The Role of Ionic Interaction at the Muscarinic Receptor

Abstract

It is commonly assumed by pharmacologists that the positively charged cationic group in such agonists as acetylcholine, noradrenaline and histamine is essential for their agonist action. The evidence in favour of this view is far from conclusive. Barlow & Hamilton (1962) examined the effect ofpH on the activity of nicotine in producing synaptic block of the rat phrenic nerve-diaphragm preparation. When compared with the quaternary nicotine N-methiodide the activity of nicotine was related to thepH in the way to be expected if the nicotinium ion were more active than the non-protonated form, but neither the range ofpH values studied nor the precision of the results were sufficient to establish the activity (if any) of the non-protonated nicotine relative to the nicotinium ion. Hamilton (1963) has shown a similar effect of pH in the action of nicotine on the frog rectus abdominus muscle, but in this case also the activity of the non-protonated form was not established. Ariens, Simonis & van Rossum (1964) were unable to discover any clear relationship between pH and the action of arecoline on the frog rectus, but did not extend their study over a wide enough pH range to obtain decisive results. Rocha e Silva (1961) examined the effect ofpH on the action of histamine. There was a clear pH-dependence below pH 7 but the action was independent of pH above 7. These results are compatible with the response to histamine being dependent on a group with a pKa 7.0. However, since the dissociation constants of the ionizable groups in histamine are pKaj, 5.9; pKa2, 9.7, the pH-dependence cannot be attributed to the change in histamine ionization and is probably due to a change in the charge on the receptors.An alternative approach to the problem is to study the activity of analogues of the agonist in which the basic nitrogen atom has been replaced by an atom with a different electron shell so that a cation is not formed. Banister & Whittaker (1951) examined the action of 3,3-dimethylbutyl acetate, the analogue of acetylcholine in which the basic nitrogen atom has been replaced by a carbon atom. On the frog rectus, this drug had about 1/12,000th the activity of acetylcholine and, on the guinea-pig ileum, the activity was small and maximal responses were not obtained. The analogue of histamine in which the primary amino-group is replaced by a hydroxyl group, H-(2-hydroxyethyl)-imidazole, is reported to have little or no histamine-like activity (Vartiainen, 1935;Schneedorf & Ivy, 1935;Grossman, Robertson & Rosiere, 1952). These latter pieces of evidence do suggest that uncharged analogues may have some activity and it seemed that the problem merited further examination in view of its importance for the theoretical interpretation of drugreceptor interactions.

The effect of ionic strength on cardiac muscarinic receptors

“…However, the differences in the structure of the cationic groups in carbachol and NMS could be enough to provide this difference. Nevertheless, it may be recalled that there is evidence that for antagonists that are tertiary bases the difference in effectiveness between the charged and uncharged forms is not very large, whereas in agonists uncharged forms seem to have very much reduced activity (Burgen, 1965).…”

Synaptic block of a transmitter‐induced potassium conductance in Aplysia neurones.

“…A gain of 10 in this amplifier provided an output of 1 mV/nA of current in the current electrode. Arecoline concentrations that have been given do not take into account the fact that somewhat over half of the compound in the sea water bathing the ganglion (pH 7 8) is in the un-ionized, and hence inactive, form (see Burgen, 1965). Consequently, arecoline is a much more effective agonist than it would appear to be from the values given, which have not involved a calculation of the predicted active concentration.…”

Actions of Some Esters of 3,3‐dimethylbutan‐1‐ol (The Carbon Analogue of Choline) on the Guinea‐pig Ileum

“…In an assessment of 'the role of ionic interaction at the muscarinic receptor', Burgen (1965) compared the estimated affinities of compounds containing quaternary nitrogen with those of analogues containing quaternary carbon, acetylcholine and 3,3-dimethylbutylacetate, benziloylcholine and 3,3-dimethylbutylbenziloate, in order to calculate the contribution made by the charged nitrogen atom to the binding of agonists and antagonists. Burgen had obtained results which suggested that the different activities of acetylcholine and its carbon analogue were due chiefly to differences in affinity and that the absence of the positive charge therefore reduced affinity more than 3000-fold.…”

“…An 3,3-Dimethylbutylacetate was prepared from the alcohol and acetic anydride as described by Burgen (1965). It had b.p.…”