In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

SUMMARY1. The action of catecholamines on the transport and the distribution of Na and K and the resting membrane potential (EM) has been investigated in soleus muscles isolated from fed rats.2. In a substrate-free Krebs-Ringer bicarbonate buffer adrenaline (ADR) (6 x 106 M) increased 22Na efflux by 83 %, 42K influx by 34 %, and Em by 10%. Similar effects were exerted by noradrenaline (NA), phenylephrine, salbutamol and isoprenaline. The effects of ADR on Na-K transport and EM were suppressed by ouabain (10-3 M) and propranolol (10-5 M), but not by thymoxamine (10-5 M) or tetracaine (10-4 M).3. Following 90 min of incubation in the presence of ADR (6 x 106 M), the intracellular K/Na-ratio was increased threefold. NA produced almost the same change, and both catecholamines seem to induce a new steady-state distribution of Na and K which can be maintained for several hours in vitro.4. The effect of ADR on 22Na efflux and EM could be detected at concentrations down to 6 x IO-and 6 x 10-10 M, respectively, and halfmaximum increase was obtained at around 2 x 10-8 M. NA was at least one order of magnitude less potent.5. The effect of low concentrations of ADR on 22Na efflux was potentiated by theophylline (2 mM). When added together, dibutyryl-cyclic AMP and theophylline mimicked the action of ADR on 22Na efflux, 42K influx, Na/K content and EM. Ouabain (10-3 M) also suppressed the effect of dibutyryl-cyclic AMP and theophylline on Na-K transport.6. Following the addition ofouabain (10-3M), EM rapidly dropped from a mean of -71 to -63 mV, and then showed a slow linear fall for up to 4hr. 7. The hyperpolarization induced by ADR was associated with a decrease in membrane conductance, 22Na influx and 42K efflux. The time course and the response to ouabain suggests that all of these effects are secondary to stimulation of the active coupled transport of Na and K. T. CLAUSEN AND J. A. FLATMAN 8. It is concluded that in rat soleus muscle, the active Na-K transport is electrogenic and susceptible to stimulation by catecholamines via beta-adrenoceptors. This effect is mediated by adenyl cyclase activation and may account for the increase in EM and the intracellular K/Na ratio.

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SUMMARY1. The role of extracellular and intracellular Ca2+ in pancreatic enzyme secretion has been assessed by correlating the exchange of 45Ca with amylase secretion in the isolated uncinate pancreas of baby rats.2. The rate coefficient of 45Ca efflux from pre-loaded glands declined continually (indicating that 45Ca is retained in several different pools) and probably reflects changes in the concentration of cytoplasmic free KCa, which is determined by the rate at which 45Ca is released from intracellular organelles into the cytoplasm.3. The rate coefficient of45Ca release was not influenced by extracellular Ca2+ or Mg&+ concentrations.4. Cholecystokinin-pancreozymin (CCK-PZ) and acetylcholine accelerated the release of both 45Ca and amylase in a dose-dependent fashion, even when extracellular Ca2+ was reduced to 0.1 mm, but did not affect the initial rate of KCa uptake by the tissue.5. In Ca2+-free media (containing 0.5 mM-EGTA) basal amylase secretion slowly declined and stimulated secretion was virtually abolished, but the accelerated release of 45Ca was maintained.6. These observations indicate that natural stimuli of pancreatic enzyme secretion alter 45Ca distribution in the cell by a process which is independent of extracellular Ca2+ and which is associated with amylase secretion provided that the plasma membrane has not been depleted of R. M. CASE AND T. CLAUSEN substitution also increased 45Ca uptake. Thus, under special conditions, secretion may be stimulated when increased amounts of Ca2+ are made available from extracellular sources.9. Hyperosmolarity (known to increase 45Ca release in muscle) also accelerated 45Ca release and amylase secretion.10. 2,4-Dinitrophenol markedly accelerated 45Ca efflux but did not stimulate amylase secretion, indicating that a rise in cytoplasmic Ca2+ will not initiate secretion if energy metabolism is impaired.11. CCK-PZ slightly increased the rate coefficient of 42K release, indicating a changed membrane permeability.12. The stimulatory effects of CCK-PZ and acetylcholine were suppressed during Na+-substitution by Li+, suggesting that the Na+ concentration gradient across the membrane is important in secretion.13. It is concluded that the primary action of CCK-PZ and acetylcholine may be to increase the influx of Na+ into the cell by changing membrane permeability. This in turn is responsible for the release of Ca2+ from intracellular stores (probably endoplasmic reticulum), leading to a rise in Ca2+ concentration close to the structures involved in enzyme secretion. Secretion then follows provided that ATP is available and the plasma membrane is not depleted of Ca2+.

SUMMARY1. The action of insulin on the transport and the distribution of Na and K has been studied in rat soleus muscles incubated at 300 C in glucose-free Krebs-Ringer bicarbonate buffer.2. Measurements of the uptake and the wash-out of 22Na indicate that the muscles contain an intracellular pool of Na available for transport which is confined to the water space not available to sucrose. Ouabain (10-4-10-3M) inhibited 22Na efflux by 69 % (0.287 #tmole/g tissue wet weight per minute) and 42K-influx by 40 % (0.196 gmole/g tissue wet weight per minute). When all extracellular Na was replaced by Li, both 22Na-efflux and 42K-influx were inhibited to about the same extent and ouabain produced very little further inhibition. 2,4-dinitrophenol decreased the ouabain-resistant component of 22Na-efflux by 39 %, 3. Insulin (from 0.1 to 100 mu./ml.) increased the rate coefficient of 22Na-efflux by from 11 to 46 % within 15 min. In the presence of ouabain (10-3M), the same relative increase was obtained, indicating that the hormone stimulates the glycoside-sensitive and the glycoside-insensitive Na transport to a similar extent. The effect of insulin on 22Na-efflux was not abolished by tetracaine (0 5 x 10-3M), phlorizin (0.5 x 10-2M) or by the substitution of Na, K, Mg or Ca. In the presence of 2,4-dinitrophenol (0.5 X 10-4M) or at temperatures below 150 C, the hormone produced no detectable change in 22Na-efflux.4. Insulin increased 42K-influx from 0*525 to 0 664 ,umole/g tissue wet weight per minute. This effect was entirely blocked by ouabain but not by tetracaine. Insulin produced a 14 % transient decrease in 42K-efflux.5. The continued exposure to insulin led to a new steady state, in which the intracellular Na pool was decreased from around 10 to around 5,almole/g tissue wet weight and the K content increased by an equivalent amount.In the presence of ouabain or at low extracellular concentrations of K, T. CLAUSEN AND P. G. KOHN insulin increased the rate of 22Na-influx by around 35 %. This effect was blocked by 2,4-dinitrophenol but not by tetracaine. 6. It is concluded that insulin stimulates the active coupled transport of Na and K, possibly by increasing the relative Na-affinity of the system mediating this process.

In rat soleus muscle, high frequency electrical stimulation produced a rapid increase in intra‐cellular Na+ (Na+i) content. This was considerably larger in muscles contracting without developing tension than in muscles contracting isometrically. During subsequent rest a net extrusion of Na+ took place at rates which, depending on the frequency and duration of stimulation, approached the maximum transport capacity of the Na+–K+ pumps present in the muscle. In isometrically contracting muscles, the net extrusion of Na+ continued for up to 10 min after stimulation, reducing Na+i to values 30% below the resting level (P < 0.001). This undershoot in Na+i, seen in both soleus and extensor digitorum longus muscles, could be maintained for up to 30 min and was blocked by ouabain or cooling to 0 °C. The undershoot in Na+i could be elicited by direct stimulation as well as by tubocurarine‐suppressible stimulation via the motor endplate. It could not be attributed to a decrease in Na+ influx, to effects of noradrenaline or calcitonin gene‐related peptide released from nerve endings, to an increase in extracellular K+ or the formation of nitric oxide. The results indicate that excitation rapidly activates the Na+–K+ pump, partly via a change in its transport characteristics and partly via an increase in intracellular Na+ concentration. This activation allows an approximately 20‐fold increase in the rate of Na+ efflux to take place within 10 s. The excitation‐induced activation of the Na+–K+ pump may represent a feed‐forward mechanism that protects the Na+–K+ gradients and the membrane potential in working muscle. Contrary to previous assumptions, the Na+–K+ pump seems to play a dynamic role in maintenance of excitability during contractile activity.

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