Physiological insights into Suramin interactions with excitation-contraction coupling in skeletal muscle

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Life Sciences, Faculty of Science, Technology & Engineering, La Trobe University, Bundoora.

Suramin has long been used in the treatment of various human diseases. Intravenous infusions of Suramin are commonly administered to patients over extended periods of time but there are a number of significant contraindications with peripheral muscle weakness being one of the most frequently reported. Previous work has shown that even after a single infusion (300 mg per kg) Suramin remains in skeletal muscle in effective concentrations (11.6 µg per ml; 84 days) for prolonged periods. These observations provide a strong rationale for investigation of the specific effects of Suramin on skeletal muscle function. We hypothesised that Suramin can alter skeletal muscle contractile functions, providing possible explanations for the reported muscle weakness in patients. Single, mechanically skinned fibres from the EDL muscle of Long Evans Rats provided an experimental preparation with exquisite direct access to and potential control of the excitation, Ca2+ handling and contractile machinery of the muscle fibre. Various Suramin concentrations (2 µM - 500 µM) were used in these studies, reflecting the full range of concentrations that may be evident in human treatment regimes. We identified a number of significant Suramininduced effects on muscle function. At high doses (100 µM, 500 µM) Suramin was able to remove the regulatory complex Troponin from the contractile proteins. Furthermore, at all concentrations Suramin inhibited myosin ATPase activity, resulting in significant reductions in total muscle force. Suramin also affected the function of the sarcoplasmic reticulum (SR) by directly activating the release of Ca2+ from the SR and by antagonising the CaM RYR1 interactions involved in Ca2+ release and the pump activity (SERCA) underpinning Ca2+ uptake. Finally, the coupling between DHPR RYR, essential for coupling of membrane excitation to contraction, was affected by Suramin (10 µM) through a reversible inhibition of the T-tubular sodium potassium ATPase. The implications of these findings are discussed in relation to their potential to contribute to the contraindications of muscle weakness in clinical applications.