K+ transport in the caterpillar intestine epithelium: role of osmolytes for the K+-secretory capacity of the tobacco hornworm midgut

Abstract

The midgut of the tobacco hornworm, Manduca sexta, actively secretes potassium ions. This can be measured as short-circuit current (I-SC) with the midgut mounted in an Ussing chamber and superfused with a high-K+ saline containing as its major osmolyte 166 mM sucrose. Iso-osmotic substitution of sucrose by non-metabolisable compounds (mannitol, urea, NaCl and the polyethylene glycols 200, 400 and 600) led to a dramatic, though reversible, drop in the current. Acarbose, a specific inhibitor of invertase (sucrase) in vertebrates and insects, had no detectable influence on I-SC. Unexpectedly, after replacing sucrose iso-osmotically with the saccharides glucose, fructose, trehalose or raffinose, the K+ current could no longer be supported. However, all osmolytes smaller than sucrose (except for NaCl), metabolisable or not, initiated an immediate quite uniform but transient, increase in I-SC by about 20%, before its eventual decline far below the control value. Hypo-osmotic treatment by omission of sucrose also transiently increased the K+ current. Small osmolytes substituted for sucrose caused no transient I-SC stimulation when the epithelium had been challenged before with hypo-osmolarity; however, the eventual decline in I-SC could not be prevented. Our data seem inconsistent with a role of sucrose as energiser or simple osmolyte. Rather, we discuss here its possible role as analogous to that of sucrose in lower eukaryotes or plants, as an extra- and/or intracellular ``compatible osmolyte'' that stabilises structure and/or function of the proteins implicated in K+ transport.