Ca2+ can affect Vmax without changes in myosin light chain phosphorylation in smooth muscle

Area of Science:

• Muscle Physiology
• Biochemistry

Background:

• Smooth muscle contraction is regulated by calcium ions.
• Crossbridge cycling rate and energy usage are key determinants of muscle function.
• Myosin light chain phosphorylation (MyLCP) is a known regulator of smooth muscle contraction.

Purpose of the Study:

• To investigate the effects of elevated extracellular calcium ([Ca2+]o) on crossbridge cycling rate and energy usage in rabbit taenia coli.
• To determine if changes in crossbridge cycling rate and energy usage correlate with myosin light chain phosphorylation (MyLCP) under varying calcium conditions.

Main Methods:

• Measurements of maximum velocity of shortening (Vmax) and high energy phosphate usage (delta approximately P) were performed.
• Isometric tetanus was induced in rabbit taenia coli at 18 degrees C.
• The degree of phosphorylation of the 20,000-dalton light chain of myosin (MyLCP) was assessed.

Main Results:

• Elevated [Ca2+]o (4.5 mM) significantly increased the rate of delta approximately P during both force development and maintenance compared to normal Krebs medium (1.9 mM Ca2+).
• Vmax increased in high calcium conditions, consistent with increased delta approximately P rates, but MyLCP showed no significant change.
• Both Vmax and delta approximately P decreased with tetanus duration, independent of MyLCP levels.

Conclusions:

• Ca2+-dependent and time-dependent alterations in Vmax and energy usage can occur in smooth muscle without corresponding changes in MyLCP.
• Crossbridge cycling rate modulation may involve Ca2+-dependent mechanisms beyond MyLCP.