K+ transport in Malpighian tubules ofTenebrio molitorL.: a study of electrochemical gradients and basal K+uptake mechanisms

Abstract

SUMMARYMalpighian tubules of the mealworm Tenebrio molitor were isolated for intracellular measurement of basolateral (Vbl) and,indirectly, apical (Vap) membrane potentials. In control Ringer (50 mmol l-1 K+, 140 mmol l-1Na+), Vbl was 24mV, cell negative, and Vap was 48 mV, cell negative with reference to the lumen. Ion substitution experiments involving K+ and Na+indicated that both Vbl and Vap were sensitive to the bathing K+ concentration, with the change in Vap being 60-77% that of Vbl. A 10-fold drop in bath [K+] irreversibly decreased fluid secretion rates from 6.38±0.95 nl min-1 (mean ± S.E.M.) to 1.48±0.52 nl min-1 (N=8). In the presence of 6 mmol l-1 Ba2+, a blocker of basal K+ channels,fluid secretion rates reversibly decreased and the hyperpolarization of both Vbl and Vap seen in 50 mmol l-1 and 140 mmol l-1 K+ indicated a favourable electrochemical gradient for basal K+ entry. In 5 mmol l-1 K+, Ba2+ induced two different responses: Vbl either hyperpolarized by approximately 10 mV or depolarised by approximately 14mV, according to the electrochemical gradient for K+, which was either inward or outward in low bath[K+]. Rubidium, a `permeant' potassium substitute, caused a hyperpolarization of Vbl, indicating the specificity of K+ channels found in Tenebrio tubule cells. Other possible K+ uptake mechanisms located in the basolateral membrane were investigated. Blocking of the putative electroneutral Na+/K+/2Cl- cotransporter by 10 μmol l-1 bumetanide reversibly decreased fluid secretion rates, with no detectable change in membrane potentials. Ouabain (1 mmol l-1), an Na+/K+-ATPase inhibitor, irreversibly decreased fluid secretion rates but had no effect on electrical potential differences either in the absence or presence of Ba2+. The results implicate K+ channels, the Na+/K+/2Cl-contransporter and the Na+/K+-ATPase in basal K+ and fluid transport of Tenebrio tubule cells.

Intracellular Na+, K+ and Cl− activities in Acheta domesticus Malpighian tubules and the response to a diuretic kinin neuropeptide

“…A similar Ba 2+ -sensitive K + conductance has been identified in other insects (Morgan and Mordue, 1983;O'Donnell and Maddrell, 1984;Baldrick et al, 1988;Leyssens et al, 1992;Neufeld and Leader, 1998;Wiehart et al, 2003;Ianowski and O'Donnell, 2004;Scott et al, 2004). There was no evidence of a significant Na + conductance, but a basolateral Cl -conductance was revealed after K + channels were blocked by Ba 2+ .…”

“…Bumetanide-sensitive Na + /K + /2Cl -cotransport has been demonstrated in the Malpighian tubules of R. prolixus and D. melanogaster (Ianowski et al, 2002;Ianowski and O'Donnell, 2004), and contributes to ion uptake in T. oceanicus (Xu and Marshall, 1999a;Marshall and Clode, 2009), A. aegypti (Hegarty et al, 1991;Scott et al, 2004), Manduca sexta (Audsley et al, 1993;Reagan, 1995), L. migratoria (Al-Fifi et al, 1998), Tenebrio molitor (Wiehart et al, 2003) and F. polyctena (Leyssens et al, 1994). A putative Na + /K + /2Cl -cotransporter has been cloned from the Malpighian tubules of M. sexta (Reagan, 1995), but mRNA encoding its homologue (CG31547) (Pullikuth et -cotransport is driven by the Na + electrochemical gradient ( bNa /F) across the basolateral membrane, which is maintained by active Na + extrusion into the lumen and bath.…”

Mechanisms of K+ transport across basolateral membranes of principal cells in Malpighian tubules of the yellow fever mosquito,Aedes aegypti

“…Effects of the loop diuretics bumetanide and furosemide on insect Malpighian tubules have been reported in the mealworm (Wiehart et al, 2003b), blowfly (O'Donnell and Maddrell, 1984), fruit fly (Linton and O'Donnell, 1999), tobacco hornworm (Audsley et al, 1993;Reagan, 1995), ant (Leyssens et al, 1994), locust (Baldrick et al, 1988) and cricket (Xu and Marshall, 1999b). The consensus of these studies is that bumetanide and furosemide inhibit Na + /K + /2Cl -and/or K + /Clcotransport systems (Baldrick et al, 1988;Gillen and Bowles, 2001;Leyssens et al, 1994) in the basolateral membrane, thereby preventing the entry of NaCl and/or KCl into the cell.…”

Basolateral ion transport mechanisms during fluid secretion byDrosophilaMalpighian tubules: Na+ recycling,Na+:K+:2Cl– cotransport and Cl– conductance

“…These results lead to proposals of vectorial K + transport through K + channels in Malpighian tubules of T. molitor (Wiehart et al, 2003b), A. aegypti (Masia et al, 2000), L. migratoria (Hyde et al, 2001), H. maori (Neufeld and Leader, 1998) and D. melanogaster (Dow et al, 1994a,b).…”

“…Equivalent circuit analysis shows that increasing basolateral membrane resistance with Ba 2+ will cause the loop current to drive the basolateral membrane potential more negative Pannabecker et al, 1992;Weltens et al, 1992;Masia et al, 2000;Wiehart et al, 2003b). In F. polyctena and A. aegypti tubules it has been proposed that the loop current is carried by inward K + flow through basolateral K + channels Weltens et al, 1992;Masia et al, 2000).…”