The T-SR junction in contracting single skeletal muscle fibers.
The junction between the T system and sarcoplasmic reticulum (SR) of frog skeletal muscle was examined in resting and contracting muscles. Pillars, defined as pairs of electron-opaque lines bounding an electron-lucent interior, were seen spanning the gap between T membrane and SR. Feet, defined previously in images of heavily stained preparations, appear with electron-opaque interiors and as such are distinct from the pillars studied here. Amorphous material was often present in the gap between T membrane and SR. Sometimes the amorphous material appeared as a thin line parallel to the membranes; sometimes it seemed loosely organized at the sites where feet have been reported. Resting single fibers contained 39 +/- 14.3 (mean +/- SD; n = 9 fibers) pillars/micrometer2 of tubule membrane. Single fibers, activated by a potassium-rich solution at 4 degrees C, contained 66 +/- 12.9 pillars/micrometer2 (n = 8) but fibers contracting in response to 2 mM caffeine contained 33 +/- 8.6/micrometer2 (n = 5). Pillar formation occurs when fibers are activated electrically, but not when calcium is released directly from the SR; and so we postulate that pillar formation is a step in excitation-contraction coupling.
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002110:Caffeine / Q000494:pharmacology; D066298:In Vitro Techniques; D007365:Intercellular Junctions / Q000648:ultrastructure*; D008854:Microscopy, Electron; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure*; D011188:Potassium / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000648:ultrastructure; D014982:Xenopus laevis / Q000502:physiology
D002110:Caffeine; D011188:Potassium