Affinity and efficacy of racemic, (+)‐, and (−)‐methacholine in muscarinic inhibition of [³H]‐noradrenaline release

Abstract

Mackay (1966). KA of( + )-, (± )-, and (-)-methacholine were 2.5, 4 and 440 ZlM, resulting in an isomeric affinity ratio KA (+ )/KA (-) of 180.The discrepancy between the isomeric IC50 ratio and the isomeric KA ratio is explained by a higher intrinsic efficacy of the (+ )-enantiomer compared to the (-)-enantiomer. Thus, (+ )-methacholine has to occupy fewer receptors to induce a given inhibition of release than its antipode as revealed by a plot of fractional receptor occupancy vs response. 4 The results show that, in the effector system of presynaptic muscarinic inhibition, methacholine enantiomers differ greatly not only in affinity for the receptor, but also to some extent in the efficiency of signal transmission, and both parameters contribute to the high isomeric potency ratio. The activity of the racemate is fully accounted for by the activity of the (+)-enantiomer.

Dissociation constants and relative efficacies of acetylcholine, (+)‐ and (‐)‐methacholine at muscarinic receptors in the guinea‐pig ileum

“…By applying the method of Furchgott & Bursztyn (1967) (Table 1). However, Fuder & Jung (1985) noted that the KA value of (±)-MCh was slightly higher when PrBCM was used instead ofphenoxybenzamine. It therefore appears that (+ )-MCh has similar affinity for presynaptic muscarinic receptors in the rat heart and postsynaptic receptors in the guinea-pig ileum.…”

“…Cholinesterase inhibition had no apparent effect on the potency ratio of the MCh enantiomers although it increased the relative potency of ACh. Fuder & Jung (1985) recently determined the KA values of (+)-and (-)-MCh at presynaptic muscarinic receptors mediating inhibition of noradrenaline release in rat heart. The KA values were determined in the absence of a cholinesterase inhibitor as physostigmine had no effect on the inhibitory potency of( + )-MCh.…”

“…It therefore appears that (+ )-MCh has similar affinity for presynaptic muscarinic receptors in the rat heart and postsynaptic receptors in the guinea-pig ileum. Fuder & Jung (1985), using the method of Mackay (1966), obtained a KA value of 440 LM for (-)-MCh. Their value was slightly, but not significantly lower than that similarly determined at ileal muscarinic receptors (Table 1).…”

Cholinesterase activity and exposure time to acetylcholine as factors influencing the muscarinic inhibition of [3H]‐noradrenaline overflow from guinea‐ pig isolated atria

“…Thus, the lack of effect of exogenous acetylcholine cannot result from abundant amounts of endogenous acetylcholine causing a maximum degree of muscarinic inhibition. Moreover, methacholine reduced the evoked overflow (Figure 4) (Fuder & Jung, 1985) can be hydrolyzed by acetylcholinesterase (Beckett et al, 1963).…”

“…After removing I ml of the fluid to determine the total tritium content, the [3H]-noradrenaline content of the samples was determined (Graefe et al, 1973;Fuder et al, 1982a). Noradrenaline values were corrected for minor cross contaminations (Fuder et al, 1982a (Fuder & Jung, 1985) 100 ± 0%, n = 3). In contrast, tetrodotoxin 3 pM reduced the evoked total tritium overflow by only 57.6 ± 7.5% (n = 3).…”