K+ current stimulation by Cl- in the midgut epithelium of tobacco hornworm (Manduca sexta). I. Kinetics and effect of Cl(-)-site-specific agents

The antidiuretic neurohormone RhoprCAPA-2 downregulates fluid transport across the anterior midgut in the blood-feeding insect Rhodnius prolixus

Osmotic balance in insects is regulated by the excretory system, consisting of Malpighian tubules and the gut under the control of diuretic and antidiuretic factors. Terrestrial insects must conserve water, and antidiuresis is the norm, only interrupted by brief diuretic periods. Surprisingly, little is known about antidiuresis in insects. Two antidiuretic strategies have been described. The first antidiuretic mechanism involves the reabsorption of fluid from the primary urine in the hindgut. More recently, a second antidiuretic strategy was reported, consisting of inhibition of primary urine formation by the Malpighian tubules. Recently, we isolated, characterized, and cloned the gene encoding for the antidiuretic neurohormone (the neuropeptide RhoprCAPA-2) acting on the Malpighian tubules of Rhodnius prolixus. Here we describe a third, novel mechanism central to the antidiuretic strategy of R. prolixus, the inhibition of ion and fluid transport across the anterior midgut by RhoprCAPA-2. Our results show that RhoprCAPA-2 (1 micromol/l) reduces serotonin-stimulated fluid transport from 83 +/- 11 to 12 +/- 12 nl/min and equivalent short-circuit current from 20 +/- 4 to 5 +/- 0.7 microA/cm(2) in diuretic hormone-stimulated anterior midgut. RhoprCAPA-2 appears to function independently of intracellular cGMP or Ca(2+) in the midgut. Thus, the antidiuretic neurohormone RhoprCAPA-2 has multiple target tissues, and we hypothesize that RhoprCAPA-2 functions to coordinate the transport activity of the anterior midgut and Malpighian tubules so that the rate of fluid transport into the haemolymph by the anterior midgut matches the transport rate of Malpighian tubules to maintain the volume and ion composition of haemolymph.

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

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.

K+ current stimulation by Cl- in the midgut epithelium of tobacco hornworm (Manduca sexta). II. Analysis of Ba(2+)-induced K+ channel conduction noise

Chloride-stimulated K+ secretion by Manduca sexta midgut (5th-instar larvae) was measured as K(+)-carried short-circuit current of the tissue mounted in an Ussing chamber. "Microscopic" parameters, such as single-channel current and channel density for the rate-determining passive transport step across the basolateral goblet cell membrane (i.e. K+ channels), were estimated by means of current-fluctuation analysis of the K+ channel blockade by haemolymph-side Ba2+ ions. Ba2+ was equally effective with Cl- or gluconate (Glu-) as the principal ambient anion. The Ba(2+)-induced K+ channel conduction noise is reflected by a Lorentzian, or relaxation, noise component in the power spectrum of the K+ current fluctuations. A reduced Lorentzian plateau value, but an unchanged corner frequency, were observed when Cl- was replaced by Glu-. The results from the analysis of a "two-state" model of K+ channel block by Ba2+, with respect to the anion-replacement effects, suggest that the observed changes in K+ current and Lorentzian plateau value mirror a complex change of the underlying parameters: Cl- omission reduces single channel current but increases channel density so that the product of single channel current and channel density is smaller in Glu- than in Cl-. It seems likely that basolateral K+ channels (1) are subject to anionic gating ligands, and (2) depend on anions with respect to the rate of K+ transfer through an open K+ channel.