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Local anesthetics inhibit receptors coupled to phosphoinositide signaling in Xenopus oocytes
1Department of Physiology and Biophysics, University of Tennessee, Memphis, 894 Union Avenue, Memphis, TN 38163 USA.
Abstract:
Effects and the mechanism of action of quaternary amine local anesthetics on ligand- and voltage-activated ion currents were studied using voltage-clamped ovarian follicles and oocytes from Xenopus laevis. The fast inward and slow outward currents in response to acetylcholine were unaltered by procaine, whereas the oscillatory and smooth inward chloride currents (ICl) were abolished. Potassium currents (IK) elicited by norepinephrine and oscillatory ICl elicited by lysophosphatidic acid were blocked. Procaine caused a noncompetitive inhibition of oscillatory ICl mediated by heterologously expressed neurotransmitter receptors from the rat brain. Threefold differences were found in the procaine sensitivity of the 5-HT2a and 5-HT2c receptors. The rank order of intrinsic inhibitory activity of local anesthetics was: procaine > lidocaine > dibucaine > tetracaine. Extra- or intracellular application of procaine did not alter the Ca2+-activated Cl- current, indicating that neither the endogenous voltage-gated Ca2+ nor the Ca2+-activated Cl- channels account for the inhibition. Procaine caused only a slight reduction in ICl elicited by photolysis of caged inositol 1,4,5-trisphosphate (InsP3) and did not abolish ICl triggered by GTP[gamma-S]-induced direct activation of G proteins. For receptors coupling to the phosphoinositide/Ca2+ signal transduction pathway, the primary and physiologically relevant site of procaine action appears to be on the extracellular surface, upstream from the G protein, presumably on the receptor.
Insights
Quaternary amine local anesthetics like procaine block specific chloride ion currents (ICl) in Xenopus oocytes. The drug primarily acts on the receptor
Area of Science:
- Neuroscience
- Pharmacology
- Cellular Biology
Background:
- Local anesthetics are widely used for their ability to block ion channels.
- The precise mechanisms by which quaternary amine local anesthetics affect various ion currents are not fully understood.
- Understanding these mechanisms is crucial for developing safer and more effective anesthetic agents.
Purpose of the Study:
- To investigate the effects of quaternary amine local anesthetics on ligand- and voltage-activated ion currents.
- To elucidate the mechanism of action of procaine on ion currents in Xenopus laevis.
- To determine the primary site of action for procaine in specific signaling pathways.
Main Methods:
- Voltage-clamped ovarian follicles and oocytes from Xenopus laevis.
- Application of acetylcholine, norepinephrine, and lysophosphatidic acid to elicit ion currents.
- Heterologous expression of neurotransmitter receptors from rat brain.
- Photolysis of caged inositol 1,4,5-trisphosphate (InsP3) and GTP[gamma-S] activation of G proteins.
Main Results:
- Procaine abolished oscillatory and smooth inward chloride currents (ICl) but did not affect acetylcholine-induced currents.
- Potassium currents (IK) and lysophosphatidic acid-elicited ICl were blocked by procaine.
- Procaine non-competitively inhibited oscillatory ICl mediated by expressed neurotransmitter receptors, with varying sensitivity for 5-HT2a and 5-HT2c receptors.
- Procaine's action was primarily on the extracellular surface, upstream from the G protein, affecting receptors in the phosphoinositide/Ca2+ signaling pathway.
Conclusions:
- Procaine selectively inhibits certain ion currents, particularly chloride currents, in Xenopus oocytes.
- The primary site of procaine action is on the extracellular side of receptors involved in the phosphoinositide/Ca2+ signaling pathway.
- These findings provide insights into the molecular mechanisms of local anesthetic action and their potential targets.