Ion Transport by Turtle Colon: A Role for Volume-Sensing Transporters in the Basolateral Membrane

Abstract

It is now widely appreciated that cells have evolved a variety of mechanisms for regulating intracellular composition. Prominent among these are a wide range of so-called volume regulatory mechanisms which permit the cell to adjust its solute composition, and hence its water content, in the face of potentially injurious changes in extracellular osmolarity. Lymphocytes, for example, possess well-characterized transport mechanisms that, when activated, promote solute loss or gain and resist osmotically induced cell swelling or shrinking (Grinstein et al. 1984). Epithelial cells face an additional hazard, the “other problem” which is encountered as a direct result of high rates of transcellular solute movement. The cell must be protected from the potentially enormous changes in cell composition which could result if solute influx and efflux become “unbalanced”. Schultz (1981) has used the graphic term “flush through” to describe the problem confronting salt-transporting epithelial cells. It is likely, given the extremes of throughput encountered by epithelial cells, that they have evolved a variety of mechanisms for avoiding the disaster of “flush through”. In this review we will consider two components of a potential volume regulatory mechanism which we have found to be prominent in the cells of the reptilian colon, a basolateral K⁺ conductance (Dawson 1987; Dawson and Chang 1990) and a basolateral Na/H antiporter (Post and Dawson 1992). Although it is a bit presumptuous to refer to these as volume “sensors”, it is clear that both are activated (or inactivated) by changes in cell volume.