Potassium, Na+,K+-pumps and fatigue in rat muscle.
During contractile activity, skeletal muscles undergo a net loss of cytoplasmic K(+) to the interstitial space. During intense exercise, plasma K(+) in human arterial blood may reach 8 mm, and interstitial K(+) 10-12 mm. This leads to depolarization, loss of excitability and contractile force. However, little is known about the effects of these physiological increases in extracellular K(+) ([K(+)](o)) on contractile endurance. Soleus muscles from 4-week-old rats were mounted on transducers for isometric contractions in Krebs-Ringer bicarbonate buffer containing 4-10 mm K(+), and endurance assessed by recording the rate of force decline during continuous stimulation at 60 Hz. Increasing [K(+)](o) from 4 to 8 or 10 mm and equilibrating the muscles for 40 or 20 min augmented the rate of force decline 2.4-fold and 7.2-fold, respectively (P < 0.001). The marked loss of endurance elicited by exposure to 8 or 10 mm K(+) was alleviated or significantly reduced by stimulating the Na(+),K(+)-pumps by intracellular Na(+) loading, the beta(2)-agonist salbutamol, adrenaline, calcitonin gene related peptide, insulin or repeated excitation. In conclusion, excitation-induced increase in [K(+)](o) is an important cause of high-frequency fatigue, and the Na(+),K(+)-pumps are essential for the maintenance of contractile force in the physiological range of [K(+)](o). Recordings of contractile force during continuous stimulation at 8-10 mm K(+) may be used to analyse the effects of agents or conditions influencing the excitability of working isolated muscles.
Institute of Physiology and Biophysics, University of Aarhus, Ole Worms Allé 1160, DK-8000 Arhus C, Denmark. tc@fi.au.dk
Torben
T
Clausen
Ole Baekgaard
OB
Nielsen
D000318:Adrenergic beta-Agonists; D000420:Albuterol; D000818:Animals; D015740:Calcitonin Gene-Related Peptide / Q000378:metabolism; D004558:Electric Stimulation; D004837:Epinephrine / Q000378:metabolism; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism; D018763:Muscle Fatigue / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000378:metabolism*; D010807:Physical Endurance / Q000502:physiology; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D000318:Adrenergic beta-Agonists; D007328:Insulin; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D015740:Calcitonin Gene-Related Peptide; D000420:Albuterol; D011188:Potassium; D004837:Epinephrine