Placental transport studied by means of isolated plasma membrane vesicles

Abstract

It has recently become possible to isolate relatively easily pure preparations of membranes from the brush border as well as from the basal surface of the term human trophoblast . Such membranes reseal to form closed structures which are called 'vesicles'. Many studies on solute transport have been performed using such vesicles because they permit experimental investigation of transport systems, and of their driving forces and regulation at the level of the membrane transport proteins responsible for membrane solute transport. Such an analysis cannot readily be performed using other experimental methods. In the present article several transport systems present in the human placenta which have been studied by means of vesicle experiments will be reviewed. Attention will also be drawn to the limitations and potential pitfalls of such experiments. QUESTIONS TO BE ASKED WHEN USING MEMBRANE VESICLES As with all scientific analysis the first and most important question to be asked is 'what is the experimental model being used for?' In most instances, studies with vesicles are used to analyse, at the membrane level, physiological phenomena which have been described in more complex systems, for example in vivo or using perfused placentas or in experiments on intact trophoblast in vitro. They also can be used to establish the presence and properties of transport systems which previously have not been studied, although in this case the physiological significance of experimental findings may prove difficult to establish. In this instance the use of vesicle methodology is no different than the use, for example, of enzyme biochemistry to study metabolic processes; indeed transport systems be they carrier-, pump-or channel-mediated may be considered as 'vectorial enzymes' separating two fluid phases and it is the investigation of the properties of such 'enzymes' that is the aim of these experiments. Importantly vesicles also allow the investigation of mechanisms of control of transport, for example by altering the extent of transport protein phosphorylation. Assuming that the experimental aim has been carefully thought about, more practical problems then need to be considered. The first of these concerns the purity of the membranes isolated. Since the work of Smith et al. (1974) brush-border-membrane vesicles have been readily available using very straightforward biochemical methods (see for example, Kudo et al. 1987; Glazier et al. 1988). More recently C. H. Smith and colleagues (Kelly et al. 1983) have developed a method for the isolation of basal membrane vesicles which has been successfully employed as will be discussed later. Marker enzyme (or receptor) analysis, using alkaline phosphatase (EC 3.1.3.1) for the