Comparison between strontium and calcium uptake by the fragmented sarcoplasmic reticulum

Area of Science:

• Biochemistry
• Cell Biology
• Muscle Physiology

Background:

• The sarcoplasmic reticulum (SR) is crucial for regulating intracellular calcium levels in muscle cells.
• Understanding ion transport mechanisms, particularly ATP-dependent ion uptake, is vital for comprehending muscle function and dysfunction.

Purpose of the Study:

• To investigate the characteristics of strontium (Sr2+) uptake by fragmented sarcoplasmic reticulum.
• To compare strontium uptake with calcium (Ca2+) uptake and elucidate the role of the Sr2+-activated ATPase.

Main Methods:

• Utilized fragmented sarcoplasmic reticulum vesicles to study ATP-supported ion transport.
• Measured strontium and calcium uptake rates under varying conditions, including ATP presence, inorganic phosphate (Pi), ADP, and internal/external ion concentrations.
• Determined transport ratios and analyzed the effects of ion exchange and ATP depletion.

Main Results:

• ATP-dependent strontium uptake is monophasic and faster than fast calcium uptake.
• Strontium accumulation is proportional to external concentration and ATP hydrolysis, with a transport ratio of one in the absence of oxalate.
• Strontium uptake is not activated by Pi; however, strontium can activate slow calcium uptake while inhibiting fast calcium uptake, and ion-induced release is partial and transient.

Conclusions:

• The Sr2+-activated ATPase exhibits distinct transport kinetics for strontium compared to calcium.
• Energy conversion during strontium transport is efficient, unlike calcium transport under conditions of inefficient ATP phosphorylation.
• Strontium and calcium can partially and transiently release each other from the sarcoplasmic reticulum, indicating complex regulatory interactions.