Reversal of cardiac vagal effects of physostigmine by adjunctive muscarinic blockade.
Pre-treatment with reversible acetylcholinesterase (AChE) inhibitors is an effective strategy for reducing lethality following organophosphate nerve agent exposure. AChE inhibition may have unwanted cardiac side effects, which could be negated by adjunctive anti-cholinergic therapy. The aims of the present study were to examine the concentration-dependent effects of physostigmine on cardiac responses to vagus nerve stimulation (VNS), to test whether adjunctive treatment with hyoscine can reverse these effects and to assess the functional interaction and electrophysiological consequences of a combined pre-treatment. Studies were performed in an isolated innervated rabbit heart preparation. The reduction in heart rate with VNS was augmented by physostigmine (1-1000nmol/L), in a concentration-dependent manner - with an EC50of 19nmol/L. Hyoscine was shown to be effective at blocking the cardiac responses to VNS with an IC50of 11nmol/L. With concomitant perfusion of physostigmine, the concentration-response curve for hyoscine was shifted downward and to the right, increasing the concentration of hyoscine required to normalise (to control values) the effects of physostigmine on heart rate. At the lowest concentration of hyoscine examined (1nmol/L) a modest potentiation of heart rate response to VNS (+15±3%) was observed. We found no evidence of cardiac dysfunction or severe electrophysiological abnormalities with either physostigmine or hyoscine alone, or as a combined drug-therapy. The main finding of this study is that hyoscine, at concentrations greater than 10-8M, is effective at reversing the functional effects of physostigmine on the heart. However, low-concentrations of hyoscine may augment cardiac parasympathetic control.
Cardiovascular Division, King's College London, UK. Electronic address: james.winter@kcl.ac.uk.
James
J
Winter
Defence Science and Technology Laboratory, Porton Down, Wiltshire, UK.
Alexandra R
AR
Cook
Defence Science and Technology Laboratory, Porton Down, Wiltshire, UK.
Dawn H
DH
Patient
Defence Science and Technology Laboratory, Porton Down, Wiltshire, UK.
Stevan R
SR
Emmett
Defence Science and Technology Laboratory, Porton Down, Wiltshire, UK.
John
J
Tattersall
Cardiovascular Division, King's College London, UK.
Michael J
MJ
Shattock
D000818:Animals; D002800:Cholinesterase Inhibitors / Q000627:therapeutic use*; D004305:Dose-Response Relationship, Drug; D004338:Drug Combinations; D006321:Heart / Q000187:drug effects / Q000502:physiology; D006339:Heart Rate / Q000187:drug effects*; D066298:In Vitro Techniques; D018727:Muscarinic Antagonists / Q000494:pharmacology*; D010830:Physostigmine / Q000627:therapeutic use*; D011817:Rabbits; D012601:Scopolamine / Q000494:pharmacology*; D055536:Vagus Nerve Stimulation / Q000379:methods*
D002800:Cholinesterase Inhibitors; D004338:Drug Combinations; D018727:Muscarinic Antagonists; D010830:Physostigmine; D012601:Scopolamine
Acetylcholinesterase; Eserine; Hyoscine; Organophosphate nerve agents; Physostigmine; Scopolamine; Vagus nerve