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.

SUMMARY1. We have made simultaneous measurements of membrane potential and wall tension in rat 200 ,sum mesenteric arteries.2. The resting membrane potential was -59-2 + 0-4 mV and stable (218 measurements, fifty-two vessels).3. With maximal exogenous noradrenaline stimulation (10 JM) the membrane depolarized to about -34 mV. During the onset of tension development oscillations (period about 6 see) in both tension and membrane potential were often seen; the membrane potential changes led the tension changes by about 1-2 sec.4. In the presence of increased K+ (e.g. 40 mM), vessels had an increased noradrenaline sensitivity, and here noradrenaline stimulation produced little change in membrane potential.5. With maximal K+ stimulation (85 mM), in the presence of phentolamine (1 SM), the membrane depolarized to about -17 mV, the tension being about 70 % of the maximal noradrenaline response.6. In the presence of phentolamine (1 /UM), noradrenaline caused hyperpolarization without tension development. The hyperpolarization was inhibited by propranolol and mimicked by isoprenaline.7. The results suggest that in these small vessels membrane potential variations are not essential to, but have an important modulating influence on, the tension response to exogenous noradrenaline.

During excitation of muscle fibres the action potentials are associated with a marked increase in Na¤ influx and K¤ efflux. The activity of the Na¤-K¤ pump will compensate for these fluxes, but it is well documented that the capacity of this transport system may not be large enough to prevent progressive reductions in the transmembrane gradients for Na¤ and K¤ (Sreter & Woo, 1963;Juel, 1986;Nielsen & Overgaard, 1996). It has been hypothesized that such reductions may eventually cause muscle fatigue due to lowered excitability (Bigland-Ritchie et al. 1979;Sj ogaard, 1990). It is well known from experiments with isolated muscles that large increases in [K¤]ï lead to a decrease in contractile

Action potential (AP) excitation requires a transient dominance of depolarizing membrane currents over the repolarizing membrane currents that stabilize the resting membrane potential. Such stabilizing currents, in turn, depend on passive membrane conductance (Gm), which in skeletal muscle fibers covers membrane conductances for K+ (GK) and Cl− (GCl). Myotonic disorders and studies with metabolically poisoned muscle have revealed capacities of GK and GCl to inversely interfere with muscle excitability. However, whether regulation of GK and GCl occur in AP-firing muscle under normal physiological conditions is unknown. This study establishes a technique that allows the determination of GCl and GK with a temporal resolution of seconds in AP-firing muscle fibers. With this approach, we have identified and quantified a biphasic regulation of Gm in active fast-twitch extensor digitorum longus fibers of the rat. Thus, at the onset of AP firing, a reduction in GCl of ∼70% caused Gm to decline by ∼55% in a manner that is well described by a single exponential function characterized by a time constant of ∼200 APs (phase 1). When stimulation was continued beyond ∼1,800 APs, synchronized elevations in GK (∼14-fold) and GCl (∼3-fold) caused Gm to rise sigmoidally to ∼400% of its level before AP firing (phase 2). Phase 2 was often associated with a failure to excite APs. When AP firing was ceased during phase 2, Gm recovered to its level before AP firing in ∼1 min. Experiments with glibenclamide (KATP channel inhibitor) and 9-anthracene carboxylic acid (ClC-1 Cl− channel inhibitor) revealed that the decreased Gm during phase 1 reflected ClC-1 channel inhibition, whereas the massively elevated Gm during phase 2 reflected synchronized openings of ClC-1 and KATP channels. In conclusion, GCl and GK are acutely regulated in AP-firing fast-twitch muscle fibers. Such regulation may contribute to the physiological control of excitability in active muscle.

An increased extracellular K+ concentration ([K+]0) is thought to cause muscle fatigue. We studied the effects of increasing [K+]0 from 4mM to 8-14mM on tetanic contractions in isolated bundles of fibres and whole soleus muscles from the rat. Whereas there was little depression of force at a [K+]0 of 8-9mM, a further small increase in [K+]0 to 11-14mM resulted in a large reduction of force. Tetanus depression at 11mM [K+]0 was increased when using weaker stimulation pulses and decreased with stronger pulses. Whereas the tetanic force/resting membrane potential (EM) relation showed only moderate force depression with depolarization from -74 to -62mV, a large reduction of force occurred when EM fell to-53mV. The implications of these relations to fatigue are discussed. Partial inhibition of the Na+-K+ pump with ouabain (10(-6 )M) caused additional force loss at 11mM [K+]0. Salbutamol, insulin, or calcitonin gene-related peptide all stimulated the Na+-K+ pump in muscles exposed to 11mM [K+]0 and induced an average 26-33% recovery of tetanic force. When using stimulation pulses of 0.1ms, instead of the standard 1.0-ms pulses, force recovery with these agents was 41-44% which was significantly greater (P < 0.025). Only salbutamol caused any recovery of EM (1.3mV). The observations suggest that the increased Na+ concentration difference across the sarcolemma, following Na+-K+ pump stimulation, has an important role in restoring excitability and force.