Agonist-evoked Ca(2+) wave progression requires Ca(2+) and IP(3)

Area of Science:

• Cellular Physiology
• Smooth Muscle Biology
• Calcium Signaling

Background:

• Smooth muscle cells generate calcium (Ca2+) waves in response to inositol trisphosphate (IP3)-generating agonists.
• The precise mechanism governing the propagation of these Ca2+ waves remains incompletely understood.

Purpose of the Study:

• To investigate the mechanism of Ca2+ wave progression in voltage-clamped smooth muscle cells.
• To determine the roles of IP3 and Ca2+ in initiating and propagating these waves.

Main Methods:

• Utilized localized photolysis of caged IP3 and caged Ca2+ buffer diazo-2 in single smooth muscle cells.
• Evoked Ca2+ waves using the IP3-generating agonist carbachol (CCh).
• Manipulated Ca2+ buffer capacity and IP3 levels to assess wave dynamics.

Main Results:

• Ca2+ waves initiated over a large cellular region (approx. 30 microm) and propagated with a narrower wavefront (approx. 9 microm).
• Wave progression required both Ca2+ and IP3; increasing Ca2+ buffering halted waves.
• Colliding IP3-evoked Ca2+ increases diffused passively, indicating wave propagation is not solely Ca2+-dependent positive feedback.
• Waves propagated when IP3 was elevated globally, suggesting initiation requires a substantial IP3 increase over a large area.

Conclusions:

• Smooth muscle Ca2+ waves initiate over a surprisingly large cellular length.
• Active propagation of the Ca2+ wave front necessitates the involvement of both IP3 and Ca2+.
• The findings elucidate key aspects of intracellular Ca2+ signaling dynamics in smooth muscle.