title: Exacerbated potassium-induced paralysis of mouse soleus muscle at 37°C vis-à-vis 25°C: implications for fatigue. K+ -induced paralysis at 37°C.
abstract: The main aim was to investigate the effects of raised [K+](o) on contraction of isolated non-fatigued skeletal muscle at 37°C and 25°C to assess the physiological significance of K+ in fatigue. Mouse soleus muscles equilibrated at 25°C had good mechanical stability when temperature was elevated to 37°C. The main findings at 37°C vis-à-vis 25°C were as follows. When [K+](o) was raised from 4 to 7 mM, there was greater twitch potentiation, but no significant difference in peak tetanic force. At 10 mM [K+](o) there was (1) a faster time course for the decline of peak tetanic force, (2) a greater steady-state depression of twitches and tetani, (3) an increase of peak force over 50-200 Hz (whereas it decreased at 25°C), (4) significant tetanus restoration when stimulus pulse duration increased from 0.1 to 0.25 ms and (5) greater depolarisation of layer-2 fibres, with no repolarisation of surface fibres. These combined data strengthen the proposal that a large run-down of the K+ gradient contributes to severe fatigue at physiological temperatures via depolarisation and impaired sarcolemmal excitability. Moreover, terbutaline, a β(2)-adrenergic agonist, induced a slightly greater and more rapid, but transient, restoration of peak tetanic force at 10 mM [K+](o) at 37°C vis-à-vis 25°C. A right shift of the twitch force-stimulation strength relationship at 10 mM [K+](o) was partially reversed with terbutaline to confer the protective effect. Thus, catecholamines are likely to stimulate the Na+ -K+ pump more powerfully at 37°C to restore excitability and attenuate, but not prevent, the detrimental effects of K+.
authors.affiliation: School of Sport and Recreation, Faculty of Health and Environmental Sciences, AUT University, Auckland, New Zealand. simeon.cairns@aut.ac.nz
authors.forename: Simeon P
authors.initials: SP
authors.lastname: Cairns
authors.forename: John P
authors.initials: JP
authors.lastname: Leader
authors.forename: Denis S
authors.initials: DS
authors.lastname: Loiselle
mesh_terms: D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D005260:Female; D008564:Membrane Potentials / Q000502:physiology; D051379:Mice; D023421:Models, Animal; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000503:physiopathology*; D010243:Paralysis / Q000139:chemically induced* / Q000503:physiopathology*; D011188:Potassium / Q000009:adverse effects* / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000502:physiology; D013696:Temperature; D013726:Terbutaline / Q000494:pharmacology
chemical_list: D000318:Adrenergic beta-Agonists; D000254:Sodium-Potassium-Exchanging ATPase; D013726:Terbutaline; D011188:Potassium