An analysis of the mode of action of carbachol on the chick biventer cervicis nerve--muscle preparation

Effects of locally administered anticholinesterase agents on the secretory response of human eccrine sweat glands to acetylcholine and carbachol

The effects of locally administered physostigmine and di-isopropylphosphorofluoridate (DFP) were compared on the secretory response of sweat glands to intradermally injected acetylcholine and carbachol in healthy male volunteers (physostigmine: six subjects; DFP: one subject). The response to acetylcholine reached its peak within 10 s of injection and then rapidly declined, whereas the response to carbachol increased steadily reaching a peak between 5 and 7 min after injection. The response to acetylcholine was potentiated in the presence of both physostigmine and DFP, whilst the response to carbachol was not significantly affected by either of these drugs. The difference in the time-course of responses to acetylcholine and carbachol may be attributed to differences in the susceptibility of the two drugs to metabolism by acetylcholinesterase; carbachol, unlike acetylcholine, being virtually immune to metabolism by this enzyme. It is concluded that the response to carbachol is mediated by a direct stimulatory action on post-synaptic muscarinic receptors rather than by activation of pre-synaptic nicotinic receptors leading to the release of endogenous acetylcholine.

Carbachol-methohexitone interactions at the chick neuromuscular junction: experiments with the moving fluid electrode technique

The moving fluid electrode was used to record simultaneously the depolarization and contracture responses produced by drug action. Carbachol (CCh) (5.5 X 10(-6)-5.5 X 10(-4)M) acetylcholine (ACh) (5.5 X 10(-6)-1.1 X 10(-2) M) produced large depolarization and contracture responses in the isolated chick biventer cervicis (BVC) muscle. These responses were greatly reduced by methohexitone (8.8 X 10(-5) M). The concentration-response curves for ACh and carbachol in the control Krebs solution were shifted in the presence of methohexitone to the right in a non-competitive manner. The present results indicate that both ACh and carbachol act postjunctionally by depolarizing the membrane and producing contractures in the chick biventer cervicis muscle.

The action of barbiturates on contractile responses of canine and feline bronchial smooth muscle

Pentobarbitone, thiopentone and phenobarbitone in concentrations ranging from 10(-4) M to 10(-3) M reduced the contractile responses of isolated canine and feline trachealis muscle to electrical field stimulation without affecting contractile responses to acetylcholine. The order of potency was thiopentone, pentobarbitone and phenobarbitone. Experiments using carbachol and neostigmine or physostigmine suggested that the barbiturates act by preventing the release of acetylcholine from nerve endings. In vivo, pentobarbitone (2.5-20 mg/kg i.v.) produced dose-related reductions in the increase in total lung resistance induced by vagal stimulation in chloralose-anaesthetised cats.

Antagonism of cholinomimetics by troxypyrrolidinium in guinea-pig atria and longitudinal ileal muscle: comparison with hemicholinium-3

The inhibitory effect of troxypyrrolidinium (trox) (10(-6) to 2 x 10(-3) M) on responses to carbachol (CCh) and acetylcholine (ACh) was investigated in the electrically stimulated left atrium and longitudinal ileal muscle of the guinea pig. In both tissues, trox exhibited antimuscarinic activity causing parallel shifts of the concentration--response curves to both agonists with no depression of maximum responses. Responses to CCh were inhibited by trox (5 x 10(-4) M) to a greater extent than responses to ACh and this difference was maintained following inhibition of cholinesterases with dyflos. In the guinea-pig atrium using CCh as agonist dose ratios produced by the higher concentrations of trox (greater than 5 x 10(-5) M) were less than expected resulting in a non-linear Arunlakshana--Schild (A-S) plot and this effect of trox was maintained in the presence of mecamylamine (2 x 10(-5) M). In longitudinal ileal muscle flattening of the A-S plot with high concentrations of trox did not occur. Although hemicholinium-3 (HC-3) produces a non-linear A-S plot for antimuscarinic activity in atria the A-S plot obtained with the longitudinal ileal strip using HC-3 (2 x 10(-5) to 2 x 10(-3) M) did not exhibit flattening at high concentrations. The dose ratios obtained with HC-3 (5 x 10(-4) M) using CCh as agonist were significantly greater than those obtained with ACh. It is suggested that trox, like HC-3, acts as a metaffinoid antagonist at the muscarinic receptor.

Carbachol-induced sensitivity changes in skeletal muscle and their mechanism of action

Pretreatment of carbachol (in vitro), in various doses (5.5 pM to 550 nM), produced a significant decrease in the sensitivity of the frog rectus abdominis muscle to acetylcholine (ACh), in a dose-dependent manner. This action was observed with a 60 min incubation period. Contractions induced by potassium chloride (KCl) were unaffected by carbachol. Physostigmine (10 micrometer) potentiated the decrease in sensitivity induced by carbachol, whereas human plasma antagonised it. Low calcium (0.27 mM) in the medium and hexamethonium (300 micrometer) antagonised the sensitivity decrease induced by carbachol. These results suggest that carbachol decreases the sensitivity to ACh by releasing ACh from the presynaptic nerve terminals. The results further suggest that carbachol may release ACh by depolarising the presynaptic nicotinic receptors.