A. Rutledge

The actions of peppermint oil and menthol on calcium channel dependent processes in intestinal, neuronal and cardiac preparations

The activities of menthol and peppermint oil were determined in guinea‐pig ileal smooth muscle, in rat and guinea‐pig atrial and papillary muscle, in rat brain synaptosomes and in chick retinal neurones by pharmacological 45Ca2+ uptake and radioligand binding assays. Menthol is a major constituent of peppermint oil and is approximately twice as potent as peppermint oil as an inhibitor of K+ depolarization‐induced and electrically stimulated responses in ileum and electrically stimulated atrial and papillary muscles. IC50 values in the ileal preparation ranged from 7.7 to 28.1 μg ml−1 and in the cardiac preparations from 10.1 to 68.5 μg ml−1. Similar potencies were demonstrated against K+ depolarization‐induced 45Ca2+ uptake in synaptosomes and against K+ depolarization and Bay K 8644‐induced uptake in chick retinal neurons. IC50 values for menthol inhibition of K+ and Bay K 8644 responses in the retinal neurons were 1.1 × 10−4 M (17.2 μg ml−1) and 1.75 × 10−4 M (26.6 μg ml−I), respectively, and for peppermint oil were 20.3 and 41.7 μg ml−1 respectively. Both menthol and peppermint oil inhibited specific [3H]nitrendipine and [3H]PN 200–110 binding to smooth and cardiac muscle and neuronal preparations with potencies comparable to, but slightly lower than, those measured in the pharmacological and 45Ca2+ uptake experiments. Binding of menthol and peppermint oil, studied at 78 μg ml−1, was competitive against [3H]nitrendipine in both smooth muscle and synaptosome preparations. The data indicate that both menthol and peppermint oil exert Ca2+ channel blocking properties which may underlie their use in irritable bowel syndrome. Ca2+ channel antagonism may not be the only pharmacological effect of menthol and peppermint oil contributing to intestinal smooth muscle relaxation.

The binding interactions of Ro 40-5967 at the L-type Ca2+ channel in cardiac tissue

Ro 40-5967 [(1S,2S)-2-[2[3-(2-benzamidopropyl]- methylamino]ethyl]-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-naphthyl- methoxyacetate] is a new Ca2+ channel antagonist active at L-type channels. Radioligand binding studies in cardiac tissue show that Ro 40-5967 does not inhibit 1,4-dihydropyridine binding, but does inhibit diltiazem, desmethoxyverapamil and SR 33557 binding with IC50 values of 8 x 10(-9), 10(-8) and 5 x 10(-8) M, respectively. Equilibrium and kinetic binding studies showed that Ro 40-5967 inhibited both desmethoxyverapamil and SR 33557 binding in an apparently competitive manner. Ro 40-5967 defines an additional and possibly unique antagonist binding site on the L-type voltage-gated Ca2+ channel.

Effect of an homologous series of aliphatic alcohols on neuronal and smooth muscle voltage-dependent Ca2+ channels

The acute inhibitory actions of alcohol on K(+)-stimulated 45Ca2+ uptake into synaptosomes shows regional variation in sensitivity throughout the brain, suggesting the possibility of a selective action on a specific Ca2+ channel subtype. This was examined by comparing the effects of a homologous series of aliphatic alcohols on synaptosomal Ca2+ channels with their actions on K(+)-stimulated Ca2+ channels in guinea-pig intestinal longitudinal muscle, which have been demonstrated to be of the L-type. K(+)-stimulated contraction of and [3H]nitrendipine binding to smooth muscle were both inhibited by the alcohols at similar concentrations, with the potency increasing with chain length. In synaptosomes, however, K(+)-stimulated 45Ca2+ uptake was 5-30 times more sensitive to the inhibitory actions of alcohol than were [3H]nitrendipine and [125I]omega-conotoxin binding. These observations suggest that K(+)-stimulated 45Ca2+ uptake is mediated by a non-L non-N type channel which is more sensitive to the acute effects of alcohols. This is supported by the observation that K(+)-stimulated 45Ca2+ uptake which is insensitive to L- and N-channel antagonists was inhibited by funnel web spider venom.

Short-term regulation of neuronal calcium channels by depolarization.

The 1,4-dihydropyridine-sensitive voltage-gated Ca2+ channel is widely distributed in excitable cells. The channel and its several associated drug binding sites are known to be up- and downregulated by a variety of homologous and heterologous influences including membrane depolarization. The neurosecretory GH4C1 cell line possesses L-type channels. Depolarization of these cells by elevated K+ increases the binding affinity of 1,4-dihydropyridines and decreases the number of 1,4-dihydropyridine binding sites and functional channels. There is a coordinate upregulation of the number of muscarinic receptors. This membrane potential- and Ca(2+)-calmodulin-dependent process of channel downregulation may involve internalization of the channel heteromeric complex or, more plausibly, a dissociation of the complex and a concomitant loss of both binding and permeation functions.

Interactions of analogs of the 1,4-dihydropyridine tiamdipine in cardiac and smooth muscle

Two series of 1,4-dihydropyridines related to tiamdipine, 2-(2-aminoethylthio)methyl-3-carboethoxy-5-carbomethoxy-6- methyl-4-(3-nitrophenyl)-1,4-dihydropyridine, have been evaluated for their pharmacologic and radioligand binding properties in smooth and cardiac muscle. In the tiamdipine series the influence of phenyl ring substitution, 3-Cl, 3-MeO and 3-CF3, was greatly reduced relative to the N-formyl and neutral nifedipine derivatives. Consistent with our previous observations onset and offset of action were greatly reduced by the presence of the amine side chain. In tiamdipine analogs also bearing an asymmetric substituent at C-2, chirality at C-4 was determinant for activity.