An examination of the effects of osmotic pressure changes upon transmitter release from mammalian motor nerve terminals

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Abstract
When the frequency of miniature end-plate potentials (mepps) was measured at neuromuscular junctions in rat diaphragm nerve preparations in vitro bathed in solutions having osmolarities between 200 and 700 m-os-moles/1., mepp frequency was transiently increased by exposure to osmotic gradients exceeding 75 m-osmoles/1., and then declined, within 1 hr. to a steady level slightly higher than the control level of frequency. Smaller osmotic gradients caused a maintained increase in mepp frequency. Epp quantal content was initially increased and later profoundly decreased upon exposure of preparations to solutions with an osmotic pressure of 500 or 600 m-osmoles/l. but was unaffected by less hypertonic solutions. Variation of the Ca or Mg content of the bathing solutions did not alter these effects of osmotic pressure on the early transient increase in mepp frequency or epp quantal content but affected the late steady increase in mepp frequency. The value of the transient increase in mepp frequency was exponentially related to the osmotic gradient in the range 0-300 m-osmoles/1. with a Qio of 1.95 (range il-34[degree]C). Greater osmotic gradients did not further increase mepp frequency. Variation of the ionic strength of the bathing medium did not influence osmotic effects upon frequency. The discrepancy between the effects of osmotic gradients upon spontaneous and nerve-impulse induced transmitter release was explained by an occlusion of the osmotic effects by depolarization of nerve terminals. Time-course studies showed that in the presence of 20 mM-KCl the mepp frequency increase in response to an increase in osmotic pressure was small and was followed by a reduction in frequency to below control levels while osmotic pressure changes had no immediate effect upon mepp frequency in solutions containing 30 mM-KCl. Increased osmotic gradients could release transmitter by a mechanism independent of Ca and of nerve terminal depolarization. The initial transient effects of changes of osmotic gradient upon transmitter release are related to flow of water through the nerve terminal membrane, while the later effects are related to nerve terminal volume changes.