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.
Clausen
Torben
T
Institute of Physiology and Biophysics, University of Aarhus, Ole Worms Allé 1160, DK-8000 Arhus C, Denmark. tc@fi.au.dk
Nielsen
Ole Baekgaard
OB
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