[Changes in transmitter release processes in the rat neuromuscular junction as affected by calcium metabolic...

Spontaneous synaptic activity, miniature end plate potentials (MEPP), was recorded intracellularly in rat phrenic diaphragmatic preparations isolated 8-10 days after removal of the external lobes of the parathyroid glands, with the total blood serum calcium being decreased from 2.75 +/- 0.07 to 1.50 +/- 0.09 mM. The frequency of MEPP was high in both normocalcium (2 mM) and hypocalcium (1 mM) media and complete removal (1 mM EGTA) of Ca2+ from the preparation-washing solution. In the latter case one could observe two synaptical population with different levels of secretion activation. Besides, if judged from the distribution of MEPP amplitudes and the appearance of "spontaneous EPP", the pattern of transmitter exocytosis also experiences changes. It is assumed that under experimental endogenous hypocalcemia there develop functional restitutions in the axonal terminal, leading to the accumulation in them of the transmitter and (or) Ca2+.