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K⁺ transport in Malpighian tubules of Tenebrio molitor L.: a study of electrochemical gradients and basal K⁺ uptake mechanisms

U. I. M. Wiehart¹, S. W. Nicolson¹ and E. Van Kerkhove^2,*

¹ Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa
² Laboratory of Physiology, Biomed CMK, Limburgs Universitair Centrum, B3590 Diepenbeek, Belgium

View larger version (11K): [in a new window]   Fig. 1. Fluid secretion rates of Tenebrio Malpighian tubules in response to different bath K+ concentrations. Increasing the K+ concentration to 140 mmol l-1 K+ stimulated fluid secretion rates significantly (P<0.02). A 10-fold decrease in the K+ concentration irreversibly decreased fluid secretion rates (P=0.01). Data are presented as means ± S.E.M. (n=8 for both secretion assays).

View larger version (14K): [in a new window]   Fig. 2. (A) The effect of changing the external K+ concentration on basolateral (Vbl) and transepithelial (Vte) membrane potentials in a paired experiment. Vbl hyperpolarizes in a low K+ concentration and depolarizes in a high K+ concentration. Vte hyperpolarizes as the K+ concentration increases. (B) Vbl as a function of log bath [K+] shows a slope of 28 mV decade-1; y=28x-70. Each point represents means ± S.E.M. of Vbl.

View larger version (10K): [in a new window]   Fig. 3. Summary of the potential profiles across Tenebrio Malpighian tubules measured in three different K+ concentrations, mostly during unpaired experiments. Vbl, basolateral membrane potential; Vap, apical membrane potential; Vte, transepithelial membrane potential.

View larger version (13K): [in a new window]   Fig. 4. The reversible effect of 6 mmol l-1 barium on (A) the fluid secretion rate and (B) the basolateral membrane potential (Vbl) of isolated Malpighian tubules (means ± S.E.M., N=8 tubules). The horizontal bar in A indicates the time of exposure to barium.

View larger version (15K): [in a new window]   Fig. 5. Two different responses of basolateral membrane potential (Vbl) to barium in the presence of a low K+ concentration. (A) Significant hyperpolarization of the Vbl was seen in eight impalements (P<0.01) and (B) a depolarization was seen in 13 impalements (P<0.001). The break in the line in B was due to disturbances; time elapsed was 3 min.

View larger version (13K): [in a new window]   Fig. 6. Summary of the changes in basolateral membrane potential (Vbl) and transepithelial membrane potential (Vte) in response to the addition of 6 mmol l-1 barium observed in different bath K+ concentrations. The solid line indicates the normal potential profile and the dotted line indicates the potential profile in the presence of barium. Asterisks indicate significant differences (P<0.05) between mostly unpaired experiments.

View larger version (12K): [in a new window]   Fig. 7. The effect of 10 µmol l-1 bumetanide on the secretion rates of Tenebrio tubules (means ± S.E.M., N=14 tubules). The horizontal bar indicates the time of exposure to bumetanide.

View larger version (12K): [in a new window]   Fig. 8. The irreversible effect of 1 mmol l-1 ouabain on the secretion rates of Tenebrio tubules. Each point represents the means ± S.E.M. for eight tubules (P<0.01). The horizontal bar indicates the time of exposure to ouabain.