The effect of beta 2-adrenoceptor activation on ion-shifts and fatigue in mouse soleus muscles stimulated in vitro.
The resting membrane potential (RMP), the intracellular free Na+ concentration ([Na+]i) and the intracellular free K+ concentration ([K+]i), were measured with double-barrelled ion-selective microelectrodes in mouse soleus muscles in vitro. In addition, the Na+ contents and K+ contents have been measured with the flame photometric technique. At rest the beta 2-selective adrenoceptor agonist terbutaline (10(-5) M) increased the membrane potential and [K+]i, and decreased [Na+]i when compared with control muscles. During a 5 min stimulation period the muscles, which were incubated with the beta 2-adrenoceptor agonist, showed a smaller depolarization, a smaller decrease in [K+]i and a smaller increase in [Na+]i than stimulated control muscles. This difference was probably associated with an increased rate of Na-K-pumping in the beta 2-adrenoceptor stimulated muscles. The beta 2-agonist treated muscles were more resistant to fatigue than control muscles. This effect was significant with 10(-6) M terbutaline (25 degrees C). A depolarization obtained by increasing [K+]o was shown to reduce the maximal tension. It is postulated, that the K+ shifts, which are responsible for the depolarization during muscle activity, are one of the mechanisms underlying muscle fatigue.
Juel
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Zoophysiological Laboratory B, August Krogh Institute, University of Copenhagen, Denmark.
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D004558:Electric Stimulation; D005110:Extracellular Space / Q000032:analysis; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D009119:Muscle Contraction / Q000187:drug effects; D009132:Muscles / Q000187:drug effects* / Q000378:metabolism; D009994:Osmolar Concentration; D011188:Potassium / Q000032:analysis; D015221:Potassium Channels / Q000187:drug effects*; D012964:Sodium / Q000032:analysis; D015222:Sodium Channels / Q000187:drug effects*; D013726:Terbutaline / Q000494:pharmacology*
D000318:Adrenergic beta-Agonists; D015221:Potassium Channels; D015222:Sodium Channels; D012964:Sodium; D013726:Terbutaline; D011188:Potassium