Key result
High-affinity Ca2+-ATPase unlikely solely drives ATP-dependent Ca2+ uptake due to differing radiation and inhibitor sensitivities.
Why the study?
The relationship and distinct roles of high-affinity Ca2+-ATPase and ATP-dependent Ca2+ uptake in dog antrum smooth muscle plasma membrane were unclear.
The study demonstrates that the high-affinity Ca2+-ATPase in dog antrum smooth muscle plasma membranes is distinct from the protein responsible for ATP-dependent Ca2+ uptake.
Answers are grounded in this paper and cite their sources.
Questions Ca2+-ATPase exclusivity in antral smooth muscle Ca2+ handling; leaves open the dominant ATP-dependent uptake mechanism for further basic research.
The plasma membrane-enriched fraction from dog antrum smooth muscle is enriched in ATP-dependent azide-insensitive Ca2+ uptake (0.3-0.4 microM Ca2+ required for half-maximal activity), a high-affinity Ca2+-ATPase (Km of 0.3-0.8 microM for Ca2+), a low-affinity Ca2+-ATPase (Km for 250-400 microM for Ca2+), and a Mg2+-ATPase. Studies using membranes washed with EDTA and assay media treated with Chelex 100 showed that the high-affinity Ca2+-ATPase did not depend on contaminating Mg2+. Thus, whereas the ATP-dependent Ca2+ uptake had an absolute requirement for Mg2+, the Ca2+-ATPases did not. Studies using gamma-irradiation showed that the protein responsible for the ATP-dependent Ca2+ uptake was inactivated at significantly lower doses of radiation than the three ATPases. The Ca2+ uptake and the high-affinity Ca2+-ATPase also differed in their inhibition by calmodulin antagonists and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Thus it is unlikely that the high-affinity Ca2+-ATPase by itself is responsible for the ATP-dependent Ca2+ uptake.
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Grover et al. (1985) studied this question. The high-affinity Ca2+-ATPase in dog antrum smooth muscle plasma membrane is unlikely to be solely responsible for ATP-dependent Ca2+ uptake due to differing sensitivities to radiation and inhibitors.
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