Key result
Increasing ionic strength reduces cardiac muscarinic receptor binding ~4-fold, decreasing carbachol affinity.
Why the study?
The effect of ionic strength on the binding characteristics of cardiac muscarinic receptors was not fully understood.
Ionic strength alters muscarinic binding in vitro; leaves open physiological relevance to cardiac function or drug effects in vivo.
The binding of N-methylscopolamine (NMS) and carbachol to muscarinic receptors in the rat heart has been measured as a function of ionic strength (mu). The binding of NMS was reduced by 3.69 fold for a 10 fold increase in ionic strength. The binding of carbachol was affected in two ways. Firstly, the proportions of the subsites were changed. Above mu = 0.5 M, the superhigh (SH) subsite was converted into the low (L) subsite and above mu = 0.8 M, the high (H) subsite was also converted into the L subsite. Therefore, at high ionic strength, no agonist-determined subsites can be detected. In addition, increase in ionic strength reduced the binding of carbachol to all subsites and to a much greater extent than for NMS.
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Arnold S. V. Burgen (1986) studied this question. Ionic strength was evaluated on Binding of N-methylscopolamine (NMS) and carbachol to muscarinic receptors. Increasing ionic strength reduced the binding of N-methylscopolamine by 3.69-fold and converted high-affinity carbachol subsites into low-affinity subsites in rat heart muscarinic receptors.
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