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Ischemic stimulation of iodoacetic acid (IAA)-treated rat extensor digitorum longus (EDL) muscles produced contractures. Similar ischemic stimulation of control EDL muscles did not result in contracture. At the onset of contracture, ATP concentration was not reduced, phosphocreatine concentration was reduced > 75%, ADP concentration was increased 9-fold, Ca2+ concentration ([Ca2+]) was increased approximately 11-fold, and inorganic phosphate concentration increased less in IAA-treated muscles than in stimulated control muscles. To test whether contracture resulted from elevated [Ca2+] and/or increased Ca2+ sensitivity of the contractile proteins, this laboratory made skinned fiber-activating solutions that simulated four different conditions: unstimulated IAA-treated and control muscles, IAA-treated muscles at contracture, and ischemically simulated control muscles. Skinned EDL fibers had lower single-fiber tensions and reduced Ca2+ sensitivities in activating solutions that mimicked the conditions in stimulated control muscles compared with activating solutions that simulated the conditions in unstimulated muscles. In contrast, the maximum tension was maintained and Ca2+ sensitivity was increased in activating solutions that simulated contracture. Tension at contracture resulted from increased intracellular [Ca2+] and increased myofibrillar Ca2+ sensitivity compared with the Ca2+ sensitivity of stimulated control fibers.

The effects of iodoacetic acid (IAA) and ischemic contraction were studied in rat extensor digitorum longus muscles. Ischemic stimulation of IAA-treated muscles produced contracture. We measured total muscle water content, distribution of water between intracellular and extracellular spaces, creatine concentration ([Cr]), creatine phosphate concentration ([PCr]), [ATP], [Pi], intracellular pH, and intracellular Ca2+ concentration ([Ca2+]i) at the onset of contracture. [ADP] was calculated from the equilibrium of the creatine kinase reaction using the measured values of [ATP], [PCr], [Cr], and pH. At the onset of contracture there was a 75% reduction of [PCr], a 12-fold increase in [ADP], and an 11-fold increase in [Ca2+]i compared with unstimulated IAA-treated muscles. [ATP] was not depleted at contracture compared with unstimulated IAA-treated muscles, and [Pi] increased less in muscles at contracture compared with stimulated control muscles. The persistent tension in contractures probably resulted from increased [Ca2+]i combined with increased myofibrillar Ca2+ sensitivity due to elevated [ADP] and relatively reduced intracellular acidification and [Pi].

Na current density and membrane capacitance were studied with the loose patch voltage clamp technique on rat fast- and slow-twitch skeletal muscle fibers at three different regions on the fibers: 1) the end plate border, 2) greater than 200 microns from the end plate (extrajunctional), and 3) on the end plate postsynaptic membrane. Fibers were treated with collagenase to improve visualization of the end plate and to enzymatically remove the nerve terminal. The capacitance of membrane patches was similar on fast- and slow-twitch fibers and patches of membrane on the end plate had twice the capacitance of patches elsewhere. For fast- and slow-twitch fibers, the sizes of the Na current normalized to the area of the patch were as follows: end plate greater than end plate border greater than extrajunctional. For both types of fibers, the amplitudes of the Na current normalized to the capacitance of the membrane patch were as follows: end plate approximately end plate border greater than extrajunctional. At each of the three regions, the Na current densities were larger on fast-twitch fibers and fast-twitch fibers had a larger increase in Na current density at the end plate border compared with extrajunctional membrane.

Muscle fibers from the lateral gastrocnemius or intercostal muscles of 7 normal adult males were chemically skinned (sarcolemma disrupted) and isolated fibers were divided into two parts for histochemical determination of fiber type and physiologic studies. The Ca- and Sr-induced tension relationships and maximum contraction velocities were measured. Slow twitch fibers developed tension at lower concentrations of Ca or Sr than fast twitch fibers. The difference between fast and slow twitch fibers was greatest when Sr was the activating cation. Fast and slow twitch fibers was greatest when Sr was the activating cation. Fast and slow twitch fibers generated similar maximum tensions. The contraction velocities of fast twitch fibers were more than two-fold greater than slow twitch fibers. Fast-oxidative-glycolytic (FOG, type IIA) and fast-glycolytic (FG, type IIB) fibers had similar Ca- and Sr-tension relationships and contraction velocities.

The effects of iodoacetic acid (IAA) and ischemic contraction were studied in rat extensor digitorum longus muscles. Ischemic contraction of IAA-treated muscles produced contracture. The onset of contracture was not associated with a change in sarcolemmal electrical properties or reduction in intracellular [ATP]; however, [creatine phosphate] was reduced by 75% and free [ADP] was increased by 665%. Continued stimulation of IAA-treated fibers resulted in depolarization, loss of membrane excitability, further depletion of creatine phosphate, and reduction in [ATP]. The effects seen in IAA-treated muscle did not appear to result from a direct action of IAA on the surface membrane, contractile proteins, or excitation-contraction coupling. The contractures in IAA-treated muscle may have resulted from increased Ca sensitivity of the contractile proteins, increased myoplasmic [Ca], or both. Both effects may have resulted from increased [ADP]. In addition, the reduced acidification during ischemic contraction of IAA-treated fibers compared with control fibers may have further increased the Ca sensitivity of IAA-treated fibers compared with controls.

Lateral gastrocnemius muscle biopsies from a 26-year-old man with hyperkalemic periodic paralysis and a 23-year-old man with hypokalemic periodic paralysis were studied. Both patients came from families in which older relatives had developed a vacuolar myopathy in association with their periodic paralysis. Muscle fibers were chemically skinned, and individual fibers were studied with a low-compliance strain gauge. The tension generated by fibers was studied in baths with calcium concentrations from 10(-8) mol/L to 2.5 x 10(-5) mol/L. The Ca-tension relationships and maximal tensions (normalized to fiber cross-sectional area) of fast and slow twitch fibers were indistinguishable from those found in fibers from 5 normal subjects. The results reinforce earlier findings which suggested that loss of Ca-induced myofibril contraction was not the cause of paralysis in periodic paralysis.

We studied the myofibrillar calcium affinity and mechanical threshold of single muscle fibers from the extensor digitorum longus (EDL) and soleus muscles of male Sprague-Dawley rats treated with daily intramuscular injections of dexamethasone, 1.5 mg/kg, for 14 days. The strength-duration and pCa-tension relationships of EDL fibers were not altered by glucocorticoid treatment. However, in soleus fibers the mechanical threshold was increased and the myofibrillar calcium sensitivity was reduced by dexamethasone treatment. Glucocorticoid treatment did not change the maximum tension per cross-sectional area of single-skinned EDL or soleus fibers. The glucocorticoid-induced atrophy and impaired force-generating capacity of fast-twitch muscle are probably not caused by impaired excitation-contraction coupling.

Sodium currents were recorded in rat fast and slow twitch muscle fibers. Changes in the membrane potential around the resting potential produced slow changes in the sodium current amplitude due to alterations of the slow inactivation process that was increased by steady depolarization and removed by prolonged hyperpolarization. In contrast, classical fast inactivation was not operative around the resting potential, and depolarizations of greater than 20 mV were required to close half of the channels by fast inactivation. Because slow inactivation is operative around the resting potential of mammalian muscle fibers, it may partially explain why small depolarizations, such as those that occur in some patients with periodic paralysis, can reduce excitability.

This study investigated the influence of muscle source and fiber type on the calcium sensitivity of skinned rat skeletal muscle fibers from predominantly slow muscles [soleus (SOL) and adductor longus (AL)], mixed muscle [posterior gracilis (PG)], and predominantly fast-twitch muscle [extensor digitorum longus (EDL)]. Fibers were characterized histochemically and by one-dimensional protein gel electrophoresis, and calcium-tension relationships were determined. Fiber type and muscle source had significant effects on the negative log of the calcium concentration associated with half-maximal tension (pCa1/2). Slow-twitch fibers had larger values of pCa1/2 than did fast-twitch fibers. Slow-twitch fibers from the predominantly slow muscles, SOL and AL, had similar values of pCa1/2 but slightly smaller values than from the mixed muscle, PG. Fast-glycolytic (FG) fibers from the predominantly fast muscle, EDL, had a higher pCa1/2 than fibers from the mixed fiber type muscle, PG. There were no differences between the pCa1/2 associated with FG and fast-oxidative-glycolytic fibers.