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Modulation of the Ca(2+)-sensitivity in phasic and tonic smooth muscle
1Laboratorium voor Fysiologie, Gasthuisberg O/N, K U Leuven.
Verhandelingen - Koninklijke Academie Voor Geneeskunde Van Belgie
|January 1, 1992
Summary
Cytoplasmic calcium (Ca2+) regulates smooth muscle force, but this relationship isn't always direct. Muscle type and agonists influence Ca2+ sensitivity, impacting force development.
Area of Science:
- Physiology
- Cell Biology
- Pharmacology
Background:
- Cytoplasmic free calcium concentration ([Ca2+]i) is a key regulator of smooth muscle contraction.
- Myosin light chain phosphorylation is a critical step in the contractile process.
- Differences exist between phasic and tonic smooth muscle in their regulation of force.
Purpose of the Study:
- To investigate the relationship between [Ca2+]i, myosin light chain phosphorylation, and force development in different smooth muscle types.
- To explore the mechanisms underlying Ca2+ desensitization and sensitization in smooth muscle contraction.
- To compare the Ca2+ sensitivity and regulatory mechanisms in phasic (guinea-pig ileum) and tonic (rabbit pulmonary artery) smooth muscle.
Main Methods:
- Measurement of [Ca2+]i using the fluorescent indicator fura-2.
- Assessment of myosin light chain phosphorylation.
- Measurement of force development in isolated smooth muscle preparations (guinea-pig ileum and rabbit pulmonary artery).
- Experiments conducted on intact and permeabilized muscle preparations.
- Application of various agonists to modulate Ca2+ sensitivity.
Main Results:
- A close temporal link and good correlation were observed between [Ca2+]i rise and force initiation/maintenance, supporting Ca2+ as a primary regulator.
- Dissociation between [Ca2+]i and force occurred in tonic rabbit pulmonary artery, but not in phasic guinea-pig ileum.
- Pronounced Ca2+ desensitization was observed in guinea-pig ileum during prolonged depolarization, linked to myosin light chain phosphatase activity.
- Agonists increased the Ca2+ sensitivity of the contractile apparatus, with varying potentiation observed between different agonists.
- Agonist-induced sensitization may involve phosphorylation of a protein phosphatase inhibitor, thus inhibiting myosin light chain phosphatase.
Conclusions:
- While [Ca2+]i is a primary regulator, smooth muscle force is not invariably tightly coupled to it.
- Differences in myosin light chain kinase/phosphatase activity ratios contribute to the distinct Ca2+-sensitivities of tonic and phasic smooth muscles.
- The Ca2+-sensitivity of the contractile apparatus can be modulated by agonists, indicating complex regulatory mechanisms beyond direct Ca2+ levels.
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