Me. Kriebel et al., HYPERTONIC TREATMENT REVERSIBLY INCREASES THE RATIO OF GIANT SKEW MINIATURE END-PLATE POTENTIALS TO BELL-MINIATURE END-PLATE POTENTIALS, Neuroscience, 71(1), 1996, pp. 101-117
Miniature endplate potentials were recorded from single frog muscle fi
bers before, during and after treatment with hypertonic saline (200-50
0 mM NaCl or Na gluconate added to frog saline). Miniature endplate po
tential amplitude distributions were plotted from small muscle fibers
so that the modes and ratios of the skew-miniature endplate potential
to bell-miniature endplate potential classes could be defined. Muscle
fibers were voltage clamped with two electrodes to determine the input
resistance before, during and after treatment. Input resistance incre
ased from two to 100 times during treatment and rapidly fell towards c
ontrol values (no more than 30% greater) when preparations were return
ed to normal frog saline. Short duration treatments with 200-300 mM hy
pertonic salines immediately increased frequencies (100-fold) of both
skew-miniature endplate potential and bell-miniature endplate potentia
l classes. Preparations when returned to normal frog saline after a fe
w minutes of treatment showed control miniature endplate potential dis
tributions within minutes. One to two hour treatments left only the sk
ew-miniature endplate potential class and with hour-long recovery peri
ods bell-miniature endplate potentials reappeared and ratios of skew-m
iniature endplate potential to-bell-miniature endplate potential class
es returned to control values. Treatment with 500 mM NaCl added to fro
g saline immediately increased the percentage of skew-miniature endpla
te potentials (from 2 to 50%) with little or no increase in overall mi
niature endplate potential frequencies. The mode of the skew-miniature
endplate potential class was unchanged after hypertonic treatment, wh
ereas that of the bell-miniature end plate potential class either rema
ined about the same size or decreased depending on the duration of tre
atment. The number and percentage of giant-miniature endplate potentia
ls belonging to the skew-miniature endplate potential class increased
as a function of the duration of 200-300 mM hypertonic saline treatmen
ts. Most giant-miniature endplate potentials had a slow rising phase w
ith a foot and/or breaks demonstrating a composite structure. Sequenti
ally recorded giant-miniature endplate potentials had similar initial
slopes indicating either repetitive releases from single sites or rele
ases from cooperative sites. After hypertonic treatment the bell-minia
ture endplate potential size was never more than that expected with th
e increase (under 30%) in input resistance. The results presented here
are completely different from those of Yu and Van der Kloot [(1991) J
. Physiol. 433, 677-704] who reported that the bell-miniature endplate
potential amplitude was increased two- to four-fold after hypertonic
treatment. The wide range of results in the ratio of skew-miniature en
dplate potential to bell-miniature endplate potential classes is discu
ssed in regards to the quantal hypothesis which is based on a single c
lass of immutable amounts of transmitter; and, a hypothesis based on a
dynamical process that meters transmitter in subunit amounts to contr
ol miniature endplate potential size and class during release.