Calcium and depolarization dependence of twin-pulse facilitation of synaptic release at nerve terminals of crayfish and frog muscle

Spatial variation in membrane excitability modulated by 4-AP-sensitive K+ channels in the axons of the crayfish neuromuscular junction

Current-clamp recordings were made from the primary (1°) and secondary (2°) branching points (BPs) of axons at the crayfish neuromuscular junction. Action potential (AP) firing initiated by current injected at the 2° BP showed strong adaptation or high-frequency firing at threshold current, whereas AP firing frequency at the 1° BP exhibited a gradual rise with increasing current amplitude. The voltage threshold for AP (V(TH)) was higher at the 2° BP than the 1° BP. 4-Aminopyridine (4-AP) at 200 μM increased AP amplitude and duration at both BPs but reduced threshold current at the 2° BP more than at the 1° BP. This blocker lowered V(TH) at both BPs, but the difference between the BPs remained. Firing patterns evoked at the 2° BP became similar to those evoked at the 1° BP in 4-AP. Thus 4-AP-sensitive channels may be more concentrated in the distal axon and control AP initiation and firing patterns there. Orthodromic APs between the two BPs were also compared. There was no difference in AP amplitude between the two BPs, but AP half-width recorded at the 2° BP was longer than that at the 1° BP. AP duration at both BPs increased gradually, by ∼17%, during a 100-Hz, 500-ms train (in-train rise). Normalized AP half-widths revealed a smaller fractional in-train rise at the 2° BP. Thus, although distal APs were broader, AP duration there was under more stringent control than that of the proximal axon. 4-AP increased AP amplitude and duration of the entire orthodromic train and reduced the magnitude of the in-train rise in AP half-width at both BPs. However, this blocker did not uncover a clear difference between the two BPs. Thus 4-AP-sensitive channels concentrated in distal axon may be essential in preventing unintended firing and modulating AP waveform without interfering with orthodromic AP propagation.

Historical view and physiology demonstration at the NMJ of the crayfish opener muscle

Here we present some of the key important discoveries made with the opener neuromuscular (NMJ) preparation of crustaceans and illustrate that there is still much to learn from this model preparation. In understanding the history one can appreciate why even today this NMJ still offers a rich playground to address questions regarding pre- and post-synaptic function and plasticity. The viability and ease of access to the terminal for intracellular as well as extracellular electrophysiology and imaging are significant advantages. The mechanisms behind the modulation of vesicular kinetics and fusion within the high- and low-output terminals are begging for investigation. The preparation also offers a testable model system for computational assessments and manipulations to examine key variables in theoretical models of synaptic function, for example calcium dynamics during short-term facilitation. The synaptic complexity of active zone and statistical nature of quantal release is also an open area for future investigation both experimentally and computationally.

5-HT offsets homeostasis of synaptic transmission during short-term facilitation

In this study, we approach the topic of vesicle recruitment and recycling by perturbing neurotransmission at the crayfish neuromuscular junction with altered electrical activity and the presence of the neuromodulator serotonin (5-HT). After induction of short-term facilitation (STF) with stimulus pulse trains (40 Hz, 20 pulses), the amount of synaptic transmission can be maintained at a relatively constant level, producing a plateau in the amplitude of the excitatory postsynaptic potentials (EPSPs) throughout the remaining stimuli within a train of a few hundred milliseconds. With an increase in the frequency of the stimuli within a train (60 Hz, 20 pulses), an altered plateau of larger EPSP amplitudes occurs. This suggests that differential rates of vesicle recruitment can be rapidly reached and maintained. Exposure of nerve terminals to 5-HT further enhances the EPSP amplitudes to yet a higher plateau level. The effect of 5-HT is more pronounced for 40-Hz pulse trains than for 60-Hz trains. This suggests that 5-HT can recruit vesicles into the readily releasable pool (RRP) and that the recruitment is limited at higher stimulation frequencies. The attainment of a larger amplitude in the plateaus of the EPSPs at 60 Hz compared with 40 Hz also suggests that the rapid induction of STF enhances the entry of vesicles into the RRP. By direct quantal counts, mean quantal content increases linearly during STF, and 5-HT offsets the linear release. We propose that 5-HT and electrically induced recruitment of vesicles from a reserve pool to the RRP may share similar recruitment mechanisms.

Myofibrillar protein isoform expression is correlated with synaptic efficacy in slow fibres of the claw and leg opener muscles of crayfish and lobster

In the crayfish and lobster opener neuromuscular preparations of the walking legs and claws, there are regional differences in synaptic transmission even though the entire muscle is innervated by a single excitatory tonic motor neuron. The innervation of the proximal fibres produced larger excitatory postsynaptic potentials (EPSPs) than those of the central fibres. The amplitudes of the EPSPs in the distal fibres were intermediate between those of the proximal and central regions. These differences in EPSP amplitudes were correlated with differences in short-term facilitation between the three regions. When given a 10- or 20-pulse train of stimuli, the proximal fibres showed greater short-term facilitation initially, often followed by a maximization of short-term facilitation towards the end of a train. In contrast, the central fibres showed a linear increase in short-term facilitation throughout a stimulus train. The distal fibres showed intermediate short-term facilitation compared with the other two regions. Analysis of myofibrillar isoforms showed that levels of troponin-T1 (TnT1), a 55 kDa isoform expressed in slow-tonic (S2) fibres, were correlated with synaptic properties. Proximal fibres had the highest levels of TnT1, with lower levels in distal fibres; central fibres lacked TnT1, which is characteristic of slow-twitch (S1) fibres. In addition, differences in troponin-I isoforms correlated with TnT1 levels between the proximal, central and distal regions. The correlation between slow fibre phenotype and strength of innervation suggests a relationship between synaptic structure and expression of troponin isoforms.

Influence of serotonin on the kinetics of vesicular release

The mechanisms by which synaptic vesicles are transported and primed to fuse with the presynaptic membrane are important to all chemical synapses. Processes of signal transduction that affect vesicular dynamics, such as the second-messenger cascades induced by neuromodulators, are more readily addressed in assessable synaptic preparations of neuromuscular junctions in the crayfish. We assessed the effects of serotonin (5-HT) through the analysis of the latency jitter and the quantal parameters: n and p in the opener muscle of the walking leg in crayfish. There is an increase in the size of the postsynaptic currents due to more vesicles being released. Quantal analysis reveals a presynaptic mechanism by an increase in the number of vesicles being released. Latency measures show more events occur with a short latency in the presence of 5-HT. No effect on the frequency or size of spontaneous release was detected. Thus, the influence of 5-HT is presynaptic, leading to a release of more vesicles at a faster rate.

Effects of mobile buffers on facilitation: experimental and computational studies

Facilitation is an important form of short-term plasticity that occurs in most synapses. At crayfish neuromuscular junctions, basal transmission and facilitation were significantly reduced after presynaptic introduction of "fast" high-affinity calcium buffers, and the decay of facilitation was accelerated. The existence of residual calcium during facilitation was also demonstrated. Computational modeling of three-dimensional buffered Ca(2+) diffusion and binding to secretory and facilitation targets suggest that the facilitation site is located away from a secretory trigger mediating exocytosis; otherwise, the facilitation site would be saturated by each action potential. Our simulations account for many characteristics of facilitation and effects of exogenous buffer, and suggest that facilitation is caused by residual calcium gaining access to a site distinct from the secretory trigger through restricted diffusion.

Differential facilitation of high- and low-output nerve terminals from a single motoneuron

In the crayfish opener neuromuscular preparation, regional differences in synaptic transmission are observed among the terminals of a single motoneuron. With a single stimulus, the high-output terminals of the proximal region of the muscle produce a larger excitatory postsynaptic potential than do the low-output terminals of the central region of the muscle. We tested the hypothesis that the low-output terminals exhibit more facilitation than do high-output terminals for twin-pulse, train, and continuous-stimulation paradigms. Previous studies have not employed several stimulation paradigms to induce facilitation among high- and low-output terminals of a single motoneuron. We found that the high-output terminals on the proximal fibers facilitate more than the low-output terminals on the central muscle fibers, in contrast with previous studies on similar muscles. The difference in measured facilitation is dependent on the stimulation paradigm. These results are important because ultrastructural differences between these high- and low-output terminals are known and can be used for correlation with physiological measurements. Short-term facilitation is a form of short-term memory at the synaptic level, and the processes understood at the crayfish neuromuscular junction may well be applicable to all chemical synapses.

Study of the inhibitor of the crayfish neuromuscular junction by presynaptic voltage control

The inhibitor of the crayfish opener muscle was investigated by a presynaptic voltage control method. Two microelectrodes were inserted into the inhibitor and the amplitude and duration of presynaptic depolarization were controlled by a voltage-clamp amplifier. The inhibitory postsynaptic potential (IPSP) was measured from a muscle fiber located near the presynaptic voltage electrode. Nonlinear summation of IPSP amplitudes was corrected after chloride equilibrium potential was measured. With the use of 5-ms presynaptic pulses, the depolarization-release coupling (D-R) curve constructed from IPSP peak amplitudes (IPSPcor) had a threshold of about -35 mV and reached its maximal level at -5 to -10 mV. Depolarization beyond the maximum led to a suppression of neurotransmitter release. When transmitter release during a presynaptic pulse was completely suppressed, IPSPs activated by tail current could be identified with an average synaptic delay of 2.5 ms. Transmitter secretion triggered by a calcium current activated during the 5-ms pulses (IPSPon) was also measured on the rising phase of an IPSP, at 2.5 ms after the end of the 5-ms pulses. D-R coupling plots measured from IPSPon exhibited a more pronounced suppression than that obtained from IPSPcor. The effect of presynaptic pulse duration on the level of transmitter release was analyzed. Transmitter release increased with increasing duration and was nearly saturated by 20-ms pulses depolarized to 0 mV. The following conditions were identified as necessary to obtain a consistent D-R curve with a clear suppression: 1) small animals, 3.8 cm head to tail, 2) 15 degrees C, 3) 40 mM tetraethylammonium and 1 mM 4-aminopyridine, 4) an extracellular calcium concentration of < or = 10 mM. In addition, a consistent correlation was found among the branching pattern of the inhibitor, the placement of the presynaptic electrode, and the characteristics of the D-R curves. An ideal presynaptic electrode configuration involved placing the voltage electrode in a secondary branch, approximately 100 microns from the main branch point, and placing the current electrode at the branch point. Postsynaptically, optimal recordings were obtained from muscle fibers innervated by a single branch of the inhibitor that originated from a point near the presynaptic voltage electrode. A cable-release model was constructed to evaluate the relationship between the shape of the D-R coupling curves and the space constants of the presynaptic terminals. A comparison between the model and the D-R coupling curves suggested that the space constant of an inhibitor branch on a muscle fiber is > or = 8 times longer than its actual length. Therefore the upper limit estimate of the space constant of a typical preparation is approximately 3 mm. Results reported here outline morphological and physiological conditions needed to achieve optimal control of the presynaptic branch of the crayfish inhibitor. The cable-release model quantitatively defines the extent of presynaptic voltage control.

Immunoglobulin G from a patient with Miller-Fisher syndrome rapidly and reversibly depresses evoked quantal release at the neuromuscular junction of mice

A neuromuscular blocking factor has been described in the serum of patients with Miller-Fisher syndrome (MFS). We here examined the effect of immunoglobulins (Ig) on neuromuscular transmission in mice recording quantal endplate currents by means of a perfused macro-patch-clamp electrode. Ig and IgM- and IgG-fractions from an anti-GQ1b-positive patient with typical MFS were highly purified. After application of MFS-IgG, quantal release decreased 1000-fold within 2 min. Returning to control solution the average release came back to the baseline level within 4 min. In contrast, control-IgG and MFS-IgM did not cause any blocking effect. The very fast and fully reversible presynaptic blockade of release caused by the highly purified IgG-fraction may be one factor producing muscle weakness in MFS.

Factors influencing the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction

1. The effects of several conditions and agents on the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction in low-Ca2+ high-Mg2+ bathing solutions were examined. 2. Twin-pulses gave two endplate potential (epps) with first (m2) and second (m2) quantal contents. The ratio of m2/m1 was taken as a measure of the degree of facilitation. 3. The mean value of this ratio was > 1. Individual ratios fluctuated widely at junctions with smaller values of m1 but were focused around 1 at junctions with larger values of m1. Thus, some populations of junctions with smaller values of m1 contributed to an increment in the mean ratio. 4. The mean ratio was virtually constant irrespective of changes in the spontaneous and evoked release of transmitter at temperatures between 20 and 36 degrees C and at external concentrations of Ca2+ from 0.4 to 0.8 mM. 5. 4-Aminopyridine(4-AP) slightly but significantly increased this ratio with increases in m1 and m2 at temperatures of 24 and 36 degrees C. Ouabain slightly but significantly reduced the ratio, with increases in m1 and m2. The steadiness of the ratio was maintained in the presence of caffeine, high K+, neomycin or omega-conotoxin irrespective of changes in m1 and m2, except in the case of omega-conotoxin. 6. Spontaneous output at 36 degrees C increased in the presence of 4-aminopyridine, ouabain, caffeine, high K+ or neomycin. 7. These results indicate that maintenance of a stable value of the ratio of m2 to m1 is a dominant feature.(ABSTRACT TRUNCATED AT 250 WORDS)

Time course of transmitter release calculated from simulations of a calcium diffusion model

A three-dimensional presynaptic calcium diffusion model developed to account for characteristics of transmitter release was modified to provide for binding of calcium to a receptor and subsequent triggering of exocytosis. When low affinity (20 microM) and rapid kinetics were assumed for the calcium receptor triggering exocytosis, and stimulus parameters were selected to match those of experiments, the simulations predicted a virtual invariance of the time course of transmitter release to paired stimulation, stimulation with pulses of different amplitude, and stimulation in different calcium solutions. The large temperature sensitivity of experimental release time course was explained by a temperature sensitivity of the model's final rate limiting exocytotic process. Inclusion of calcium tail currents and a saturable buffer with finite binding kinetics resulted in high peak calcium transients near release sites, exceeding 100 microM. Models with a single class of calcium binding site to the secretory trigger molecule failed to produce sufficient synaptic facilitation under this condition. When at least one calcium ion binds to a different site having higher affinity and slow kinetics, facilitation again reaches levels similar to those seen experimentally. It is possible that the neurosecretory trigger molecule reacts with calcium at more than one class of binding site.

Multiplicative and additive Ca(2+)-dependent components of facilitation at mouse endplates

1. Facilitation of endplate potentials (EPPs) and frequency of miniature endplate potentials (MEPPs) were studied, in the presence of low Ca2+/raised Mg2+, in isolated mouse hemidiaphragm, using pseudo-random sequences of nerve stimulation and automated (computer) counting of MEPPs and quantal components of EPPs. 2. The facilitation in quantal content of EPPs (m) produced by one or more antecedent stimuli was accompanied by facilitation of MEPP frequency (fm) that was similar in magnitude and substantially less than expected if facilitation reflects persistent (residual) intraterminal Ca2+. The time course of 'phasic' quantal release, associated with the EPP, was little if at all altered with facilitation. 3. The magnitude and time course of facilitation was consistent with two distinct presynaptic processes, each manifest both in m and fm, (i) an effect to multiply transmitter release, and (ii) residual Ca2+ which adds to Ca2+ brought in by nerve impulses. These have distinct time courses. 4. After loading nerve terminals with bis (O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), facilitation of m and fm became very small. 5. At sufficiently low Ca2+/raised Mg2+ facilitation of m and fm became very small although latency histograms showed clear EPPs. However, the multiplicative component of facilitation became maximal at Ca2+/Mg2+ concentrations giving an average m value less than 0.1, corresponding to about 5% of normal Ca2+ entry per pulse. At lower Ca2+, facilitation was restored when EPPs were made larger using 4-aminopyridine. 6. With EPPs elicited by brief 'direct' nerve terminal depolarizations, facilitation was graded with pulse intensity (and m) and could be much less than with EPPs with similar m evoked by nerve stimuli at lower Ca2+ and/or higher Mg2+. 7. It was concluded that fast facilitation is primarily multiplicative and reflects activity within the nerve terminal of a Ca(2+)-sensitive process distinct from that generating Ca(2+)-dependent release.

Neurotransmitter release: Facilitation and three-dimensional diffusion of intracellular calcium

In order to account for the time courses of both evoked release and facilitation, in the framework of the Ca2+ hypothesis, Fogelson and Zucker (1985, Biophys. J. 48, 1003-1017) suggested treating diffusion of Ca2+, once it enters through the Ca2+ channels, as a three-dimensional process (three-dimensional diffusion model). This model is examined here as a refined version of the "Ca(2+)-theory" for neurotransmitter release. The three-dimensional model was suggested to account for both the time course of release and that of facilitation. As such, it has been examined here as to its ability to predict the dependence of the amplitude and time course of facilitation under various experimental conditions. It is demonstrated that the three-dimensional diffusion model predicts the time course of facilitation to be insensitive to temperature. It also predicts the amplitude and time course of facilitation to be independent of extracellular Ca2+ concentration. Moreover, it predicts that inhibition of the [Na+]o in equilibrium with [Ca2+]i exchange does not alter facilitation. These predictions are not upheld by the experimental results. Facilitation is prolonged upon reduction in temperature. The amplitude of facilitation declines and its duration is prolonged upon increase in extracellular Ca2+ concentration. Finally, inhibition of the [Na+]o in equilibrium with [Ca2+]i exchange prolongs facilitation but does not alter the time course of evoked release after an impulse.

Actions of lead on transmitter release at mouse motor nerve terminals

The actions of lead (Pb2+) on transmitter release were studied at neuromuscular junctions in mouse diaphragm in vitro. The quantal content of end-plate potentials (EPPs) was reduced by Pb2+ in a dose-related manner consistent with inhibition of Ca2+ entry into nerve terminals, with a half-maximal effect at 1.4 microM (in 0.5 mM Ca2+ and 2 mM Mg2+). Pb2+ also inhibited the increased frequency of MEPPs (fMEPP where MEPPs denotes miniature EPPs) produced by Ba2+ in the presence of raised K+, blocking the calculated Ba2+ entry half-maximally at 170 microM. However, at concentrations of 50-200 nM, Pb2+ often increased fMEPP in 20 mM K+ in the presence of Ca2+ and acted to promote the irreversible effect of lanthanum (La3+) to raise fMEPP. In nominally Ca(2+)-free solution with 20 mM K+, brief (1 min) application of Pb2+ (20-320 microM) caused rapid dose-dependent reversible rises in fMEPP. With prolonged exposure to Pb2+, fMEPP rose and then slowly declined; after removal of Pb2+, once fMEPP had fallen to low levels, fMEPP responded nearly normally to Ca2+ or ethanol, but not to Pb2+ itself. In 5 mM K+, 0 mM Ca2+ and varied [Pb2+] (where [] denotes concentration), nerve stimulation caused no EPPs, but prolonged tetanic stimulation produced increases in fMEPP graded with [Pb2+] that persisted as a "tail"; results were consistent with growth of fMEPP with the 4th power of intracellular Pb2+ and removal of intracellular Pb2+ with a time constant of about 30 s.(ABSTRACT TRUNCATED AT 250 WORDS)

Evoked phasic release in frog nerve terminals obtained after block of Ca2+ entry by Cd2+

Cutaneous pectoris muscles of frogs were isolated, mounted in a chamber and superfused with Ringer's solution. With a macro-patch-clamp electrode placed on a section of a motor nerve terminal, quantal synaptic currents were elicited by depolarizing pulses and recorded. The electrode tip and the section of the terminal recorded from were perfused rapidly by Ringer's solution alone or containing 20-500 microM Cd2+ to block Ca2+ inflow. Separate superfusion of the muscle and the rest of the terminal with normal or elevated Ca2+ Ringer's solution provided a sufficiently high resting Ca2+ concentration in the terminal even when Ca2+ was blocked by Cd2+. The depolarization level of maximal Ca2+ inflow into the terminal was found by measuring maximal test pulse facilitation, Fc. In control solution as well as in the case of Cd2+ block, the rate of phasic release after depolarizing pulses rose further when depolarization was increased past the level of Fc, and reached a saturation level which was maintained at estimated depolarizations up to +200 mV. Block of Ca2+ inflow by Cd2+ decreased release substantially, but did not suppress it. The depression of release was greater in the range of large Ca2+ inflow (around Fc) than for very large depolarizations. The time course of phasic release was unaltered by blockage of Ca2+ inflow. It is concluded that Ca2+ inflow contributes to the promotion of evoked release only in the depolarization range in which Ca2+ inward current is large.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of Ca2+ inflow at nerve terminals of frog muscle blocks facilitation while phasic transmitter release is still considerable

Action potentials were triggered in the motor nerve by a suction electrode and calcium currents (iCa) in the nerve terminals were measured by means of a perfused macro-patch-clamp electrode on the distal portion of the end-plates. Postsynaptic currents were blocked by adding d-tubocurarine, whereas presynaptic Na+ (iNa) and K+ (iK) currents were blocked by adding tetrodotoxin (TTX), tetraethylammonium and 3,4-diaminopyridine, respectively, to the perfusate of the electrode. The current components which could be suppressed by addition of Cd2+ to the perfusate were taken as presynaptic iCa. The observed effects on the presynaptic current components were very similar to those reported previously. If the electrode was perfused with Ringer's solution containing the blockers for iNa and iK, the same, obviously complete block of iCa was obtained by 50 and 100 microM Cd2+, an average of 96% block by 20 microM Cd2+ and 50% block by about 5 microM Cd2+. Using the same type of electrode and similar locations on motor nerve terminals, postsynaptic quantal currents and twin-pulse facilitation (Fd) were elicited by variable-duration (0.5-3 ms) depolarizing pulses. When the electrode was perfused with Ringer's solution containing TTX, 20 microM Cd2+ added to the perfusate reduced the rate of phasic release of quanta insignificantly for short depolarizing pulses and by a factor of about 10 for longer pulses. Fd was blocked almost completely. Addition of 50 microM Cd2+ to the perfusate had a greater depressive effect on release after short depolarizing pulses and reduced release after longer pulses by a factor of about 100.(ABSTRACT TRUNCATED AT 250 WORDS)

Twin pulse facilitation in dependence on pulse duration and calcium concentration at motor nerve terminals of crayfish and frogs

Phasic release from motor-nerve terminals of crayfish and frogs was elicited and recorded by means of a macro-patch-clamp electrode through which the terminal was depolarized in graded pulses. The tip of the electrode was perfused and the Ca concentration around the terminal, Cae, was controlled independent from that in the superfusion of the muscle, Cab. Release increased with pulse duration with a double-logarithmic slope of 5 to 9 in crayfish and frogs, which represents a form of "early facilitation" (Katz and Miledi 1968). In crayfish, this relation was shifted to longer pulse durations on lowering Cae, while in frogs, in addition, the saturation level of release was suppressed at low Cae. Responses to twin pulses with intervals of 7-10 ms showed facilitation, Fd. When pulse duration of the twin pulses was increased, starting from about 0.5 ms, Fd increased to a maximum, but declined for longer pulses which elicited release approaching the saturation range. On lowering Cae, the maximum of Fd, Fd, increased in amplitude and was shifted to larger pulse durations. Also reduction of Cab increased Fd. The effects of pulse duration and of Cae and Cab on Fd are predicted by the residual Ca theory of facilitation, if it is assumed that changes of Cae produce corresponding changes in Ca inflow during depolarization, and if the resting intracellular Ca concentration is influenced by the extracellular Ca concentration. The large values of early facilitation can not be explained by the residual Ca theory of facilitation and may indicate the action of another depolarization dependent factor which joins in the control of release.

Shifts in the voltage dependence of synaptic release due to changes in the extracellular calcium concentration at nerve terminals on muscle of crayfish and frogs

The rate of release of transmitter quanta, elicited by variable depolarization pulses applied to a nerve terminal by means of a macro-patch-clamp electrode, was measured in muscles of crayfish and frogs. The electrode was perfused with solutions containing different Ca concentrations, Cae. The bath was superfused separately, usually with solutions containing nominally no Cab and elevated Mgb. A fixed depolarization pulse followed the variable test pulse within 7-10 ms, and facilitation, Fc, of release after the fixed pulse was determined as a measure of Ca-inflow during the test pulse. As described before, Fc always showed a peak, Fc, at depolarization amplitudes of the test pulse below the saturation level of release. When Cae was changed, the depolarization levels generating Fc shifted in a negative direction if Cae was lowered, and in a positive direction if Cae was increased. These shifts agreed with the known dependence of the effective membrane potential (controlling e.g. Ca inward current) on Cae which is due to shielding of surface changes by Ca2+ (cf. Hille 1984). Changes of Cab, at constant Cae, did not affect the depolarization dependence of Fc. It is concluded that Ca inflow is not the only factor controlling quantal release, and that at least in depolarizations beyond those eliciting Fc another potential dependent factor increases release while Ca inflow presumably falls.