The concentration of free Ca 2+ in the sarcoplasmic reticulum of frog cut twitch skeletal muscle fibers estimated with tetramethylmurexide

The contractile response in skeletal muscle is controlled by the\ud time course and magnitude of Ca2+ release from the sarcoplasmic\ud reticulum (SR) into the myoplasm. In order to gain an understanding\ud of how SR Ca2+ release is regulated, it is often useful to estimate\ud the permeability of the SR to Ca2+ as this provides a measure of\ud the extent of activation of SR Ca2+ release channels. The permeability\ud of the SR is proportional to the rate of Ca2+ release normalized\ud by the driving force for release which is primarily determined by\ud the concentration of free calcium in the SR (denoted [Ca2+]SR).\ud In past studies (e.g. [1–4]), the driving force for Ca2+ release was\ud taken to be the concentration of total calcium in the SR (denoted\ud [CaT]SR) rather than [Ca2+]SR owing to the lack of information about\ud [Ca2+]SR. The primary aims of the experiments in this article were\ud to establish the resting value of [Ca2+]SR (denoted [Ca2+]SR,R) with\ud the absorbance indicator tetramethylmurexide (TMX) and, perhaps more importantly, the ratio of [Ca2+]SR,R and TMX’s apparent dissociation\ud constant. This latter information is needed to establish\ud the time course of [Ca2+]SR with TMX during SR Ca2+ release and,\ud thereby, the time course of the true release permeability.\u