APPS November 2002 Meeting Abstract 1361


THE EFFECT OF THE PHOSPHOLIPASE A2 INHIBITORS INDOMETHACIN AND QUINACRINE ON DEPOLARISATION-INDUCED FORCE RESPONSES IN SKINNED SKELETAL MUSCLE FIBRES OF THE RAT

Anthony J. Bakker1, Renzhi Han1, Miranda D. Grounds2, Takuya Suizu1, 1 School of Biomedical and Chemical Sciences, 2 School of Anatomy & Cell Biology, The University of Western Australia, Crawley, WA 6009, Australia.

In mechanically skinned fibres, depolarisation-induced force responses (DIFRs) can be elicited by the normal excitation-contraction coupling (ECC) process. However, ECC eventually fails and the DIFRs run down to zero. Elevations in cytosolic Ca2+ ([Ca2+]c) also abolish ECC in skinned fibres, which has led to the hypothesis that elevations in [Ca2+]c are responsible for the run down process, although the mechanism by which Ca2+ acts is unknown1. Elevated [Ca2+]c has previously been shown to induce sarcolemmal damage in skeletal muscle2, and inhibitors of the Ca2+-sensitive enzyme phospholipase A2 (PLA2) have been shown to prevent the sarcolemmal damage2. Therefore, in this study we examined the effects of PLA2 inhibitors on DIFRs and sarcoplasmic reticulum (SR) function in skinned skeletal muscle fibres.

Skinned fibres were dissected from the extensor digitorum longus muscle of Wistar rats killed by CO2 overdose. DIFRs was elicited by replacing the K+-based bath solution with one containing predominantly Na+.

The PLA2 inhibitors indomethacin (200 ÁM) and quinacrine (200 ÁM) reversed the run-down of DIFRs in skinned EDL fibres. Fibres exposed to indomethacin after complete run-down produced DIFRs that were significantly larger (2.2 ▒ 0.5 times, p=0.04, n=7) and much wider (12.4 ▒ 2.1 times, p=0.001, n=7) than initial control responses. The PLA2 inhibitors aristolochic acid (100 ÁM) and NDGA (6 ÁM) had no effect on DIFRs in skinned fibres (n=5). Indomethacin also significantly increased SR Ca2+ loading (117.2 ▒ 3.0 % of controls, p=0.0008, n=8) in the skinned fibres, and fura-2 measurements indicated that indomethacin could also induce intracellular Ca2+ release in a dose-dependent manner in C2C12 myotubes.

In conclusion, the PLA2 inhibitors indomethacin and quinacrine restore DIFRs in skinned skeletal muscle fibres after run-down, possibly by increasing ability of the SR to take up and release Ca2+. However, the recovered DIFRs do not resemble initial controls.

(1) Lamb GD, Junankar PR, Stephenson DG. Journal of Physiology. 1995;489:349-362.

(2) Duncan CJ, Jackson MJ. Journal of Cell Science. 1987;87:183-188.


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