Related Experiment Videos
Tonic potentiation and attenuation produced by membrane depolarization in guinea-pig trachealis
1Second Department of Internal Medicine, School of Medicine, Nagoya University, Japan.
Abstract:
1. We studied how membrane depolarization directly affected intracellular Ca2+ signalling when voltage-operated Ca2+ channels (VOCC) were not available in guinea-pig tracheal smooth muscle. To block VOCC, we used 3 micromol/L verapamil, which completely abolished high K+ (20-60 mmol/L)-induced contraction, and elevation of fura-2 signal. 2. Muscle tone was generated by adding Ca2+ to the extracellular Ca2+-free solution containing prostaglandin (PG)E2 (100 nmol/L) after abolishing basal tone with indomethacin (1 micromol/L). 3. In the absence of verapamil, high K+ (20-60 mmol/L) solution potentiated 2.4 mmol/l Ca2+-induced sustained contractions. Even in the presence of 3 micromol/L verapamil, replacement with 20 and 40 mmol/L K+ solution induced tonic potentiation, which was changed to attenuation with a higher K+ solution (60 mmol/L), lower extracellular Ca2+ concentration ([Ca2+]o) and pretreatment with cyclopiazonic acid (10 micromol/L), a Ca2+ sequestration inhibitor. 4. These results indicate that the balance between depolarization-dependent Ca2+ release and receptor-operated cation channel inhibition may determine whether tonic potentiation or attenuation is manifested, depending on the availability of VOCC, the magnitude of the depolarization, [Ca2+]o and Ca2+ content in the sarcoplasmic reticulum.
Insights
Membrane depolarization impacts intracellular calcium (Ca2+) signaling in airway smooth muscle, even without voltage-operated Ca2+ channels. The balance of Ca2+ release and channel inhibition determines contraction responses.
Area of Science:
- Physiology
- Pharmacology
- Cellular Biology
Background:
- Voltage-operated calcium channels (VOCC) are crucial for regulating intracellular calcium (Ca2+) signaling and smooth muscle contraction.
- Understanding Ca2+ signaling independent of VOCC is essential for comprehending airway smooth muscle physiology and dysfunction.
- Prostaglandin E2 (PGE2) and indomethacin are used to modulate smooth muscle tone and basal activity.
Purpose of the Study:
- To investigate the direct effects of membrane depolarization on intracellular Ca2+ signaling in guinea-pig tracheal smooth muscle when VOCC are blocked.
- To elucidate the mechanisms underlying Ca2+ handling and contraction in airway smooth muscle under conditions of inhibited VOCC.
Main Methods:
- Guinea-pig tracheal smooth muscle was studied using verapamil (3 µmol/L) to block VOCC.
- Muscle tone was induced by adding extracellular Ca2+ to a Ca2+-free solution containing PGE2 (100 nmol/L) after basal tone abolition with indomethacin (1 µmol/L).
- High potassium (K+) solutions (20-60 mmol/L) were used to induce depolarization, and fura-2 signal was measured to assess intracellular Ca2+ levels.
Main Results:
- In the absence of VOCC, high K+ induced tonic potentiation of contractions and Ca2+ signals, but this was attenuated at 60 mmol/L K+.
- Attenuation was also observed with lower extracellular Ca2+ concentrations ([Ca2+]o) and after pretreatment with cyclopiazonic acid (10 µmol/L), a Ca2+ sequestration inhibitor.
- Depolarization-induced responses were dependent on the magnitude of depolarization, [Ca2+]o, and sarcoplasmic reticulum Ca2+ content.
Conclusions:
- Membrane depolarization can influence intracellular Ca2+ signaling and airway smooth muscle tone even when VOCC are unavailable.
- The observed potentiation or attenuation of contraction depends on a balance between depolarization-induced Ca2+ release and inhibition of receptor-operated cation channels.
- These findings highlight the complex regulation of Ca2+ homeostasis in airway smooth muscle, influenced by VOCC availability and sarcoplasmic reticulum Ca2+ stores.