AJP - Cell Physiology, Vol 248, Issue 5 449-C456, Copyright © 1985 by American Physiological Society
Calcium pump, high-affinity Ca2+-ATPase, and other ATPases in dog antrum smooth muscle plasma membrane
A. K. Grover, C. Y. Kwan and P. J. Oakes
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.
