ABSTRACT
Objective · To study the electrophysiological effect of (S)-OTS·HCl on the heart. Methods · The conventional intracellular recording, electrocardiograph (ECG) and Langendorff cardiac perfusion technique were employed to investigate the effect of (S)-OTS·HCl on in-vivo and in-vitro hearts of guinea pigs and rabbits. Results · (S)-OTS·HCl could bind to M2 muscarinic receptors and dose-dependently prolong the RR intervals significantly in vivo. It had no effect on resting potential (RP), action potential amplitude (APA), and maximum upstroke velocity of phase 0 (Vmax) of ventricular myocytes. Instead, 1×10-5 mol/L (S)-OTS·HCl could shorten the action potential duration at 50 percent repolarization (APD50) and APD90 to 91.6% and 90.9%, respectively. And the spontaneous depolarization rate of phase 4 (SDR) of sinus nodes was reduced to its 13.7% when rabbit sinus nodes were exposed to 1×10-7 mol/L (S)-OTS·HCl. (S)-OTS·HCl could inhibit Ca2+channel effectively. It decreased APA and Vmax of sinus nodes and attenuated the cardiac contractility in vitro. Conclusion · (S)-OTS·HCl is a potent cholinergic agonist and has negative chronotropic, dromotropic, and inotropic effects on hearts via binding to M2 muscarinic receptors.
ABSTRACT
BACKGROUND: After peripheral nerve injury in human, a syndrome of events (spontaneous pain, allodynia and hyperalgesia) may be observed that includes no response of morphine and dependency of this pain state on intact sympathetic function. Spinally delivered 2-adrenoceptor agonist and cholinergic agonist or cholinesterase inhibitors have been shown to have actions attenuating the hyperalgesia in rat models of nerve injury-induced pain. Using a fixed-dose analysis and an isobolographic paradigm, the spinal interaction between the 2-adrenoreceptor agonist, clonidine and cholinergic agonist, carbachol is characterized in rat model of nerve injury-induced tactile hyperalgesia. METHODS: Male Sprague Dawely rats were anesthetized with halothane, and the left L5 and L6 spinal nerve were ligated (Chung model). After recovery, a polyethylene tubing catheter was implanted into lumbar intrathecal space. After recovery from catheter implantation, intrathecal dose-response curves were established for the antiallodynic effect of carbachol (0.1, 0.3, 1.0, 3.0 microgram) and clonidine (0.3, 1.0, 3.0, 10 microgram) alone to obtain the ED50 for each agent. ED50 fractions (1/2, 1/4, 1/8, 1/16) of drug combinations of carbachol-clonidine were administered and thresholds for left hind limb paw withdrawal to von Frey hair application were assessed. The ED50 of carbachol-clonidine combination was established and isobolographic analysis of the drug interactions was carried out.c RESULTS: Intrathecal carbachol and clonidine alone produced dose-dependent reductions of tactile allodynia: ED50 of 66 ng (12~367 ng) and 39 ng (1~1452 ng), respectively. With the fixed dose analysis, the log dose-response curves showed a left shift that considerably exceeds the theoretical curves made by a simple sum of the effects of carbachol alone and clonidine. With the isobolographic analysis, ED50 of mixture was found to be statistically less than the theoretical additive ED50 of mixture. CONCLUSION: The experiments suggest that intrathecal carbachol and clonidine alone produce a dose dependent antagonism on touch evoked allodynia and intrathecal carbachol is synergistic when combined with intrathecal clonidine.