Ca2+ can control vascular smooth-muscle thin filaments without caldesmon phosphorylation

Area of Science:

• Biochemistry
• Molecular Biology
• Physiology

Background:

• Vascular smooth muscle contraction is regulated by Ca2+-dependent myosin MgATPase activation.
• Caldesmon is a protein in thin filaments implicated in this Ca2+-dependent regulation.
• Potential mechanisms include caldesmon interaction with Ca2+-binding proteins or phosphorylation by Ca2+-dependent kinases.

Purpose of the Study:

• To investigate the role of caldesmon phosphorylation in the Ca2+-dependent activation of vascular smooth muscle thin filaments.
• To determine if caldesmon phosphorylation kinetics correlate with the rapid Ca2+ activation of myosin MgATPase.

Main Methods:

• Measurement of Ca2+ activation of aorta thin filaments.
• Quantification of caldesmon phosphorylation levels and kinetics in response to Ca2+.

Main Results:

• Ca2+ rapidly activates aorta thin filaments in under 10 seconds.
• Caldesmon phosphorylation occurs slowly, with a half-time greater than 10 minutes.
• The maximum level of caldesmon phosphorylation is low (approximately 1 molecule per 7 caldesmon molecules).

Conclusions:

• The rapid Ca2+-dependent activation of vascular smooth muscle thin filaments is not mediated by caldesmon phosphorylation.
• The hypothesis that caldesmon phosphorylation is the primary mechanism for Ca2+-dependent myosin MgATPase activation is untenable.