The use of subcellular membrane fractions in analysis of control of smooth muscle function.

Abstract

1) Use of purified subcellular membranes from smooth muscle offers an approach to study mechanisms of Ca-handling which may eliminate some problems which have delayed progress and confused interpretation of experiments with intact tissues. 2) With techniques available to date it has been possible to obtain satisfactorily purified plasma membrane vesicles and mitochondria, but endoplasmic reticulum vesicles have not been obtained in satisfactory purity. 3) Failure to isolate pure endoplasmic reticulum fractions may be caused by its similar densities to plasma membrane and to the larger quantity of plasma membrane as well as by differential damage to ER vesicles by the isolation techniques used. If oxalate-activation of ATP-dependent Ca2+-transport proves to be a property unique to ER, the resultant increase in density of ER vesicles will aid their isolation. If so, and if the causes of differential loss of this transport function during isolation can be identified and avoided, then study of the properties of purified endoplasmic reticulum should be possible. 4) Purified plasma membrane vesicles have been shown to possess(soL) a) an ATP-dependent pump capable of extruding Ca2+ from cell and lowering the internal Ca2+ to about 10−7 M; b) a Na−Ca2+ exchange system which appears to have a limited velocity and capacity compared to heart and to nerve, but so far it has not been studied unter optimal conditions (e.g. with a maintained Na+ gradient); c) a pH-dependent, ATP-dependent, high affinity Ca binding which may participate in excitation-contraction coupling; d) a pH-independent, low affinity Ca-binding which has not been studied in detail.