The effects of Suramin on Ca2+ activated force and sarcoplasmic reticulum Ca2+ release in skinned fast-twitch skeletal muscle fibers of the rat

Abstract

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 kg^-1) Suramin remains in skeletal muscle in effective concentrations (11.6 μg mL^-1; 84 days) for prolonged periods. These observations provide a strong rationale for investigation of the specific effects of Suramin on skeletal muscle function. Single mechanically skinned fibers were directly exposed to Suramin (10, 100 or 500 μmol L^-1) for defined durations (2-10 min) in controlled physiological solutions that mimic the intracellular ionic environment of a fiber. Suramin treatment (10-500 μmol L^-1) directly affected the contractile apparatus in a dose-dependent manner causing a decrease in Ca²⁺-sensitivity (pCa50 = -log (Ca²⁺) concentration, where 50% of maximum Ca²⁺- activated force is produced) by 0.14 to 0.42 pCa units and reduction in maximum Ca²⁺-activated force by 14 to 62%. Suramin treatment (100 μmol L^-1 for 10 min and 500 μmol L^-1 for 2 min) also caused development of a Ca²⁺-independent force corresponding to 2.89 ± 4.33 and 16.77 ± 7.50% of pretreatment maximum Ca²⁺-activated force, respectively. Suramin treatment (100 μmol L^-1, 2 min) also increased the rate of sarcoplasmic reticulum (SR) Ca²⁺ release without significant changes in SR Ca²⁺ uptake. We report new functional effects for Suramin related to alterations in both the contractile apparatus and SR Ca²⁺-handling of skeletal muscle that may contribute to the peripheral muscle weakness noted in human pharmacological treatments.

Keywords: EC‐coupling; mechanically skinned; skeletal muscle; suramin.

Figure 1

(A) Representative relationship of relative force (percentage of maximal activated force) as a function of calcium concentration (pCa). (B) Mathematical derivation from force‐pCa curve of the Ca²⁺‐force relationship.

Figure 2

Representative force recording of the change in force level in an EDL mechanically skinned muscle fiber following activation in solutions of different pCa before and after a 2 min exposure to 500 μmol L⁻¹ Suramin. Each consecutive arrow indicates a change to a solution with a higher free [Ca²⁺] (lower pCa) (pCa values from left to right 8.40, 6.40, 6.22, 6.02, 5.88, 5.75, 5.48, and 4.50). [Ca²⁺] was increased until a stable maximum force level was achieved. Note the depression of Ca²⁺‐activated force levels following Suramin treatment. The significant elevation in resting force (during the wash period) and the depression of Ca²⁺‐activated force levels following Suramin treatment were comparable to longer treatment at 100 μmol L⁻¹ Suramin.

Figure 3

Force‐pCa relationships for individual extensor digitorum longus mechanically skinned muscle fibers exposed to 10 μmol L⁻¹ Suramin (2 min). (A) Changes in fiber sensitivity to Ca²⁺ occurring from Suramin exposure. All force values are represented as a percentage of force produced at pCa 4.5 in at either the control or Suramin protocol. (B) Change is absolute force following Suramin exposure. All values are represented as the percentage relative to pCa 4.5 pre‐treatment (Control). Control values denoted as □; Suramin treatment denoted as ▵.

Figure 4

The conversion of representative force transients (% of maximum) into free Ca²⁺ transients for a single mechanically skinned extensor digitorum longus muscle fiber after 120 sec Ca²⁺ loading of the SR. Representative force transients for (A) control and (C) post–Suramin treatment (100 μmol L⁻¹, 2 min) conditions. The calculated free calcium transients (μmol L⁻¹) for (B) control and (D) force transients, using the conversion equation (1).

Figure 5

Suramin effects on key characteristics of Ca²⁺‐transients derived from caffeine‐induced force transients in single mechanically skinned extensor digitorum longus muscle fibers. (A and B) Peak concentration of Ca²⁺ transients (% of 120 sec Control); (C and D) Rate of change in Ca²⁺ (measured 20–80% of peak level) (% of 120 sec Control, measured as mol s⁻¹); and (E and F) The relative area under free calcium transients. (A, C and E) Control (no Suramin); (B, D and F) 100 μmol L⁻¹ Suramin (2 min). Results were obtained from six rats.