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A non-specific Ca2+ (or Mg2+)-stimulated ATPase in rat heart sarcoplasmic reticulum
1Division of Pharmacology and Toxicology, Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, Canada.
Abstract:
ATPase activity in rat heart sarcoplasmic reticulum was stimulated in a concentration-dependent manner by both Ca2+ and Mg2+ in the complete absence of the other cation. Increasing concentrations of Mg2+ produced an apparent inhibition of the Ca2(+)-dependent ATP hydrolysis. CDTA (trans-1,2-diaminocyclo-hexane-N,N,N',N'-tetraacetate) had no effect on these responses. The results indicate the presence of a low affinity non-specific divalent cation-stimulated ATPase in rat heart sarcoplasmic reticulum. However, sarcoplasmic reticulum vesicles transported Ca2+ with a high affinity (K0.5 Ca2+ = 0.41 microM) suggesting the presence of a high affinity Ca2(+)-transporting ATPase. Calmodulin did not stimulate rat heart sarcoplasmic reticulum ATPase activity over a range of Ca2+ and Mg2+ concentrations and failed to stimulate membrane phosphorylation and Ca2+ transport into sarcoplasmic reticulum vesicles. Calmodulin antagonists trifluoperazine and compound 48/80 did not affect the ATPase activity. Catalytic subunit of cAMP-dependent protein kinase was also ineffective in stimulating the ATPase activity. These results suggest the presence of an ATPase activity in rat heart sarcoplasmic reticulum with different properties from the high affinity Ca2(+)-pumping ATPase previously characterized in dog heart and other species.
Insights
Rat heart sarcoplasmic reticulum contains a low-affinity, non-specific divalent cation-stimulated ATPase. This differs from the high-affinity Ca2+-transporting ATPase found in other species, as calmodulin and cAMP-dependent protein kinase did not stimulate activity.
Area of Science:
- Biochemistry
- Cardiovascular Physiology
- Molecular Biology
Background:
- Sarcoplasmic reticulum (SR) is crucial for calcium (Ca2+) handling in cardiac muscle.
- Understanding SR ATPase activity is vital for cardiac function.
- Previous studies identified high-affinity Ca2+-transporting ATPases in SR of various species.
Purpose of the Study:
- To characterize the ATPase activity in rat heart sarcoplasmic reticulum.
- To investigate the role of divalent cations (Ca2+ and Mg2+) in SR ATPase activity.
- To determine if calmodulin or cAMP-dependent protein kinase modulate rat heart SR ATPase.
Main Methods:
- Assessed ATPase activity using ATP hydrolysis assays.
- Investigated the effects of varying Ca2+ and Mg2+ concentrations.
- Tested the influence of calmodulin, calmodulin antagonists, and cAMP-dependent protein kinase catalytic subunit.
- Measured Ca2+ transport into SR vesicles.
Main Results:
- Rat heart SR ATPase activity was stimulated by both Ca2+ and Mg2+ at low affinity.
- Mg2+ showed apparent inhibition of Ca2+-dependent ATP hydrolysis.
- Ca2+ transport into SR vesicles occurred with high affinity (K0.5 Ca2+ = 0.41 microM).
- Calmodulin and cAMP-dependent protein kinase did not stimulate ATPase activity, phosphorylation, or Ca2+ transport.
Conclusions:
- Rat heart SR possesses a low-affinity, non-specific divalent cation-stimulated ATPase.
- This ATPase activity differs from the high-affinity Ca2+-pumping ATPase found in other species' hearts.
- Calmodulin and cAMP-dependent protein kinase do not appear to regulate this specific ATPase activity in rat heart SR.