Inositol trisphosphate and cyclic adenosine diphosphate-ribose increase quantal transmitter release at frog motor nerve terminals: possible involvement of smooth endoplasmic reticulum

The release of chemical transmitter from nerve terminals is critically dependent on a transient increase in intracellular Ca²⁺.6., 25. The increase in Ca²⁺ may be due to influx of Ca²⁺ from the extracellular fluid¹⁵ or release of Ca²⁺ from intracellular stores such as mitochondria.1., 8., 18. Whether Ca²⁺ utilized in transmitter release is liberated from organelles other than mitochondria is uncertain. Smooth endoplasmic reticulum is known to release Ca²⁺, e.g., on activation by inositol trisphosphate or cyclic adenosine diphosphate-ribose,² so the possibility exists that Ca²⁺ from this source may be involved in the events leading to exocytosis. We examined this hypothesis by testing whether inositol trisphosphate and cyclic adenosine diphosphate-ribose modified transmitter release. We used liposomes to deliver these agents into the cytoplasmic compartment and binomial analysis to determine their effects on the quantal components of transmitter release. Administration of inositol trisphosphate (10⁻⁴ M) caused a rapid, 25% increase in the number of quanta released. This was due to an increase in the number of functional release sites, as the other quantal parameters were unaffected. The effect was reversed with 40 min of wash. Virtually identical results were obtained with cyclic adenosine diphosphate-ribose (10⁻⁴ M). Inositol trisphosphate caused a 10% increase in quantal size, whereas cyclic adenosine diphosphate-ribose had no effect. The results suggest that quantal transmitter release can be increased by Ca²⁺ released from smooth endoplasmic reticulum upon stimulation by inositol trisphosphate or cyclic adenosine diphosphate-ribose. This may involve priming of synaptic vesicles at the release sites or mobilization of vesicles to the active zone. Inositol trisphosphate may have an additional action to increase the content of transmitter within the vesicles. These findings raise the possibility of a role of endogenous inositol phosphate and smooth endoplasmic reticulum in the regulation of cytoplasmic Ca²⁺ and transmitter release.     

Calcium from internal stores triggers GABA release from retinal amacrine cells

The Ca(2+) that promotes transmitter release is generally thought to enter presynaptic terminals through voltage-gated Ca(2+)channels. Using electrophysiology and Ca(2+) imaging, we show that, in amacrine cell dendrites, at least some of the Ca(2+) that triggers transmitter release comes from endoplasmic reticulum Ca(2+) stores. We show that both inositol 1,4,5-trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) are present in these dendrites and both participate in the elevation of cytoplasmic [Ca(2+)] during the brief depolarization of a dendrite. Only the Ca(2+) released through IP(3)Rs, however, seems to promote the release of transmitter. Antagonists for the IP(3)R reduced transmitter release, whereas RyR blockers had no effect. Application of an agonist for metabotropic glutamate receptor, known to liberate Ca(2+) from internal stores, enhanced both spontaneous and evoked transmitter release.     

Ultrastructural changes in the parathyroids of Mongolian gerbils induced experimentally in vitro. Effects of variations in Ca2+ and Mg2+ concentrations

Isolated parathyroid glands from normal adult Mongolian gerbils were incubated for 15 minutes to 3 1/2 hours either at high or low concentrations of Ca2+ and Mg2+ after which they were studied ultrastructurally, using the pyroantimonate technique and x-ray analysis for identification and son concentrations were mainly composed of suppressed chief cells with moderate or high cytoplasmic density, sparsely developed endoplasmic reticulum, often large Golgi complex, occassional cytoplasmic accumulations of secretory granules, lipoid bodies, glycogen-like particles, and numerous often large mitochondria. Ca2+-containing precipitates were found mainly in mitochondria. Autophagic vacuoles contained Ca2+-loaded degenerating mitochondria. Glands exposed to low concentrations of Ca2+ and Mg2+ were mainly composed of stimulated and active chief cells; characteristic features were a moderate or low cytoplasmic density, prominent endoplasmic reticulum, small Golgi complex, medium-sized, or small mitochondria, and smooth electron lucent vacuoles with or without an association of mitochondria. Ca2+-containing precipitates were found mainly in smooth vacuoles and cytosol, but also in mitochondria and routh vacuoles. Myelin-like figures and crystalloid bodies occurred in some mitochondria, and normal or degenerating mitochondria withoug Ca2+-loading were seen in autophagic vacuoles. In addition, some stimulated chief cells exhibited double membrane-limited sequestered areas of cytoplasm with a rich content of free ribosomes and glycogen-like particles. The chief cell mitochondria seem to possess capacity for rapid accumulation of Ca2+, associated with an increase in volume at functional suppression. At stimulation of parathyroid function the endoplasmic reticulum is prominent in the active cells, and there seems to be a decrease in the volume and Ca2+-content of the mitochondria occasionally associated with degenerative changes, and a decrease also in the number of free ribosomes and glycogen-like particles in the stimulated cells.     

FK506 induces biphasic Ca2+ release from microsomal vesicles of rat pancreatic acinar cells

The effect of the immunosuppressant drug FK506 on microsomal Ca2+ release was investigated in rat pancreatic acinar cells. When FK506 (0.1-200 microM) was added to the microsomal vesicles at a steady state of ATP-dependent 45Ca2+ uptake, FK506 caused a dose-dependent and a biphasic release of 45Ca2+. Almost 10% of total 45Ca2+ uptake was released at FK506 concentrations up to 10 microM (Km=0.47 microM), and 60% of total 45Ca2+ uptake was released at FK506 concentrations over 10 microM (Km=55 microM). Preincubation of the vesicles with cyclic ADP-ribose (cADPR, 0.5 microM) increased the FK506 (< or =10 microM)-induced 45Ca2+ release (Ozawa T, Biochim Biophys Acta 1693: 159-166, 2004). Preincubation with heparin (200 microg/ml) resulted in significant inhibition of the FK506 (30 microM)-induced 45Ca2+ release. Subsequent addition of inositol 1,4,5-trisphosphate (IP3, 5 microM) after FK506 (100 microM)-induced 45Ca2+ release did not cause any release of 45Ca2+. These results indicate that two types of FK506-induced Ca2+ release mechanism operate in the endoplasmic reticulum of rat pancreatic acinar cells: a high-affinity mechanism of Ca2+ release, which involves activation of the ryanodine receptor, and a low-affinity mechanism of Ca2+ release, which involves activation of the IP3 receptor.     

Involvement of Ca2+ uptake by a reticulum-like store in the control of transmitter release.

At an identified neuro-neuronal synapse of Aplysia, 2,5-diterbutyl 1,4-benzohydroquinone, a selective blocker of the reticulum Ca2+ pump, was found to potentiate evoked quantal release of acetylcholine through an increased accumulation of Ca2+ in the presynaptic neuron during depolarization without any accompanying changes in the presynaptic Ca2+ current. We conclude that a rapid Ca2+ buffering system, similar to that associated with the endoplasmic reticulum, must be present in the nerve terminal and play a role in the control of Ca2+ which reaches the release system.     

Free radical-dependent Ca2+ signaling: role of Ca2+-induced Ca2+ release

Previously we have shown that Fe3+/ascorbate-induced Ca2+ release from scallop sarcoplasmic reticulum (SR) is due to Ca2+-channel gating by free radicals. This study is aimed at demonstrating that Ca2+-induced Ca2+ release (CICR) plays a role in this kind of Ca2+ release. Scallop SR vesicles were incubated with fluo-3 and exposed to Fe3+/ascorbate. Fluorimetric recordings showed massive Ca2+ release, with maximum rate and 50% release occurring at 30 min after exposure. Conversely, the use of the probe for reactive oxygen species dihydrorhodamine or the assay of malondialdehyde allowed oxyradical production to be traced for approximately 5 min only. Hence, although Ca2+ release started just after exposure to Fe3+/ascorbate, most release occurred after free radical exhaustion. Ruthenium red addition after Fe3+/ascorbate slowed down the Ca2+ release, whereas cyclic adenosine 5'-diphosphoribose addition accelerated it, indicating that the free radical-induced Ca2+ release from SR vesicles triggers a mechanism of CICR that dramatically increases the initial effect.     

Inhibition of mitochondrial Ca2+ uptake affects phasic release from motor terminals differently depending on external [Ca2+

We investigated how inhibition of mitochondrial Ca2+ uptake affects stimulation-induced increases in cytosolic [Ca2+] and phasic and asynchronous transmitter release in lizard motor terminals in 2 and 0.5 mM bath [Ca2+]. Lowering bath [Ca2+] reduced the rate of rise, but not the final amplitude, of the increase in mitochondrial [Ca2+] during 50-Hz stimulation. The amplitude of the stimulation-induced increase in cytosolic [Ca2+] was reduced in low-bath [Ca2+] and increased when mitochondrial Ca2+ uptake was inhibited by depolarizing mitochondria. In 2 mM Ca2+, end-plate potentials (epps) depressed by 53% after 10 s of 50-Hz stimulation, and this depression increased to 80% after mitochondrial depolarization. In contrast, in 0.5 mM Ca2+ the same stimulation pattern increased epps by approximately 3.4-fold, and this increase was even greater (transiently) after mitochondrial depolarization. In both 2 and 0.5 mM [Ca2+], mitochondrial depolarization increased asynchronous release during the 50-Hz train and increased the total vesicular release (phasic and asynchronous) measured by destaining of the styryl dye FM2-10. These results suggest that by limiting the stimulation-induced increase in cytosolic [Ca2+], mitochondrial Ca2+ uptake maintains a high ratio of phasic to asynchronous release, thus helping to sustain neuromuscular transmission during repetitive stimulation. Interestingly, the quantal content of the epp reached during 50-Hz stimulation stabilized at a similar level ( approximately 20 quanta) in both 2 and 0.5 mM Ca2+. A similar convergence was measured in oligomycin, which inhibits mitochondrial ATP synthesis without depolarizing mitochondria, but quantal contents fell to <20 when mitochondria were depolarized in 2 mM Ca2+.     

Ryanodine-, IP3- and NAADP-dependent calcium stores control acetylcholine release

Injections of inositol trisphosphate (IP3) or nicotinamide adenine dinucleotide phosphate (NAADP) into the presynaptic neurone of an identified cholinergic synapse in the buccal ganglion of Aplysia californica increased the amplitude of the inhibitory postsynaptic current evoked by a presynaptic action potential. This suggests that Ca2+ release from various Ca2+ stores can modulate acetylcholine (ACh) release. Specific blockade of the calcium-induced calcium release (CICR) mechanism with ryanodine, or of IP3-induced calcium release with heparin, abolished the effects of IP3, but not the effects of NAADP, suggesting the presence of an intracellular Ca2+ pool independent of those containing ryanodine receptors (RyR) or IP3 receptors. To reinforce electrophysiological observations, intracellular [Ca2+]i changes were measured using the fluorescent dye rhod-2. Injections of cyclic ADP-ribose (an activator of RyR), IP3 or NAADP into the presynaptic neurone induced transient increases in the free intracellular Ca2+ concentration. RyR- and IP3-induced increases were prevented by application of respective selective antagonists but not NAADP-induced increases. Our results show that RyR-dependent, IP3-dependent, and NAADP-dependent Ca2+ stores are present in the same presynaptic terminal but are differently involved in the regulation of the presynaptic Ca2+ concentration that triggers transmitter release.     

Regulation of transmitter release by synapsin II in mouse motor terminals

We investigated quantal release and ultrastructure in the neuromuscular junctions of synapsin II knockout (Syn II KO) mice. Synaptic responses were recorded focally from the diaphragm synapses during electrical stimulation of the phrenic nerve. We found that synapsin II affects transmitter release in a Ca²⁺-dependent manner. At reduced extracellular Ca²⁺ (0.5 m m ), Syn II KO mice demonstrated a significant increase in evoked and spontaneous quantal release, while at the physiological Ca²⁺ concentration (2 m m ), quantal release in Syn II KO synapses was unaffected. Protein kinase inhibitor H7 (100 μ m ) suppressed quantal release significantly stronger in Syn II KO synapses than in wild type (WT), indicating that Syn II KO synapses may compensate for the lack of synapsin II via a phosphorylation-dependent pathway. Electron microscopy analysis demonstrated that the lack of synapsin II results in an approximately 40% decrease in the density of synaptic vesicles in the reserve pool, while the number of vesicles docked to the presynaptic membrane remained unchanged. Synaptic depression in Syn II KO synapses was slightly increased, which is consistent with the depleted vesicle store in these synapses. At reduced Ca²⁺ frequency facilitation of synchronous release was significantly increased in Syn II KO, while facilitation of asynchronous release was unaffected. Thus, at the reduced Ca²⁺ concentration, synapsin II suppressed transmitter release and facilitation. These results demonstrate that synapsin II can regulate vesicle clustering, transmitter release, and facilitation.     

Control of intracellular Ca2+ activity in rat myometrium.

Four fractions enriched, respectively, in plasma membrane (PM), smooth endoplasmic reticulum (SER), rough endoplasmic reticulum (RER), and mitochondria were isolated from estrogen-dominated rat myometrium. Ca2+ uptake by these fractions was studied in order to estimate the relative potential of the corresponding organelles for controlling intracellular Ca2+ activity. Ca2+ uptake properties of the PM, SER, and RER fractions were similar except that potentiation by oxalate was in the order RER greater than or equal SER greater than PM. However, studies with the ionophores X-537A and A23187 suggested that Ca2+ was transported into the lumen of membrane vesicles of all these fractions. Unlike that of skeletal muscle sarcoplasmic reticulum, Ca2+ uptake by the myometrial fractions was not supported by high-energy compounds other than ATP. Mitochondria took up much less Ca2+ at low, and much more Ca2+ at high, free Ca2+ concentrations than did the other fractions. The amount of Ca2+ taken up in 30 s from a 1 muM free Ca2+ solution in the presence of ATP was similar for all fractions. These results suggested that mitochondria may act as an important Ca2+ control system in rat myometrium when the intracellular Ca2+ concentration is near 1 muM or higher, whereas the PM, SER, and RER may be of major importance at Ca2+ levels of 0.3 muM or lower.     

Cyclic ADP-ribose induces a larger than normal calcium release in malignant hyperthermia-susceptible skeletal muscle fibers

Malignant hyperthermia (MH) is associated with abnormal regulation of intracellular calcium in skeletal muscle fibers. Cyclic adenosine diphosphate-ribose (cADPR) is an endogenous metabolite of beta-NAD+ that induces Ca2+ release from intracellular stores in many tissues. Microinjection of cADPR (0.5 or 1 microM) increased the intracellular resting Ca2+ concentration ([Ca2+]i) in intact swine skeletal muscle in a dose-dependent manner. However, the increase in [Ca2+]i was greater in malignant-hyperthermia-susceptible (MHS) fibers than in non-susceptible (MHN) fibers. Incubation of muscle fibers in low external Ca2+ solution or in the presence of L-type Ca2+ channel entry blockers, or intracellular microinjection of heparin or ruthenium red did not modify the effect of cADPR on [Ca2+]i. Dantrolene (50 microM), a known inhibitor of intracellular Ca2+ release, decreased resting [Ca2+]i and prevented the cADPR-induced increase in [Ca2+]i. These results provide evidence: (1) for the existence of Ca2+ release mechanisms occurring via non-ryanodine or inositol 1,4,5-trisphosphate (InsP3) receptor mechanisms; (2) that MHS skeletal muscles exhibit a higher responsiveness to cADP-ribose-induced release of Ca2+ and (3) that the ability of dantrolene to block cADP-ribose-induced release of Ca2+ could be related to its pharmacologic effect on resting [Ca2+]i.     

Effect of hypertonicity on augmentation and potentiation and on corresponding quantal parameters of transmitter release

Augmentation and (posttetanic) potentiation are two of the four components comprising the enhanced release of transmitter following repetitive nerve stimulation. To examine the quantal basis of these components under isotonic and hypertonic conditions, we recorded miniature endplate potentials (MEPPs) from isolated frog (Rana pipiens) cutaneous pectoris muscles, before and after repetitive nerve stimulation (40 s at 80 Hz). Continuous recordings were made in low Ca2+ high Mg2+ isotonic Ringer solution, in Ringer that was made hypertonic with 100 mM sucrose, and in wash solution. Estimates were obtained of m (no. of quanta released), n (no. of functional release sites), p (mean probability of release), and vars p (spatial variance in p), using a method that employed MEPP counts. Hypertonicity abolished augmentation without affecting potentiation. There were prolonged poststimulation increases in m, n, and p and a marked but transient increase in vars p in the hypertonic solution. All effects were completely reversed with wash. The time constants of decay for potentiation and for vars p were virtually identical. The results are consistent with the notion that augmentation is caused by Ca2+ influx through voltage-gated calcium channels and that potentiation is due to Na+-induced Ca2+ release from mitochondria. The results also demonstrate the utility of this approach for analyzing the dynamics of quantal transmitter release.     

Calcium signaling and exocytosis in adrenal chromaffin cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+ concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+ entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+ (CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+ that modulates catecholamine release. Targeted aequorins with different Ca2+ affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+ transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]c microdomains in which the local subplasmalemmal [Ca2+]c rises abruptly from 0.1 to approximately 50 microM, triggering CICR, mitochondrial Ca2+ uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+ uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+ redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+ release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]c that regulate the early and late steps of exocytosis.     

Developmental changes in calcium content of ultrastructurally distinct subcellular compartments of preimplantation human embryos

The ultrastructural localization of mobilizable Ca2+ in different subcellular compartments of human oocytes and preimplantation embryos was studied using the potassium-pyroantimonate technique and transmission electron microscopy; the specificity was confirmed by chelation experiments and X-ray microanalysis. In unfertilized oocytes, Ca2+ was detected in small vesicles beneath the plasma membrane as well as in other forms of smooth endoplasmic reticulum (SER) and in mitochondria but not in cortical granules. In pronuclear zygotes and blastomeres of cleaving embryos, Ca(2+)-rich vesicles were no longer present close to the plasma membrane, and the entire periphery was poor in Ca(2+)-containing organelles which, however, were abundant in the perinuclear region. The uneven Ca2+ loading of SER and mitochondria from the pronuclear stage onwards suggests that Ca2+ release from both these types of organelle contributes to the embryonic Ca2+ signals. During mitosis, less Ca2+ was detected with organelles, but the antimonate reaction product was more abundant in the cytosol. These data suggest that, in addition to different forms of SER, mitochondria also act as a source of mobilizable Ca2+ in preimplantation human embryos. The previously described developmental and cell cycle related changes in the characteristics of Ca2+ signals are associated with the redistribution and structural reorganization of these organelles.      

Co-ordination of Ca(2+) signalling in mammalian cells by the new Ca(2+)-releasing messenger NAADP

Ca(2+) signalling is one of the most important means in mammalian cells of relaying the action of hormones and neurotransmitters. The great diversity of agonist-induced Ca(2+) signatures, visualized by optical imaging techniques, can be explained by the production of intracellular messengers triggering Ca(2+) release from internal stores and/or by different coupling of Ca(2+) release to Ca(2+) entry. Several messengers, such as inositol trisphosphate and cyclic ADP-ribose, have been identified to date. More recent studies have reported the important role of a newly discovered Ca(2+) releasing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). These studies have shown important interactions of these messengers in the generation of specific Ca(2+) signals. NAADP acts at a very low concentration and seems to have a key role in sensitising cyclic ADP-ribose and inositol trisphosphate receptors. These points will be discussed in the present review.     

低温存活猫肾亚细胞Ca2+浓度变化的X-射线微区分析

随着肾离体低温保存时间的延长，细胞内钙离子沉积而导致细胞损伤，影响肾脏整体功能的保存情况以及移植后肾脏的存活率，本地家猫肾脏离体后冲洗至灰白，然后置于改良Collins II保存液中低温存活，保存0，24，48，72h以后，应用Ca^2 细胞化学探针（Calcium cytochemical probe)及X－射线微区分析技术（X－ray microanalysis of microsections)检测肾小管上皮细胞亚细胞水平的钙离子浓度变化，肾脏经过不同时间体外存活以后，细胞核和内质网中钙离子浓度没有明显的变化，而线粒体和细胞溶质中钙离子浓度显著地呈现出线性升高，结果表明，随着肾脏保存时间延长，细胞溶质钙离子浓度升高源于细胞内储存的钙离子释放，正常生理条件下在细胞信息传递中充当“钙库”的线粒体和内质网在低温导致的细胞溶质内钙离子浓度上升中不表现出钙源特性。     

Can neuronal smooth endoplasmic reticulum function as a calcium reservoir?

The form of the smooth endoplasmic reticulum in dorsal root axons is described in thick (1 μm) and thin sections of rat dorsal roots previously impregnated with lead and copper. When dorsal roots were fixed in an osmium-pyroantimonate solution for the ultrastructural visualization of Ca, deposits were seen in the smooth endoplasmic reticulum and to a lesser extent in mitochondria. Following preincubation in a Ca-containing depolarizing solution dense deposits were present on the inner surface of the axolemma as well as in smooth endoplasmic reticulum and mitochondria. The presence of Ca in the smooth endoplasmic reticulum was confirmed by X-ray microanalysis. Ca-Mg activated adenosine triphosphatase activity could be localized to the smooth endoplasmic reticulum membrane using a modified Gomori procedure.The possibility is discussed that the smooth endoplasmic reticulum may function as a Ca reservoir for Ca-dependent intra-axonal processes.     

Localization of an endoplasmic reticulum calcium ATPase mRNA in rat brain by in situ hybridization

SERCA-2 is an endoplasmic reticulum Ca2+ ATPase present in brain [Gunteski-Hamblin A.-M. et al. (1988) J. biol. Chem. 263, 15032-15040]. We sought to map the distribution of this pump in the rat brain and investigate its relationship to Ca2+ uptake by brain endoplasmic reticulum. Using in situ hybridization and Northern blots with antisense oligonucleotide probes, we found that SERCA-2 is concentrated most densely in the cerebellum, especially in Purkinje cells, and in the hippocampus, with heavy labeling also in cortex, thalamus, pontine nuclei and the mitral cell layer of the olfactory bulb. 45Ca2+ uptake displayed a similar pattern with heaviest accumulation in cerebellum, hippocampus, cortex, thalamus and olfactory bulb. In corpus striatum and substantia nigra, relative 45Ca2+ accumulation was greater than SERCA-2 mRNA. Thus, SERCA-2 appears to be involved in Ca2+ uptake into endoplasmic reticulum in brain for release by inositol 1,4,5-trisphosphate and other agents.     

Non-independent quantum bumps in Limulus ventral nerve photoreceptors--a new insight in the light transduction mechanism.

Single photon-induced transient currents, called quantum bumps were stimulated by short flashes in dark-adapted ventral nerve photoreceptors of Limulus. Flash intensities were set to activate 3 or more bumps. In most cases, current bumps were activated with a constant rate. The frequency of bump occurrence was between 9 and 17 Hz. Results show that consecutive bumps are not independent and that some of them are not activated by a photon. The periodic bump activation indicates a molecular mechanism which quantifies the transmitter release not only by a light quantum, but also by a late phase of the transduction cascade. A model is proposed, in which Ca2+ ions released from intracellular stores transiently block the further Ca2+ release by inositol trisphosphate in an all-or-none manner.