[Transmembrane potential formation upon ATP hydrolysis in sarcoplasmic reticulum]

Area of Science:

• Biochemistry
• Cell Biology
• Membrane Transport

Background:

• Sarcoplasmic reticulum (SR) membrane potential is crucial for Ca2+ transport.
• Understanding the roles of different ATPases in regulating this potential is key.

Purpose of the Study:

• To investigate the electrogenic mechanisms of Ca2+ transport across the SR membrane.
• To elucidate the distinct roles of Ca-independent and Ca-dependent ATPases in potential generation.

Main Methods:

• Utilized fluorescent potential-sensitive dyes (cyanine and oxonol dyes) to monitor membrane potential.
• Employed Nernst's equation and ion gradients (MgCl2, CaCl2) to estimate potential values.
• Investigated Ca2+ transport mechanisms in fragmented SR vesicles.

Main Results:

• Ca2+ transport is driven by Ca-dependent ATPase activity, not transmembrane electrophoresis.
• Ca-independent ATPase generates a positive potential (10-30 mV) via H+ and/or Mg2+.
• Ca-dependent ATPase creates a negative potential (-20 to -40 mV) and functions as a Ca2+/H+, Mg2+-exchanger.
• Ca-independent ATPase may compensate for charge effects during Ca2+ passage.

Conclusions:

• The study proposes a model for the electrogenicity of ATPases in SR Ca2+ transport.
• Ca-dependent ATPase acts as an electrogenic exchanger, facilitating Ca2+ uptake.
• Ca-independent ATPase plays a role in maintaining charge balance across the SR membrane.