Non-specific adsorptive pinocytosis.

Unlike phagocytosis, which is typically a substrate-induced phenomenon, pinocytosis appears to be a constitutive activity in most animal cell types (see Pratten et al., 1980; Steinman et al., 1983). Pinocytic vesicles are constantly being formed from invaginations of the plasma membrane, irrespective of the availability of substrates; hence the analogies of a paternoster-style elevator (Lloyd, 1980) or an escalator (Steinman et al., 1983) have been used. Although the rate of pinosome formation is usually unaffected by substrate, constitutive pinocytosis is eminently capable of substrate selection. Substrates that enter cells by this route can do so either non-adsorptively (in the fluid phase) or adsorbed to the plasma membrane that is being internalized. Several substrates may be captured by the same pinocytic invagination, the rate of uptake reflecting in each case the concentration of the molecule in the ambient fluid, its strength of binding to the membrane binding sites, and the abundance of these sites on those regions of the plasma membrane that are internalized. In addition to this ubiquitous cellular phenomenon, there are reports of pinocytic events that are highly substratespecific: either the pinocytic invaginations form only in response to a ligand attaching to a cell-surface receptor or the pinosomes contain exclusively one receptor-bound substrate, excluding other substrates, or both. In many cases of substrate uptake by pinocytosis it is not yet known whether capture is by the constitutive mode or by a substrateinduced event. Here we suggest applying the principle known as Occam's razor. It is clearly absurd to postulate a separate class of event for every substrate, natural or synthetic, found to be captured by pinocytosis. It would be wise therefore to assume uptake of a substrate to be by constitutive pinocytosis until evidence to the contrary is found. I t has been the traditional understanding for some 2 decades that substances captured by pinocytosis are routed to the lysosomes. This view is now seen as over-simple: some pinosomes avoid the lysosomes and fuse again with the plasma membrane, releasing their contents to the same or to another extracellular compartment. Nevertheless, pinocytosis followed by delivery to the lysosomes is a major route in most cells, and one seen by many investigators to have interesting therapeutic potential in two fields. First, the lysosomal storage diseases, unlike most inborn errors of metabolism, are uniquely amenable to 'replacement therapy' with exogenous enzymes. Secondly, chemotherapeutic agents attached to macromolecules can enter cells only by pinocytosis, offering the potential for tissue-specific action if tissue-specific uptake can be achieved. Both of these novel approaches to therapy depend upon effective and if possible, cell-specific pinocytic uptake. There is therefore much current interest ir? identifying those features of a macromolecule that lead to its avid uptake by cells. The publication recently of two excellent reviews on the mechanisms and substrate specificity of pinocytosis (Besterman & Low, 1983: Steinman et a/., 1983) obviate the necessity of a lengthy account here. The remainder of the present paper concerns some important aspects of methodology and reports recent investigations on non-specific pinocytosis in the authors' laboratories.