Medknow Publications on behalf of Indian Pharmacological Society
Abstract
OBJECTIVE: To investigate the role of reactive oxygen species (ROS) on
cholinergic receptor function. MATERIALS AND METHODS : Rectus
abdominis and isolated heart preparations of frog (Rana tigirina
) were used to assess nicotinic and muscarinic receptor activity,
respectively. Thirty percent hydrogen peroxide (H2O2) solution and
Fenton mixture (Fm, 13.9 mg, 50 µM of FeSO4, 75 mg of sodium EDTA
and 50 µL of 30% H2O2 were added to 10 ml of 0.1 M K2HPO4) were
used to generate 1 mM H2O2 and hydroxyl free radicals. The responses
were recorded with acetylcholine at different phases of exposure of
tissues to ROS. Normal frog Ringer was used as a physiological
solution. Responses of acetylcholine were also recorded in the presence
of ROS before and after exposure of the tissue to an antioxidant
(ascorbic acid). RESULTS : Free-radical-mediated receptor damage was
dose (1-100 mM H2O2) and time (10-30 min) dependent when responses were
taken with 30 µg and 30 ng of ACh for nicotinic and muscarinic
receptors, respectively. There was no effect of ROS on prior exposure
of tissue to ascorbic acid (antioxidant) at a concentration of 300
µg/ml. The antioxidant has not shown any beneficial effect on
sulfhydryl groups of G-protein-coupled muscarinic receptors, which are
more susceptible and sensitive to ROS than ion-channel nicotinic
receptors where there is 96% protection with the antioxidant. Reactive
oxygen species has shown different effects on receptor function.
CONCLUSION: Free radicals continuously cause considerable damage to
the receptors. G-protein-coupled muscarinic receptors are more
susceptible than ion-channel-linked nicotinic receptors. Antioxidants
are shown to play a major role in protecting free-radical-mediated
receptor damage