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Effects of protons on muscarinic receptor cationic current in single visceral smooth muscle cells
1Department of Pharmacology and Clinical Pharmacology, St. George's Hospital Medical School, London, United Kingdom.
Abstract:
Effects of extracellular protons on muscarinic receptor-evoked cationic current (Icat) in single guinea pig ileal smooth muscle cells were studied by use of patch-clamp techniques: intracellular pH and pCa were buffered to 7.4 and 7.0, respectively, symmetrical 124 mM Cs+ solutions were used, and divalent cations were removed from the bathing solution. Increasing extracellular pH (pHo) from 7.4 to 8.4 caused a 16-mV parallel negative shift of the activation curve for Icat evoked by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) with an increase in the maximal conductance and slowed Icat relaxation on hyperpolarization; acidification to pHo 6.4 produced equivalent but opposite effects. Carbachol- and GTPgammaS-activated Icat behaved similarly, suggesting that the cationic channel rather than the muscarinic receptor was the major site of action. From 11.4 to 4.4 pHo, maximal cationic conductance was reduced progressively, and the activation curve shifted positively. Na+, K+, Ca2+, and Mg2+ had complex interactions with pHo-induced effects considered to be attributable to interaction of protons with fixed negative surface charges and, at positive potentials, to channel block.
Insights
Extracellular protons significantly alter muscarinic receptor-activated cationic currents (Icat) in guinea pig ileal smooth muscle cells. Changes in external pH affect Icat
Area of Science:
- Physiology
- Cell Biology
- Pharmacology
Background:
- Muscarinic receptors play a key role in regulating smooth muscle function.
- Cationic currents (Icat) are crucial for cellular excitability and signaling in smooth muscle.
Purpose of the Study:
- To investigate the impact of extracellular protons on muscarinic receptor-evoked cationic currents (Icat).
- To determine the specific site of action for proton effects on Icat.
Main Methods:
- Utilized patch-clamp techniques on single guinea pig ileal smooth muscle cells.
- Manipulated extracellular pH (pHo) while buffering intracellular conditions.
- Activated muscarinic receptors using guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) and carbachol.
Main Results:
- Alkaline extracellular pH (pHo 8.4) shifted the Icat activation curve negatively and increased maximal conductance.
- Acidic extracellular pH (pHo 6.4) caused opposite effects, shifting the activation curve positively.
- Proton effects were similar for GTPgammaS- and carbachol-activated Icat, indicating action at the cationic channel.
Conclusions:
- Extracellular protons modulate muscarinic receptor-evoked cationic currents in ileal smooth muscle.
- The primary site of proton action is the cationic channel, not the muscarinic receptor.
- Proton interactions with surface charges and channel block contribute to pH-dependent Icat modulation.