Affinities of the protonated and non-protonated forms of hyoscine and hyoscine N-oxide for muscarinic receptors of the guinea-pig ileum and a comparison of their size in solution with that of atropine.

1. At 37 degrees C in 0.1 M NaCl the pKa of hyoscine (10 mM) is 7.53; the non-protonated form has about one-tenth of the affinity (log K = 8.58) of the protonated form (log K = 9.58) for muscarine-sensitive receptors of the guinea-pig ileum at 37 degrees C. 2. In the same conditions the pKa of hyoscine N-oxide is 5.78 and the non-protonated form is inactive on the ileum whereas the protonated form is highly active with log K estimated to be 9.9, at least as active as hyoscine methobromide (log K = 9.85). 3. Hyoscine methobromide appears to occupy less space in water than atropine methobromide; hyoscine hydrochloride occupies less space than hyoscyamine hydrochloride: the non-protonated forms are slightly bigger. Hyoscine N-oxide hydrobromide is slightly smaller than hyoscine methobromide but the removal of the proton is accompanied by a reduction in volume, such as is seen with other zwitterions. 4. These differences in volume indicated a reduction in entropy on solution which may allow a greater increase in entropy on binding to receptors and hence greater affinity. The higher activity of hyoscine itself could also be due to the presence of the N-methyl group in the axial position, rather than equatorial as in hyoscyamine or atropine. 5. The different position of the N-methyl group may partly explain why the pKa of hyoscine is 2 units lower than that of hyoscyamine or atropine. It is also probable that the unionized form of hyoscine is stabilized by hydration. 6. Although hyoscine N-oxide is only weakly active at pH 7.6, it is present in a highly active form in the acid environment of the stomach and so might be expected to act selectively at this site.