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 µM) and quinacrine (200 µM) 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 µM) and NDGA (6 µM) 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.