A calcium conducting channel akin to a calcium pump

Area of Science:

• Biochemistry
• Cell Biology
• Membrane Biophysics

Background:

• Sarcoplasmic reticulum (SR) plays a crucial role in calcium ion (Ca2+) regulation within muscle cells.
• Understanding the properties of calcium-conducting channels in the SR is vital for comprehending muscle function and dysfunction.

Purpose of the Study:

• To characterize the biophysical and pharmacological properties of calcium conducting channels in SR membrane blebs.
• To determine if these channels are related to known calcium channels or pumps, such as ryanodine receptors or the SR calcium pump.

Main Methods:

• Electrophysiological recordings were used to study single calcium conducting channels in SR membrane blebs.
• The effects of various pharmacological agents, including ryanodine, ruthenium red, ATP, Mg2+, caffeine, Ca2+, vanadate, AlF4-, reactive red 120, and cyclopiazonic acid, were investigated.

Main Results:

• The calcium channels exhibited at least three conductance states (70 pS, 50 pS, and 37 pS).
• Channels showed weak selectivity for Ca2+ over K+ (permeability ratio ~3.4) and voltage-dependent open probability.
• Ryanodine and ruthenium red had no effect, ruling out ryanodine receptors.
• Several calcium pump inhibitors significantly affected channel activity, suggesting a link to the SR calcium pump.

Conclusions:

• The studied calcium conducting channels in SR membrane blebs are distinct from ryanodine receptors.
• Evidence strongly suggests these channels are a functional form of the sarcoplasmic reticulum calcium pump.