1. We have assessed the potency of a range of agonists and antagonists on the
muscarinic receptor responsible for inhibiting the Ca-current (ICa) in NG 108-15 hybrid cells. 2.
Acetylcholine (ACh),
oxotremorine-M and
carbachol were potent 'full' agonists (EC50 values were 0.11 microM, 0.14 microM and 2 microM, respectively). Maximum inhibition of peak high-threshold ICa by these agonists was 39.5%. (+/-)-
Muscarine, methylfurmethide and
arecaidine propargyl
ester (APE) were 'partial' agonists, with EC50 values of 0.54 microM, 0.84 microM and 0.1 microM, respectively. 3.
Atropine,
pirenzepine and
himbacine were potent antagonists of
muscarinic inhibition of ICa, with apparent pKB values of 9.8, 7.74 and 8.83, respectively.
Methoctramine was relatively weak (pKB = 7.63).
Atropine and
pirenzepine depressed maximum responses to agonists, probably because these antagonists have relatively slow dissociation rates. 4. The characteristic pharmacological profile found for the M4 receptors in these functional experiments (
himbacine high affinity,
pirenzepine moderate to high affinity,
methoctramine low affinity) corresponds well with data from earlier binding experiments (Lazareno et al., 1990). Since
mRNA hybridising to probes for the m4 receptor genotype can be detected in these cells, it is suggested that these pharmacological characteristics identify the equivalent expressed receptor subtype M4.