements.
Main Methods:
- Preparation and fractionation of smooth muscle membranes from the opossum IAS circular muscle layer.
- Differential centrifugation and sucrose density gradient centrifugation for membrane subfractionation.
- Assay of VIP binding, 5' nucleotidase activity (smooth muscle marker), and [3H]saxitoxin binding (synaptosomal marker).
Main Results:
- VIP binding was predominantly observed in membrane fractions enriched with 5' nucleotidase activity (smooth muscle).
- Significant VIP binding was also detected in fractions containing synaptosomal markers ([3H]saxitoxin binding).
- Both isolated smooth muscle cells and myenteric neurons exhibited high VIP binding.
Conclusions:
- VIP binds to both smooth muscle plasma membranes and synaptosomal membranes in the opossum IAS.
- VIP binding to smooth muscle membranes supports its function as an inhibitory neurotransmitter.
- The functional significance of VIP binding to neuronal membranes warrants further investigation.