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Valinomycin and excitation‐contraction coupling in skeletal muscle fibres of the frog.
Author(s) -
Pape P C,
Konishi M,
Baylor S M
Publication year - 1992
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1992.sp019083
Subject(s) - valinomycin , chemistry , biophysics , sarcomere , contraction (grammar) , endoplasmic reticulum , stimulation , potassium , muscle contraction , coupling (piping) , analytical chemistry (journal) , membrane potential , anatomy , myocyte , biochemistry , materials science , endocrinology , chromatography , biology , organic chemistry , metallurgy
1. Experiments were carried out on intact frog skeletal muscle fibres to study the role of H+ and K+ as counter‐ions during the release of Ca2+ from the sarcoplasmic reticulum (SR). A specific focus was to test whether valinomycin, a potassium ionophore, markedly reduces or abolishes H+ counter‐ions fluxes across the SR membrane in response to electrical stimulation. 2. Single twitch fibres, mounted on an optical bench apparatus and stretch to long sarcomere length (3.6‐4.0 microns), were activated by single action potentials (16 degrees C). Two optical signals related to excitation‐contraction coupling were measured: (i) the 'second component' of the intrinsic birefringence signal, which is closely related to the myoplasmic free [Ca2+] transient, and (ii) the transient myoplasmic alkalization (delta pH) detectable from the pH indicator Phenol Red, a signal thought to reflect the movement of protons from the myoplasm into the SR in partial electrical exchange for released Ca2+. 3. Exposure of a fibre to 5 microM‐valinomycin produced a slight, progressive decrease in the amplitude of the birefringence signal, approximately 5‐6% per hour. This result suggests that, if anything, the peak rate at which Ca2+ is released from the sarcoplasmic reticulum is slightly decreased by valinomycin. 4. The amplitude of the Phenol Red delta pH signal, measured after exposure of a fibre to valinomycin for a period of at least 60 min, averaged 0.0020 +/‐ 0.0002 (+/‐ S.E.M.); this value is slightly smaller than, but not significantly different from (P greater than 0.05; two‐tailed t test) that measured in fibres not exposed to valinomycin (0.0025 +/‐ 0.0002). This result does not support the idea that valinomycin, but virtue of increasing the flux of K+ into the SR, markedly reduces the flux of protons during Ca2+ release. 5. Our findings of minimal changes in the birefringence and delta pH signals are consistent with the idea that, at the time of Ca2+ release, the potassium conductance of the SR membrane is large and not substantially increased by the addition of valinomycin to Ringer solution.