Myoplasmic [ca], crossbridge phosphorylation and latch in rabbit bladder smooth muscle

Area of Science:

• Physiology
• Muscle Biology
• Biochemistry

Background:

• Tonic smooth muscle displays the latch phenomenon, characterized by high force with low myosin regulatory light chains (MRLC) phosphorylation, reduced shortening velocity, and lower energy use.
• The kinetics of MRLC phosphorylation and cellular activation in phasic smooth muscle remain largely uncharacterized.

Purpose of the Study:

• To investigate if calcium (Ca2+)-stimulated MRLC phosphorylation can account for agonist- or high potassium (K+)-induced contractions in fast, phasic smooth muscle.
• To determine the role of temperature in smooth muscle contraction and latch behavior.

Main Methods:

• Measurements of myoplasmic Ca2+, MRLC phosphorylation, contractile stress, and shortening velocity (assessed by 1/half-time) in rabbit urinary bladder strips.
• Experiments conducted at both 22°C and 37°C to evaluate temperature-dependent effects.

Main Results:

• High K+-induced contractions were phasic at both temperatures, with transient increases in Ca2+, MRLC phosphorylation, shortening velocity, and stress.
• Carbachol (CCh)-induced contractions at 37°C demonstrated latch behavior: sustained high stress despite decreasing Ca2+, MRLC phosphorylation, and shortening velocity.
• At 22°C, CCh induced sustained elevations in all measured parameters, and the dependence of stress on MRLC phosphorylation was steeper at 37°C than at 22°C.

Conclusions:

• Phasic smooth muscle exhibits latch behavior, similar to tonic smooth muscle.
• Latch behavior is less prominent at reduced temperatures (22°C) compared to physiological temperatures (37°C).
• Temperature significantly influences the relationship between MRLC phosphorylation and contractile force in phasic smooth muscle.