In vivo and in vitro miniature end-plate potentials at various external K concentrations

In order to obtain a nore reliable picture of transmitter release, spontaneous synaptic activity was studied in vivo in the rat soleus neuromuscular junction. The in vivo miniature end-plate potential (MEPP) recordings were compared with those obtained in vitro and, in both circumstances, MEPP frequency was measured at external potassiumconcentrations ranging from 5 to 15 mM. The influence of external potassium loading on intra- and extracellular potassium concentration also estimated by measuring the muscle ionic composition and water distribution at normal and high plasma potassium concentrations. The MEPP frequencies recorded in vivo were consistently lower than those observed in vitro for all external potassium concentrations studied (P less than0.01). These data suggest that better experimental conditions are maintained in thein vivo preparations, since the intracellular potassium increase that follows the external potassium loading can only partially justify the observed differnce and other factors, such as anesthetic effects, etc, may be ruled out.