PERTURBATION OF SARCOPLASMIC-RETICULUM CALCIUM RELEASE AND PHENOL RED ABSORBENCY TRANSIENTS BY LARGE CONCENTRATIONS OF FURA-2 INJECTED INTO FROG SKELETAL-MUSCLE FIBERS

Intact single twitch fibers from frog muscle were studied on an optical bench apparatus after micro-injection with two indicator dyes: phenol red, to monitor a previously described signal (denoted ΔpHapp; Hollingworth and Baylor. 1990. J. C, en. Physiol. 96:473-491) possibly reflective of a myoplasmic pH change following action potential stimulation; and fura-2, to monitor the associated change in the myoplasmic free calcium concentration (Δ[Ca2+]). Additionally, it was expected that large myoplasmic concentrations of fura-2 (0.5-1.5 mM) might alter ΔpHapp, since it was previously found (Baylor and Hollingworth. 1988. J. Physiol. 403:151-192) that the Ca2+-buffering effects of large fura-2 concentrations: (a) increase the estimated total concentration of Ca2+ (denoted by Δ[CaT]) released from the sarcoplasmic reticulum (SR), but (b) reduce and abbreviate Δ[Ca2+]. The experiments show that ΔpHapp was increased at the larger fura-2 concentrations; moreover, the increase in ΔpHapp was approximately in proportion to the increase in Δ[CaT]. At all fura-2 concentrations, the time course of ΔpHapp, through time to peak, was closely similar to, although probably slightly slower than, that of Δ[CaT]. These properties of ΔpHapp are consistent with an hypothesis proposed by Meissner and Young (1980. J. Biol. Chem. 255:6814-6819) and Somlyo et al. (1981.J. Cell Biol. 90:577-594) that a proton flux from the myoplasm into the SR supplies a portion of the electrical charge balance required as Ca2+ is released from the SR into the myoplasm. A comparison of the amplitude of ΔpHapp with that of Δ[CaT] indicates that, in response to a single action potential, 10-15% of the charge balance required for Ca2+ release may be carried by protons. © 1990, Rockefeller University Press., All rights reserved.