AJP - Cell Physiology, Vol 242, Issue 3 242-C249, Copyright © 1982 by American Physiological Society
Influence of ATP on sarcoplasmic reticulum function of vascular smooth muscle
G. D. Ford and M. L. Hess
A vesicular fraction isolated from bovine aorta and enriched in fragmented
sarcoplasmic reticulum (FSR) exhibited active calcium transport and ATPase
activity. By use of a hypotonic NaHCO3 extraction solution, an active
preparation was isolated that retained activity for up to 4 days. A small
but significant (P less than 0.05) Ca2+-stimulated, Mg2+-dependent ATPase
associated with calcium transport was demonstrated with a specific activity
of 0.33 mumol inorganic phosphate (Pi).mg-1.min-1. The basal Mg2+ ATPase
demonstrated Michaelis-Menten kinetics [Km(Mg2+-ATP) = 0.44 +/- 0.01 X
10(-3) M; Vmax = 2.22 +/- 0.01 mumolPi.mg-1.min-1]. The Ca2+-stimulated,
Mg2+-ATPase demonstrated apparent substrate inhibition (Ks approximately 10
mM) with no evidence for end-product (ADP) or excess added Ca2+
contributing to this inhibition. Oxalate-supported active calcium uptake
velocities also exhibited quantitatively similar substrate inhibition.
These results suggest that FSR from vascular smooth muscle contains either
two enzymes or one enzyme with two isomeric forms, one of which is
associated with the calcium uptake activity of this structure and the other
of unknown function.
