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Muscarinic receptor size on smooth muscle cells and membranes
Abstract:
The loss of [3H]quinuclidinyl benzilate ([3H]QNB) binding following high-energy radiation was used to compare the muscarinic receptor size on single smooth muscle cells isolated by collagenase digestion from the canine stomach and on plasma membranes derived from intact gastric smooth muscle without exposure to exogenous proteolysis. Radiation inactivation of galactose oxidase (68 kdaltons), yeast alcohol dehydrogenase (160 kdaltons), and pyruvate kinase (224 kdaltons) activities were used as molecular-weight standards. Radiation inactivation of [3H]QNB binding to rat brain membranes, which gave a target size of 86 kdaltons, served as an additional control. In isolated smooth muscle cells, the calculated size of the muscarinic receptor was 80 +/- 8 kdaltons. In contrast, in a smooth muscle enriched plasma membrane preparation, muscarinic receptor size was significantly smaller at 45 +/- 3 kdaltons. Larger molecular sizes were obtained either in the presence of protease inhibitors (62 +/- 4 kdaltons) or by using a crude membrane preparation of gastric smooth muscle 86 +/- 7 kdaltons).
Insights
The muscarinic receptor in canine gastric smooth muscle cells is larger than in isolated plasma membranes. This difference in size may be due to proteolysis during membrane preparation.
Area of Science:
- Pharmacology
- Cell Biology
- Biochemistry
Background:
- Muscarinic receptors are crucial for regulating gastric smooth muscle function.
- Understanding muscarinic receptor size is important for elucidating its function and potential drug interactions.
- Previous studies have yielded varying estimates of muscarinic receptor size.
Purpose of the Study:
- To compare the molecular size of muscarinic receptors in intact canine gastric smooth muscle cells versus isolated plasma membranes.
- To investigate the potential influence of proteolysis on muscarinic receptor size determination.
Main Methods:
- Utilized radiation inactivation assay to determine the target size of muscarinic receptors.
- Compared [3H]quinuclidinyl benzilate ([3H]QNB) binding loss in single smooth muscle cells and plasma membrane preparations.
- Employed molecular weight standards including galactose oxidase, yeast alcohol dehydrogenase, and pyruvate kinase.
Main Results:
- Muscarinic receptor size in isolated smooth muscle cells was determined to be 80 +/- 8 kdaltons.
- In contrast, plasma membrane preparations showed a significantly smaller muscarinic receptor size of 45 +/- 3 kdaltons.
- Protease inhibitors or crude membrane preparations yielded larger receptor sizes (62-86 kdaltons), suggesting proteolysis.
Conclusions:
- The apparent smaller size of muscarinic receptors in isolated plasma membranes is likely an artifact of proteolysis.
- The radiation inactivation method provides a more accurate assessment of in situ muscarinic receptor size in gastric smooth muscle.
- These findings highlight the importance of considering sample preparation methods when determining receptor molecular size.