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Spare cholinergic receptors in the urinary bladder
Summary
Carbachol elicits mechanical responses in rabbit detrusor muscle at much lower concentrations than those required for calcium uptake or receptor binding. This suggests a significant muscarinic receptor reserve in the bladder.
Area of Science:
- Pharmacology
- Physiology
- Urology
Background:
- Detrusor muscle contraction is mediated by muscarinic receptors.
- Understanding the concentration-response relationship of agonists is crucial for bladder function studies.
Purpose of the Study:
- To investigate the concentration-effect relationships of carbachol on rabbit detrusor muscle.
- To compare carbachol's effects on mechanical response, calcium uptake, and muscarinic receptor binding.
Main Methods:
- Studied carbachol's concentration-effect curves for mechanical response and 45Ca uptake.
- Utilized lanthanum-residual wash out method for calcium uptake.
- Performed [3H]quinuclidinyl benzilate (QNB) binding assays with Scatchard analysis on muscle homogenates.
Main Results:
- EC50 for mechanical response was 0.27 microM; for 45Ca uptake, it was 12.7 microM.
- [3H]QNB binding revealed a KD of 0.074 nM and receptor density of 47.2 fmol/mg protein.
- Carbachol displacement of [3H]QNB had an EC50 of 135 microM and a Ki of 42 microM.
Conclusions:
- Mechanical response requires significantly lower carbachol concentrations than 45Ca uptake or receptor occupancy.
- A substantial muscarinic receptor reserve (approximately 150-fold) exists in the rabbit detrusor muscle.