Studies of Mechanical Properties and Intracellular Ca2+ Transients in Frog Single Muscle Fibres

Abstract: Intracellular Ca2+ transients, monitored by the calcium-sensitive dye, fluo-3, were studied during isometric twitches and tetani in intact single muscle fibres of the frog (sarcomere length, 2.2 mm; temperature, 2-5 or 10-12 ºC). The Ca2+ transient during twitch stimulation was very brief when compared with the isometric force. In a population of fibres studied, a greater amplitude of the Ca2+ transient was associated with a more rapid decay of [Ca2+]i. The twitch force was found to be inversely related to the rate of decay of [Ca2+]i. Evidence is presented to show that the decay phase, rather than the amplitude, of the Ca2+ transient is an important determinant of the mechanical response during an isometric twitch. [Ca2+]i declined steeply during the initial phase of force relaxation, but exhibited a secondary rise during the pseudoexponential fall of tension. The latter phenomenon can be attributed to enhanced release of calcium from the contractile system induced by non-uniform length changes during relaxation. BAPTA, a high-affinity calcium chelator, was loaded into the myofibrillar space and was found to reduce the state of activation of the contractile system. However, this effect of BAPTA was not strictly correlated to the change of the Ca2+ transient. It is proposed that BAPTA, in addition to its calcium-chelating effect, also interferes with the binding of calcium to troponin. 2,3-Butanedione monoxime (BDM, £5 mM) markedly reduced the isometric force but only marginally affected the Ca2+ transient, suggesting that BDM mainly affects the crossbridge kinetics. Experiments were performed to further explore the nature of the biphasic shape of the force-velocity relation.