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K⁺ transport in Malpighian tubules of Tenebrio molitor L.: is a K_ATP channel involved?

U. I. M. Wiehart¹, G. Klein², P. Steels², 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 (14K): [in a new window]   Fig. 1. Effect of glibenclamide on fluid secretion by tubules of Tenebrio. Glibenclamide was tested at 0.1 mmol l-1 (triangles) and 0.5 mmol l-1 (circles) in control Ringer (50 mmol l-1 K+). Secretion rates recovered after stimulation with Tenmo-DH37 (100 nmol l-1). The horizontal bars indicate the time of exposure to glibenclamide (broken bar) and to Tenmo-DH37 (solid bar). Data are presented as means ± 1 S.E.M. for 7-8 tubules.

View larger version (10K): [in a new window]   Fig. 2. Response of basolateral membrane potential (Vbl) to glibenclamide (Glib.). In a low [K+] bath (5 mmol l-1 K+) Vbl responded to glibenclamide by either (A) a small but significant hyperpolarization of 3.6±1.2 mV (P=0.01, n=8) or (B) a significant depolarization of 9±1.5 mV (P=0.008, n=4). Addition of 6 mmol l-1 Ba2+ reinforced the initial response of glibenclamide.

View larger version (12K): [in a new window]   Fig. 3. (A) Effect of ouabain on basolateral membrane potential (Vbl) in low bath [K+] (5 mmol l-1) measured in the presence of Ba2+. (B) Summary of the response of Vbl to ouabain. Data are presented as means ± 1 S.E.M. (n=16, P<0.001).

View larger version (13K): [in a new window]   Fig. 4. Typical experiments showing the effect of ouabain on basolateral membrane potential (Vbl) in the presence of glibenclamide (Glib.) and Ba2+. The hyperpolarization (A) or depolarization (B) of glibenclamide and Ba2+ is followed by a depolarization of Vbl with the addition of 1 mmol l-1 ouabain. The experiments were carried out in low bath [K+] (5 mmol l-1).

View larger version (15K): [in a new window]   Fig. 5. (A) Response of basolateral membrane potential (Vbl) to different bathing K+ concentrations in the presence of Ba2+. The depolarization of Vbl when changing the [K+] from 5 mmol l-1 to 140 mmol l-1 is followed by a repolarization after 3-8 min. (B) Summary of the effect of Ba2+ in various [K+]. Data are presented as means ± 1 S.E.M. (n=6).

View larger version (8K): [in a new window]   Fig. 6. Slow response of basolateral membrane potential (Vbl) to different bathing K+ concentrations in the presence of glibenclamide (Glib.) and Ba2+. The depolarization of Vbl when changing the [K+] from 5 mmol l-1 to 140 mmol l-1 is no longer followed by a repolarization (n=6).