Ionic movements mediated by monensin in frog skeletal muscle

Na+,K+-pump stimulation improves contractility in isolated muscles of mice with hyperkalemic periodic paralysis

In patients with hyperkalemic periodic paralysis (HyperKPP), attacks of muscle weakness or paralysis are triggered by K⁺ ingestion or rest after exercise. Force can be restored by muscle work or treatment with β₂-adrenoceptor agonists. A missense substitution corresponding to a mutation in the skeletal muscle voltage-gated Na⁺ channel (Na_v1.4, Met1592Val) causing human HyperKPP was targeted into the mouse SCN4A gene (mutants). In soleus muscles prepared from these mutant mice, twitch, tetanic force, and endurance were markedly reduced compared with soleus from wild type (WT), reflecting impaired excitability. In mutant soleus, contractility was considerably more sensitive than WT soleus to inhibition by elevated [K⁺]_o. In resting mutant soleus, tetrodotoxin (TTX)-suppressible ²²Na uptake and [Na⁺]_i were increased by 470 and 58%, respectively, and membrane potential was depolarized (by 16 mV, P < 0.0001) and repolarized by TTX. Na⁺,K⁺ pump–mediated ⁸⁶Rb uptake was 83% larger than in WT. Salbutamol stimulated ⁸⁶Rb uptake and reduced [Na⁺]_i both in mutant and WT soleus. Stimulating Na⁺,K⁺ pumps with salbutamol restored force in mutant soleus and extensor digitorum longus (EDL). Increasing [Na⁺]_i with monensin also restored force in soleus. In soleus, EDL, and tibialis anterior muscles of mutant mice, the content of Na⁺,K⁺ pumps was 28, 62, and 33% higher than in WT, respectively, possibly reflecting the stimulating effect of elevated [Na⁺]_i on the synthesis of Na⁺,K⁺ pumps. The results confirm that the functional disorders of skeletal muscles in HyperKPP are secondary to increased Na⁺ influx and show that contractility can be restored by acute stimulation of the Na⁺,K⁺ pumps. Calcitonin gene-related peptide (CGRP) restored force in mutant soleus but caused no detectable increase in ⁸⁶Rb uptake. Repeated excitation and capsaicin also restored contractility, possibly because of the release of endogenous CGRP from nerve endings in the isolated muscles. These observations may explain how mild exercise helps locally to prevent severe weakness during an attack of HyperKPP.

Linkage of aerobic glycolysis to sodium-potassium transport in rat skeletal muscle. Implications for increased muscle lactate production in sepsis

Although a linkage between aerobic glycolysis and sodium-potassium transport has been demonstrated in diaphragm, vascular smooth muscle, and other cells, it is not known whether this linkage occurs in skeletal muscle generally. Metabolism of intact hind-leg muscles from young rats was studied in vitro under aerobic incubation conditions. When sodium influx into rat extensor digitorum longus (EDL) and soleus muscles was facilitated by the sodium ionophore monensin, muscle weight gain and production of lactate and alanine were markedly stimulated in a dose-dependent manner. Although lactate production rose in both muscles, it was more pronounced in EDL than in soleus. Monensin-induced lactate production was inhibited by ouabain or by incubation in sodium-free medium. Preincubation in potassium-free medium followed by potassium re-addition also stimulated ouabain-inhibitable lactate release. Replacement of glucose in the incubation medium with pyruvate abolished monensin-induced lactate production but exacerbated monensin-induced weight gain. Muscles from septic or endotoxin-treated rats exhibited an increased rate of lactate production in vitro that was partially inhibited by ouabain. Increases muscle lactate production in sepsis may reflect linked increases in activity of the Na+, K+-ATPase, consumption of ATP and stimulation of aerobic glycolysis.

Characteristics of Na(+)-Ca2+ exchange in frog skeletal muscle

1. Fluxes studies were carried out to investigate the Na(+)-dependent outward movement of Ca2+ in intact frog sartorius muscle from Leptodactylus ocellatus, a preparation for which published data on the subject are sparse. 2. Under normal resting conditions the Na(+)-Ca2+ exchange was not readily detectable. 3. When muscles were exposed to 4 mM caffeine, the rate of fractional loss of Ca2+ (kCa,o) increased by about 50%. Most of this increase exhibits characteristics typical of the Na(+)-Ca2+ antiport working in the forward mode found in other cells. 4. The increase in kCa,o promoted by caffeine was decreased by: (a) 72% in the absence of external Na+ (Nao+); (b) 73% in Na(+)-loaded muscles ([Na+]i = 98 mM); (c) 70% when fibres were depolarized to -27 mV ([K+]o = 50 mM); and (d) 80% in the presence of 5 mM amiloride. 5. Ni2+ (5 mM), an inhibitor of the Na(+)-Ca2+ exchanger current, unexpectedly increased the caffeine-promoted rise in kCa,o. This effect of Ni2+ was associated with a concomitant caffeine-stimulated Ni2+ influx. In the absence of caffeine Ni2+ did not affect kCa,o. 6. It was concluded that: (a) under resting conditions the sarcolemmal Ca2+ pump suffices to handle the cytosolic calcium concentration ([Ca2+]i); (b) Na(+)-Ca2+ activity becomes apparent when [Ca2+]i is substantially increased by caffeine-induced Ca2+ release from the sarcoplasmic reticulum; and (c) the blocking effect of Ni2+ on the current generated by a Na(+)-Ca2+ exchange with a coupling ratio > 2 might actually represent a shift of the antiport mode toward an electroneutral 1 Ni(2+)-1Ca2+ exchange.

Na(+)-K+ pump stimulation elicits recovery of contractility in K(+)-paralysed rat muscle

1. This study explores the role of active electrogenic Na(+)-K+ transport in restoring contractility in isolated rat soleus muscles exposed to high extracellular potassium concentration ([K+]o). This was done using agents (catecholamines and insulin) known to stimulate the Na(+)-K+ pump via different mechanisms. 2. When exposed to Krebs-Ringer bicarbonate buffer containing 10 mM K+, the isometric twitch and tetanic force of intact muscles decreased by 40-69%. The major part of this decline could be prevented by the addition of salbutamol (10(-5) M). In the presence of 10 mM K+, force could be restored almost completely within 5-10 min by the addition of salbutamol or adrenaline and partly by insulin. 3. In muscles exposed to 12.5 mM K+, force declined by 96%. Salbutamol (10(-5) M), adrenaline (10(-6) M) and insulin (100 mU ml-1) produced 57-71, 61-71 and 38-47% recovery of force within 10-20 min, respectively. The effects of these supramaximal concentrations of salbutamol and insulin on force recovery were additive. Salbutamol and adrenaline produced significant recovery of contractility at concentrations down to 10(-8) M (P < 0.005). 4. In soleus, the same agents stimulated 86Rb+ uptake and decreased intracellular Na+. These actions reflect stimulation of active Na(+)-K+ transport and both showed a highly significant correlation to the recovery of twitch as well as tetanic force (r = 0.80-0.88; P < 0.001). 5. The force recovery induced by salbutamol, adrenaline and insulin was suppressed by pre-exposure to ouabain (10(-5) M for 10 min or 10(-3) M for 1 min) as well as by tetrodotoxin (10(-6) M). 6. The observations support the conclusion that the inhibitory effect of high [K+]o on contractility in skeletal muscle can be counterbalanced by stimulation of active electrogenic Na(+)-K+ transport, the ensuing increase in the clearance of extracellular K+ and in the transmembrane electrochemical gradient for Na+.