Effects of guandinoethane sulfonate on contraction of skeletal muscle

Guanidinoethane sulfonic acid (GES), a chemical and biological analog of taurine, decreases rat muscle taurine content when added to drinking water. Over the same period, GES appears in muscle(8). GES supplementation is often used to study the effect of taurine depletion on physiological mechanisms, without taking into account the possible actions of GES. The purpose of the present study was to investigate the specific actions of GES on contraction of skeletal muscle. In mice EDL muscle, the time delay needed to observe a 20% force decrease after the end of a tetanic stimulation was higher in GES-supplemented than in control muscle. This observation in GES-supplemented muscle could be explained by the action of taurine or GES on several targets, beside others the rate of Ca(2+) uptake by sarcoplasmic reticulum (SR) and the Ca(2+) sensitivity of myofilaments. SR of rat EDL was isolated by successive centrifugations. The effect of 20 mM taurine or GES on the rate of Ca(2+) uptake by SR was measured with the fluorescent Ca(2+) indicator fura-2. The results show that the rate of Ca(2+) uptake by SR is not modified in the presence of taurine or GES. The Ca(2+) sensitivity of myofilaments was studied in chemically skinned fibers in the presence of 20 mM taurine or GES. Both taurine and GES increased the myofilament sensitivity to Ca(2+). Thus, the prolonged relaxation time of GES-supplemented muscle can be attributed to an increase in myofilament sensitivity to Ca(2+). This higher sensitivity is not due to a decrease in muscle taurine content but rather to an increased GES concentration.