Puerto de la Cruz, Tenerife (2003) J Physiol 548P,
P131
Poster Communications: Effects of KB-R7943 on sodium-withdrawal contractures in rat slow-twitch skeletal muscle
Wissam H. Joumaa, Aicha Bouhlel and Claude Léoty
Laboratoire de Physiologie Générale, CNRS UMR 6018, Faculté des Sciences et des Techniques, Université de Nantes, 2 rue de la Houssinire, B.P. 92208, 44322 Nantes, FranceIn skeletal muscle the evidence supporting the functional role of Na+/Ca2+exchange is based on results obtained when the extracellular medium was switched to a low-Na+ solution. Under these conditions the exchanger operates in its reverse mode transporting Ca2+ into the cell. Recently KB-R7943 (KBR) has been reported to inhibit in cardiac cells the reverse mode of Na+/Ca2+ exchange with a lower potency for the forward mode and other ion transport systems (Iwamoto et al. 1996). The aim of the present study was to test the effect of this novel agent on sodium-withdrawal contractures generated in slow skeletal muscle.Experiments were conducted on small bundles (2-4 cells) isolated from soleus muscle and mounted in the experimental chamber already described by Joumaa et al. (2002). Adult male Wistar rats were anaesthetised by an ether vapour flow and then killed by cervical dislocation. Experiments were performed in intact fibres in the presence of 2 µM cyclopiazonic acid (CPA), which was shown to improve low-Na+ contracture (Mme et al. 1997).
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In intact fibres, exposure to Na+-free medium induced repeatable transient contracture with an amplitude of 107.3 ± 4.4 mN, a time-to-peak tension of 7.9 ± 0.4 s and a time constant of relaxation of 13.1 ± 1.5 s (means ± S.E.M., n = 10). KBR induced a dose-dependent decrease in the amplitude of Na+-free contracture and up to 50 µM the effects were fully reversible. For each experiment the log dose-response curve for inhibition was fitted according to a sigmoid equation. The apparent Ki and the Hill coefficient were 11.8 ± 1.9 µM and 1.7 ± 0.4, respectively (means ± S.E.M., n = 10).
In saponin-skinned fibres, the amount of Ca2+ taken up at 1 min of loading time in pCa 7.0 solution was estimated by using the amplitude of the contracture due to caffeine application (10 mM). In the presence of KBR there was no significant change in the reduction in amplitude of caffeine contracture by CPA (control: 0.74 ± 0.05 mN; CPA: 0.59 ± 0.03 mN; KBR (20 µM): 0.57 ± 0.04 mN, n = 12, P < 0.05, Student’s paired t test).
In Triton X-100 fibres, the Ca2+ sensitivity of contractile proteins (pCa50) was not significantly modified in the presence of 2 µM of CPA and/or KBR (5-20 µM) (control: 6.312 ± 0.009; CPA: 6.294 ± 0.015; KBR (20 µM): 6.304 ± 0.008, n = 9, P > 0.05).
It is proposed that KBR could be useful to study under normal physiological conditions the role of the exchanger in the excitation-contraction coupling of mammalian skeletal muscles.
