Poneratoxin,a new toxin from an ant venom,reveals an interconversion between two gating modes of the Na channels in frog skeletal muscle fibres

The effects of synthetic poneratoxin (PoTX), a new toxin isolated from the venom of the ant Paraponera clavata, were studied under current- and voltage-clamp conditions in frog skeletal muscle fibres. PoTX induces a concentration-dependent (10^–9 M–5×10^–6 M) prolongation of the action potentials and, at saturating concentration, a slow repetitive activity developing at negative potentials. PoTX specifically acts on voltage-dependent Na channels by decreasing the peak Na current (I _Na) and by simultaneously inducing a slow I _Na which starts to activate at –85 mV and inactivates very slowly. Both the fast and the slow components of I _Na are suppressed by tetrodotoxin and reverse at the same potential corresponding to the equilibrium potential for Na ions. The fast component of I _Na has voltage dependence, activation and steady-state inactivation almost similar to those of the control I _Na. The voltage dependence of the slow Na conductance is 40 mV more negative than that of the fast one. The results suggest that PoTX affects all the Na channels and that the fast and the slow I _Na components originate from a possible PoTX-induced interconversion between a fast and a slow operating mode of the Na channels.     

The influence of recruitment order and fibre composition on the force-velocity relationship and fatiguability of skeletal muscles in the cat

Two of the lower leg muscles of the cat were examined to assess the influence of the order in which motor units are recruited (slowest to fastest or fastest to slowest) and the fibre composition on the force-velocity relationship and fatiguability of skeletal muscle. The muscles examined were the soleus, a slow twitch muscle, and the medial head of the gastrocnemius muscle, a predominantly fast twitch muscle of mixed composition. The results of these experiments showed that the force-velocity relationship was a function of both the order of motor unit recruitment and the fibre composition of the muscles. However, at either recruitment order, the ability of the computer to control the velocity of contraction and the resultant fatiguability of the muscle for these contractions was similar.     

The effect of exercise on the contractile properties of single skinned fast- and slow-twitch skeletal muscle fibres from the adult rat

The effects of long-term endurance exercise on the contractile properties of single skinned muscle fibres from adult rats, were investigated. Adult (4-month-old) male rats were subjected to a 16-week, high-intensity endurance swimming programme, where animals carried a load (corresponding to 2% of body wt), during all 2-h training sessions. At the conclusion of the training period, muscle fibres isolated from the extensor digitorum longus (EDL), and soleus (SOL), could be classified into distinct classes or fibre types on the basis of their Ca²⁺- and Sr²⁺-activated contractile characteristics. The fast-twitch EDL comprised two fibre populations, while the slow-twitch SOL was found to be composed of three distinct fibre types. Endurance swimming modified the contractile characteristics of fibres from both the EDL and SOL, but exerted greater influence on those of the SOL. This was illustrated by significant increases in the sensitivity to Ca²⁺ and Sr²⁺, and a lower threshold for contraction by these activating ions, in the exercised group. Not one of the total of 272 fibres sampled, exhibited mixed fast- and slow-twitch contractile characteristics, often associated with exercise-induced fibre type transformations. Thus, high-intensity endurance swimming induced changes in some single muscle fibre contractile properties of adult rats, but did not cause major changes in fibre type distribution.     

The effect of increased respiratory resistance on glycogen and triglyceride levels in the respiratory muscles of the rat

Summary The effect of increased respiratory resistance (stenosis of the trachea) on glycogen and triglyceride levels in the diaphragm (D) and intercostal (external-IE, internal-II) muscles was studied in the rat. Tracheal stenosis resulted in a reduction of glycogen level in the muscles. For the fed rats the reductions were: D-45 and 79%, IE-14 and 30%, II-14 and 35%, 0.5 and 3 h after stenosis, respectively. For rats fasted for 24 h the reductions were: D-64 and 86%, IE-33 and 71%, II-40 and 82%, after 0.5 and 3 h respectively. The level of triglycerides in the muscles was stable during stenosis in the fed group, whereas in the fasted group it were reduced in the diaphragm by 50% after 0.5 h, and by 52% after 3 h. It is concluded that both endogenous and blood-born energy fuels are utilized by the respiratory muscles during increased resistance breathing. The work was supported by Polish Academy of Sciences (10.4.)     

Relations between chronic stimulation-induced changes in contractile properties and the Ca2+-sequestering system of rat and rabbit fast-twitch muscles

This study compares changes in contractile properties, Parvalbumin content, and Ca²⁺-uptake by the sarcoplasmic reticulum (SR) of low-frequency stimulated rat and rabbit tibialis anterior (TA) muscles. Time to peak tension increased 1.8-fold in 35-day stimulated rabbit TA, while no change occurred in rat TA. Isometric twitch tension increased 2-fold in rabbit TA, but was unaltered in rat TA. Parvalbumin (PA) content was more than 90% reduced in rabbit TA, but only 60% in rat TA after 35 days. At this time, PA content of the stimulated rat TA was still higher than that of normal rabbit TA. Taking into account the suggested role of PA as a cytosolic Ca²⁺ buffer, its decrease could lead to an impaired free Ca²⁺-decay with a prolonged active state and a higher tension output during a single twitch. This would explain why chronic stimulation led to an increase in isometric twitch tension in rabbit TA, but not in rat TA. The 1.6-fold rise in half-relaxation time of 35-day stimulated rat and rabbit TA most likely resulted from a 50% reduced Ca²⁺-uptake by the SR, due to a still unknown modification of the Ca²⁺-transport ATPase.     

The effect of β-receptor blockade on adrenaline-induced changes in redox and energy state in m. vastus lateralis and m. soleus of the rat in vivo

The metabolic changes in blood, red (m. soleus) and white (m. vastus lateralis) skeletal muscle fibres were investigated after short-term (3 min) infusion of adrenaline with or without prior treatment with propranolol or metoprolol. The adrenaline-induced increase in plasma lactate levels was totally prevented by prior treatment with metoprolol or propranolol, whilst the β-blockers had no effect on blood glucose levels. Similar effects on lactate levels were found in the m. soleus, while metoprolol was less effective than propranolol in m. vastus lateralis. Adrenaline decreased the level of muscle creatinine phosphate and ADP, causing the equilibrium of the creatinine kinase reaction to change in the direction of ATP synthesis, although the level of ATP usually decreased. This effect was more pronounced in m. vastus lateralis compared with m. soleus. The [ATP]/[ADP] [Pi]-ratio tended to increase during infusion of adrenaline. This effect was counteracted by metoprolol but not by propranolol. The effects on the “phosphate potential” ([ATP]/[ADP] [Pi]) and the equilibrium within the creatine kinase were more pronounced in m. vastus lateralis than in m. soleus. The results demonstrate the possible role of receptors other than β-receptors, i.e. α-receptors, in mediating changes in plasma glucose levels, while plasma lactate levels are regulated by the β-adrenergic system. The role of β-receptors in mediating changes in muscle lactate levels may differ in m. soleus and m. vastus lateralis, with a relative predominance of β₂-_-receptors in m. vastus lateralis. Quantitative and qualitative differences in the adrenergic control of the energy state in the two types of muscle fibre were obvious, although it was not possible to distinguish clearly between the relative importance of α₁β₁ and β_2-receptors.