NEUROTRANSMITTER RELEASE EVOKED BY NERVE IMPULSES WITHOUT CA2-NERVE ENDINGS( ENTRY THROUGH CA2+ CHANNELS IN FROG MOTOR)

1. The requirement for extracellular Ca2+ in the process of evoked ace tylcholine (ACh) release by nerve impulses was tested at endplates in frog skeletal muscle. Ca2+-containing Lipid vesicles (Calf liposomes) were used to elevate cytoplasmic Ca2+ concentrations under conditions in which Ca2+ entry from the extracellular fluid was prevented. 2. In an extracellular solution containing no added Ca2+ and 1 mM Mg2+ ('Ca2 +-free' solution), Ca2+ liposomes promoted the synchronous release of ACh quanta, reflected electrophysiologically as endplate potentials (E PPs), in response to temporally isolated nerve impulses. 3. Motor nerv e stimulation generated EPPs during superfusion with Ca2+ liposomes in Ca2+ free solutions containing the Ca2+ channel blocker Co2+ (1 mM), and the Ca2+ chelator EGTA (2 mM). As a physiological control for Ca2 leakage from the liposomes to the extracellular fluid, the effect of Ca2+ liposomes on asynchronous evoked ACh release mediated by Ba2+ was examined. In contrast to the effects of 0.2-0.3 mM extracellular Ca2, which generated EPPs but antagonized Ba2+-mediated asynchronous ACh release, Ca2+ liposomes generated EPPs but did not reduce asynchronous release mediated by Ba2+. The effects of Ca2+ liposomes were thus not due to leakage of Ca2+ from the liposome to the extracellular fluid. 4. Morphological studies using fluorescently labelled liposomes in con junction with a confocal microscope demonstrate that lipid is transfer red from the liposomes to nerve endings and Liposomal contents are del ivered to the nerve terminal cytoplasm. 5. The results suggest that wh en intracellular Ca2+ is elevated using liposomes as a vehicle, evoked ACh release can occur in the absence of Ca2+ entry via Ca2+ channels.