Intraterminal Ca2+ and Spontaneous Transmitter Release at the Frog Neuromuscular Junction

• Published in: Journal of neurophysiology 2001-01, Vol.85 (1), p.287
• Main Authors: Angleson, J. K, Betz, W. J
• Format: Article
• Language: eng ; jpn
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.2001.85.1.287

  1 Department of Biological Sciences, University of Denver, Denver 80208; and   2 Department of Physiology and Biophysics, University of Colorado Medical School, Denver, Colorado 80262 Angleson, J. K. and W. J. Betz. Intraterminal Ca 2+ and Spontaneous Transmitter Release at the Frog Neuromuscular Junction. J. Neurophysiol. 85: 287-294, 2001. We investigated the relationship between intraterminal Ca 2+ concentration ([Ca 2+ ] i ) and the frequency of miniature end plate potentials (MEPPs) at the frog neuromuscular junction by use of ratiometric imaging of fura-2-loaded nerve terminals and intracellular recording of MEPPs. Elevation of extracellular [KCl] over the range of 2-20 mM resulted in increases in [Ca 2+ ] i and MEPP frequency. Loading terminals with the fast and slow Ca 2+ -buffers bis-( o -aminophenoxy)- N,N,N',N' -tetraacetic acid-acetoxymethyl (BAPTA-AM) and EGTA-AM resulted in equivalent reductions in the KCl-dependent increases in MEPP frequency. The [Ca 2+ ] i dependence of MEPP frequency determined by elevation of [Ca 2+ ] i due to application of 0.1-10 µM ionomycin was similar to that determined when [Ca 2+ ] i was raised by increasing extracellular KCl. Measurements in 10 mM extracellular KCl revealed that application of the phorbol ester phorbol 12 myristate 13-acetetate (PMA) caused an increase in MEPP frequency while the inactive analogue, 4 -PMA, did not. PMA application also caused an increase in [Ca 2+ ] i . The relationship between [Ca 2+ ] i and MEPP frequency in PMA was the same as was determined by the other methods of raising [Ca 2+ ] i . Under all conditions tested, our data revealed a low [Ca 2+ ] i threshold for activation of transmitter release and are consistent with a K d for [Ca 2+ ] i on the order of 1 µM.