The effects of intracellular iontophoretic injection of calcium and sodium ions on the light response of Limulus ventral photoreceptors

During intracellular iontophoretic injection of Ca⁺⁺ into Limulus ventral photoreceptor cells, there is a progressive diminution of the light response. Following Ca⁺⁺ injection, the size of the light response slowly recovers. Similarly, there is a progressive diminution of the light response during intracellular injection of Na⁺ and recovery after the injection is stopped. The rate of diminution during Na⁺ injection is greater for higher [Ca⁺⁺]_out. In solutions which contain 0.1 mM Ca⁺⁺, there is nearly no progressive decrease in the size of the light response during Na⁺ injection. Intracellular injections of Li⁺ or K⁺ do not progressively decrease the size of the light response. We propose that an increase in [Na⁺]_in leads to an increase in [Ca⁺⁺]_in and that an increase in [Ca⁺⁺]_in by any means leads to a reduction in responsiveness to light.     

Acyltransferase and acid hydrolase activities of the abalone photoreceptor cell

Summary In order to study the synthesis and degradation processes of the photoreceptor membranes in the abalone, Nordotis discus, the localization of acyltransferase and acid hydrolase activities, respectively, were determined at the electron-microscopic level. Acyltransferase activity was localized on the cytoplasmic sides of thick (>10 nm) membranes of the following organelles: a few cisternae at the trans (or concave) side of Golgi apparatus, Golgi and probably related vesicles, short tubules, curved pentalaminar disks and limiting membranes of the phagosomal multivesicular bodies; all organelles were scattered in the peri- to supranuclear cytoplasm. The phospholipids, which are major components of the photoreceptor membrane, are considered to be synthesized by these membranes. Acid phosphatase activity was localized in the lumina of Golgi cisternae and vesicles, lysosomes, and smaller multivesicular and related bodies, but not in multilamellar bodies. The matrices of the larger multivesicular bodies and of the pigment granule complexes showed arylsulfatase activity. Vesiculated and autophagocytosed photoreceptor microvilli seemed to be degraded by acid hydrolases, forming multivesicular and related bodies. Supporting cells also showed acyltransferase and acid hydrolase activities.Abbreviations used in this Paper AcP acid phosphatase - ArS arylsulfatase - AT acyltransferase - ER endoplasmic reticulum - GERL Golgi-endoplasmic reticulum-lysosomal complex - MEB meshwork body - MLB multilamellar body - MVB multivesicular body - VLB vesiculolamellar body     

Methods for monitoring Ca2+ and ion channels in the lysosome

Lysosomes and lysosome-related organelles are emerging as intracellular Ca²⁺ stores and play important roles in a variety of membrane trafficking processes, including endocytosis, exocytosis, phagocytosis and autophagy. Impairment of lysosomal Ca²⁺ homeostasis and membrane trafficking has been implicated in many human diseases such as lysosomal storage diseases (LSDs), neurodegeneration, myopathy and cancer. Lysosomal membrane proteins, in particular ion channels, are crucial for lysosomal Ca²⁺ signaling. Compared with ion channels in the plasma membrane, lysosomal ion channels and their roles in lysosomal Ca²⁺ signaling are less understood, largely due to their intracellular localization and the lack of feasible functional assays directly applied to the native environment. Recent advances in biomedical methodology have made it possible to directly investigate ion channels in the lysosomal membrane. In this review, we provide a summary of the newly developed methods for monitoring lysosomal Ca²⁺ and ion channels, as well as the recent discovery of lysosomal ion channels and their significances in intracellular Ca²⁺ signaling. These new techniques will expand our research scope and our understanding of the nature of lysosomes and lysosome-related diseases.     

Effects of membrane stabilizers on pancreatic amylase release

Summary Compounds with membrane stabilizing activity were studied as to their ability to affect pancreatic amylase release and the steps in the stimulus-secretion coupling process. Chlorpromazine, propranolol, and thymol were all found to inhibit bethanechol-stimulated amylase release and at slightly higher concentrations to induce release regardless of the presence of the secretagogue. This biphasic effect was similar to that found previously for the local anesthetic tetracaine. Release by high concentrations of propranolol and tetracaine was accompanied by ultrastructural evidence of cell damage. Membrane stabilizers at concentrations which inhibited amylase release were shown to block bethanechol-induced depolarization and stimulation of⁴⁵Ca⁺⁺ efflux although the drugs alone partially depolarized pancreatic cells. Release of amylase induced by Ca⁺⁺ introduced by the ionophore A23187 was also abolished. These findings indicate that membrane stabilizers independently inhibit the steps leading to a rise in intracellular Ca⁺⁺ and the subsequent Ca⁺⁺-activated amylase release.     

Bioelectric Control of Locomotion in the Ciliates*†

SYNOPSIS. Locomotor behavior in the ciliate protozoa is controlled by the cell membrane through electrophysiological principles already familiar in receptor, nerve, and effector cells of the metazoa. This is illustrated by the avoiding reaction (15). When the membrane of the anterior part of the ciliate receives a mechanical stimulus, as during collision, it permits a local influx of Ca⁺⁺. This constitutes a receptor current which depolarizes the remaining cell membrane by electrotonic spread. Depolarization causes a secondary transient increase in the calcium conductance of the entire cell membrane, and a general influx of Ca⁺⁺ occurs. The resulting increase in concentration of intracellular Ca⁺⁺ activates a reorientation (“reversal”) of the ciliary power stroke, causing the organism to swim backward. Forward locomotion is restored as the resting concentration of intracellular Ca⁺⁺ in the cell cortex is restored by diffusion, active extrusion, or intracellular sequestering. The control and coordination of locomotion in ciliates depend on several factors in addition to the excitable properties of the membrane. These include the sensitivities of the ciliary apparatus to intracellular concentrations of calcium and other regulating substances, the anatomical distribution of sensory receptor properties of the cell membrane, and the cable properties of the cell which permit electrotonic spread of graded potential signals without need of all-or-none conducted signals.     

An electrogenic sodium pump in Limulus ventral photoreceptor cells

A hyperpolarization can be recorded intracellularly following either a single bright light stimulus or the intracellular injection of Na⁺. This after-hyperpolarization is abolished by bathing in 5 x 10^-6 M strophanthidin or removal of extracellular K⁺. Both treatments also lead to a small, rapid depolarization of the dark-adapted cell. When either treatment is prolonged, light responses can still be elicited, although with repetitive stimuli the responses are slowly and progressively diminished in size. The rate of diminution is greater for higher values of [Ca⁺⁺]_out; with [Ca⁺⁺]_out = 0.1 mM, there is almost no progressive diminution of repetitive responses produced by either K⁺-free seawater or strophanthidin. We propose that an electrogenic Na⁺ pump contributes directly to dark-adapted membrane voltage and also generates the after-hyperpolarizations, but does not directly generate the receptor potential. Inhibition of this pump leads to intracellular accumulation of sodium ions, which in turn leads to an increase in intracellular Ca⁺⁺ (provided there is sufficient extracellular Ca⁺⁺). This increase in intracellular calcium probably accounts for the progressive decrease in the size of the receptor potential seen when the pump is inhibited.     

On the mechanism of stimulation of H+ transport in gastric mucosa by Ca++ ionophore A23187: I. Pathways for cytosolic calcium increase

The pathways for cytosolic Ca⁺⁺ increase under A23187 stimulation of H⁺ secretion were studied in the isolated gastric mucosa of the toad $\text{Bufo marinus}$. A23187 produced a more potent stimulation of secretion when added to the mucosal side which did not contain calcium. Measurements of ionophore incorporation by fluorometric methods indicated that A23187 incorporates into oxyntic cells intracellularly. The presence of divalent cations inhibited incorporation. This may be the reason for a more potent action when A23187 was added from the mucosal side. With-drawal of calcium from serosal solution largely inhibited the secretory response to A23187 added to the mucosal side. Reintroduction of calcium into the serosal side in the presence of ionophore elicited H⁺ secretion. The results are consistent with an uptake of A23187 from the mucosal side into cellular organelles and basolateral membranes. Calcium entry through the serosal side may be responsible for triggering secretion. Although A23187 likely releases calcium from intracellular stores, its rate of release may not be sufficient to bring about a full stimulation of secretion in serosal-Ca⁺⁺-free conditions.     

Ultrastructural distribution of Ca++ within neurons

Summary We used the oxalate-pyroantimonate technique to determine the ultrastructural distribution of Ca⁺⁺ in neurons of the rat sciatic nerve. The content of the precipitate was confirmed by X-ray microanalysis and appropriate controls. In the cell bodies of the dorsal root ganglia, Ca⁺⁺ precipitate was found in the Golgi, mitochondria, multivesicular bodies and large vesicles of the cytoplasm but not in lysosomes, and was prominently absent from regions of rough endoplasmic reticulum and ribosomes. It was seen in the nucleus but not in the nuclear bodies or nucleolus.Within the axon itself, Ca⁺⁺ precipitate was also found sequestered in mitochondria and smooth endoplasmic reticulum. In addition Ca⁺⁺ precipitate found diffusely throughout the axoplasm exhibited a discrete and heterogeneous distribution. In myelinated fibers the amount of precipitate decreased predictably in the axoplasm beneath the Schmidt-Lanterman clefts and in the paranodal regions at the nodes of Ranvier. This correlated with the presence of dense precipitate in the Schmidt-Lanterman clefts them-selves and in the paranodal loops of myelin.Intracytoplasmic ionic Ca⁺⁺ is maintained at 10^–7 M by balanced processes of influx, sequestration and extrusion. The irregular distribution of Ca⁺⁺ precipitate in the axoplasm of myelinated fibers suggests that there may be specific regions of preferential efflux across the axolemma.     

Pancreatic acinar cells: use of Ca++ ionophore to separate enzyme release from the earlier steps in stimulus-secretion coupling

The Ca⁺⁺ ionophore A23187 had no effect on the release of amylase by mouse pancreas fragments in the absence of Ca⁺⁺ but when Ca⁺⁺ was re-added to the medium amylase release was observed in a pattern which mimicked that produced by normal stimulants. Uptake of ⁴⁵Ca⁺⁺ by pancreatic fragments was increased by A23187. Tetracaine and dinitrophenol at concentrations which block cholinergic stimulated enzyme release blocked ionophore induced release whereas atropine did not. None of the inhibitors studied affected the ionophore induced Ca⁺⁺ uptake.     

Inhibition of rabbit neutrophil lysosomal enzyme secretion,non-stimulated and chemotactic factor stimulated locomotion by nordihydroguaiaretic acid

Nordihydroguaiaretic acid irreversibly inhibits both Ca⁺⁺ dependent and independent lysosomal enzyme release from rabbit peritoneal neutrophils induced by the chemotactic factors, formyl-methionyl-leucyl-phenylalanine and C5a in the presence of cytochalasin B. The inhibition is both concentration and time dependent. In addition, the cytochalasin B dependent release induced by arachidonic acid and the Ca⁺⁺ ionophore A23187 is similarly inhibited. Similar concentrations of NDGA also inhibit neutrophil locomotion and chemotactic factor enhanced locomotion, as measured using modified Boyden chambers. As nordihydroguaiaretic acid has been shown to be an inhibitor of lipoxygenase activity, it is possible that this pathway of arachidonic acid metabolism is important in neutrophil locomotion and in cytochalasin B dependent lysosomal enzyme release induced by secretagogues.     

Ca++ fluxes in isolated cells of rat pancreas. Effect of secretagogues and different Ca++ concentrations

Summary Secretagogues of pancreatic enzyme secretion, the hormones pancreozymin, carbamylcholine, gastrin I, the octapeptide of pancreozymin, and caerulein as well as the Ca⁺⁺-ionophore A 23187 stimulate⁴⁵Ca efflux from isolated pancreatic cells. The nonsecretagogic hormones adrenaline, isoproterenol, secretin, as well as dibutyryl cyclic adenosine 3,5-monophosphate and dibutyryl cyclic guanosine 3,5-monophosphate have no effect on⁴⁵Ca efflux. Atropine blocks the stimulatory effect of carbamylcholine on⁴⁵Ca efflux completely, but not that of pancreozymin. A graphical analysis of the Ca⁺⁺ efflux curves reveals at least three phases: a first phase, probably derived from Ca⁺⁺ bound to the plasma membrane; a second phase, possibly representing Ca⁺⁺ efflux from cytosol of the cells; and a third phase, probably from mitochondria or other cellular particles. The Ca⁺⁺ efflux of all phases is stimulated by pancreozymin and carbamylcholine. Ca⁺⁺ efflux is not significantly effected by the presence or absence of Ca⁺⁺ in the incubation medium. Metabolic inhibitors of ATP production, Antimycin A and dinitrophenol, which inhibit Ca⁺⁺ uptake into mitochondria, stimulate Ca⁺⁺ efflux from the isolated cells remarkably, but inhibit the slow phase of Ca⁺⁺ influx, indicating the role of mitochondria as an intracellular Ca⁺⁺ compartment. Measurements of the⁴⁵Ca⁺⁺ influx at different Ca⁺⁺ concentrations in the medium reveal saturation type kinetics, which are compatible with a carrier or channel model. The hormones mentioned above stimulate the rate of Ca⁺⁺ translocation.The data suggest that secretagogues of pancreatic enzyme secretion act by increasing the rate of Ca⁺⁺ transport most likely at the level of the cell membrane and that Ca⁺⁺ exchange diffusion does not contribute to the⁴⁵Ca⁺⁺ fluxes.With the technical assistance of C. Hornung.     

Role of Ca in stimulus-secretion coupling in the gastric oxyntic cell: Effect of A23187

The effects of Ca⁺⁺ ionophore A23187 on H⁺ secretion and histamine release were studied in the isolated gastric mucosa of the toad . A23187 added from the mucosal side stimulated H⁺ secretion. At high concentrations, A23187 also caused histamine release. This histamine was not sufficient to explain the effects of A23187 on H⁺ secretion. Metiamide, only partially inhibited the effect of ionophore. There was summation and/or potentiation of effects between A23187 and histamine. The results are consistent with the hypothesis that Ca⁺⁺ acts as a second messenger in stimulus-secretion coupling in the oxyntic cell. It is possible that Ca⁺⁺ and cAMP may interact as parallel second messengers in the control of gastric H⁺ secretion.     

Calcium-Induced Ultrastructural Interactions in Adrenocorticocytes

Steroidogenesis in adrenocorticocytes is closely related to intracellular [Ca²⁺]. To detect ultrastructural changes induced by growth in cytosolic [Ca²⁺], we used a rat adrenocortical cell culture, which was examined with electron microscopy and morphometric analysis. We established that either KCl-induced membrane depolarization evoking Ca²⁺ influx into the cell via voltage-operated Ca channels and Ca²⁺ ionophore, A23187, induced remarkable ultrastructural interactions between several cytosolic organelles. Lipid droplets known as key elements for Ca²⁺-induced steroidogenesis directly contacted with organelles containing the enzymes providing steroidogenic reactions (mitochondria, smooth and rough endoplasmic reticulum, nucleus, peroxisomes, and lysosomes). In most cases, the lipid droplets formed a specialized morphological structure at the sites of contact with the partner organelles. These structures are interpreted as a specialized transporting system, which provides contacts between organelles and exchange of intermediate products of the steroidogenesis process between the droplet and organelles.     

Mitochondrial and calcium perturbations in rat CNS neurons induce calpain-cleavage of Parkin: Phosphatase inhibition stabilizes pSer65Parkin reducing its calpain-cleavage

Mitochondrial impairment and calcium (Ca⁺⁺) dyshomeostasis are associated with Parkinson's disease (PD). When intracellular ATP levels are lowered, Ca⁺⁺-ATPase pumps are impaired causing cytoplasmic Ca⁺⁺ to be elevated and calpain activation. Little is known about the effect of calpain activation on Parkin integrity. To address this gap, we examined the effects of mitochondrial inhibitors [oligomycin (Oligo), antimycin and rotenone] on endogenous Parkin integrity in rat midbrain and cerebral cortical cultures. All drugs induced calpain-cleavage of Parkin to ~36.9/43.6 kDa fragments. In contrast, treatment with the proinflammatory prostaglandin J2 (PGJ2) and the proteasome inhibitor epoxomicin induced caspase-cleavage of Parkin to fragments of a different size, previously shown by others to be triggered by apoptosis. Calpain-cleaved Parkin was enriched in neuronal mitochondrial fractions. Pre-treatment with the phosphatase inhibitor okadaic acid prior to Oligo-treatment, stabilized full-length Parkin phosphorylated at Ser⁶⁵, and reduced calpain-cleavage of Parkin. Treatment with the Ca⁺⁺ ionophore A23187, which facilitates Ca⁺⁺ transport across the plasma membrane, mimicked the effect of Oligo by inducing calpain-cleavage of Parkin. Removing extracellular Ca⁺⁺ from the media prevented oligomycin- and ionophore-induced calpain-cleavage of Parkin. Computational analysis predicted that calpain-cleavage of Parkin liberates its UbL domain. The phosphagen cyclocreatine moderately mitigated Parkin cleavage by calpain. Moreover, the pituitary adenylate cyclase activating peptide (PACAP27), which stimulates cAMP production, prevented caspase but not calpain-cleavage of Parkin. Overall, our data support a link between Parkin phosphorylation and its cleavage by calpain. This mechanism reflects the impact of mitochondrial impairment and Ca⁺⁺-dyshomeostasis on Parkin integrity and could influence PD pathogenesis.     

Ca++ fluxes in resealed synaptic plasma membrane vesicles

The effect of the monovalent cations Na⁺, Li⁺, and K⁺ on Ca⁺⁺ fluxes has been determined in resealed synaptic plasma membrane vesicle preparations from rat brain. Freshly isolated synaptic membranes, as well as synaptic membranes which were frozen (?80°C), rapidly thawed, and passively loaded with K₂/succinate and ⁴⁵CaCl₂, rapidly released approximately 60% of the intravesicular Ca⁺⁺ when exposed to NaCl or to the Ca⁺⁺ ionophore A 23187. Incubation of these vesicles with LiCl caused a lesser release of Ca⁺⁺. The EC₅₀ for Na⁺ activation of Ca⁺⁺ efflux from the vesicles was approximately 6.6mM. exposure of the Ca⁺⁺-loaded vesicles to 150 mM KCl produced a very rapid (?1 sec) loss of Ca⁺⁺ from the vesicles, but the Na⁺-induced efflux could still be detected above this K⁺ - sensitive effect. Vesicles pre-loaded with NaCl (150 mM) exhibited rapid ⁴⁵Ca uptake with an estimated EC₅₀ for Ca⁺⁺ of 7–10 μM. This Ca⁺⁺ uptake was blocked by dissipation of the Na⁺ gradient. These observations are suggestive of the preservation in these purified frozen synaptic membrane preparations of the basic properties of the Na⁺Ca⁺⁺ exchange process and of a K⁺ - sensitive Ca⁺⁺ flux across the membranes.     

Ultracytochemical localization of Ca++-ATPase activity in the paraphyseal epithelial cells of the frog,Rana esculenta

Summary Ca⁺⁺-ATPase activity was studied ultracytochemically (cf. Ando et al. 1981) in the paraphysis cerebri of the frog. An intense reaction was demonstrated on the plasmalemma of the microvilli at the apical pole of paraphyseal cells; in contrast, the basolateral plasmalemma showed only a slight staining. In addition, mitochondria, gap junctions, cilia, and cytoplasmic elements (e.g., microfilaments) displayed Ca⁺⁺-ATPase activity. Variation of the Ca⁺⁺-concentration in the incubation medium from 0.1 mM to 100 mM altered the Ca⁺⁺-ATPase activity of the cell organelles. The substitution of Ca-by Mg-ions resulted in a conspicuous decrease in the enzyme activity, especially on the apical plasmalemma. Ca⁺⁺-ATPase activity is claimed to be involved in a number of extra-and intracellular functions. In comparison to the epithelium of the adjacent choroid plexus the paraphyseal epithelial cell is thought to be a principal Ca-ion regulator of the cerebrospinal fluid in frogs.Fellow of the Alexander von Humboldt Foundation     

Rhabdom size and photoreceptor membrane turnover in a muscoid fly

Summary The cross-sectional area of rhabdomeres in the compound eye of the blowfly, Lucilia, was found to remain constant under 12 h light/12 h dark cyclic lighting, and 10 days constant light or darkness. It was reduced only slightly during 3 h light after 10 days darkness (by 21%), or on exposure to 2h darkness + 1.5 h light after 10 days light (by 10%). Morphological evidence indicates that shedding of photoreceptor membrane during turnover is achieved by an extracellular route, and by pinocytosis from the bases of the microvilli. The photoreceptor membrane shed by both mechanisms appears to accumulate in multivesicular bodies. The amount of photoreceptor membrane shedding, as indicated by numbers of multivesicular bodies, is constant throughout the day and night on cyclic lighting, decreases in constant darkness, but returns to its normal level after an exposure to 3 h light subsequent to 10 days darkness.     

Intracellular localization and degradation of asialofetuin in isolated rat hepatocytes

Analysis by isopycnic and differential centrifuging of the intracellular distribution of radioactivity following uptake of ¹²⁵I-labelled asialofetuin by isolated rat hepatocytes showed that during incubations up to 1 h, most of the radioactivity was associated with structures which had a subcellular distribution pattern different from both the lysosomes and the plasma membrane. The latter two organelles were followed by means of enzyme markers. Ca²⁺ is necessary for the binding of asialofetuin to the plasma membrane, and it was also possible to differentiate between asialofetuin bound to the plasma membrane and that contained in intracellular structures by removing Ca²⁺ from the medium (by EGTA). Such experiments showed that asialofetuin became rapidly internalized. Practically all the labelled protein was located intracellularly in cells that had been incubated with asialofetuin for more that 30 min. When incubations were carried out for more that 1 h a peak appeared in the radioactivity distribution in the same place as the peak of activity of lysosomal marker enzymes. However, degradation of asialofetuin takes place in the lysosomes and this starts before the labelled protein can be found in the lysosomal fractions. Our data suggest that the rate-determining step in the cellular handling of asialofetuin is the transport of endocytized protein from the endocytic vesicles to the lysosomes.     

The Initiation of Spike Potential in Barnacle Muscle Fibers under Low Intracellular Ca++

Electrical properties of the muscle fiber membrane were studied in the barnacle, Balanus nubilus Darw. by using intracellular electrode techniques. A depolarization of the membrane does not usually produce an all-or-none spike potential in the normal muscle fiber even though a mechanical response is elicited. The intracellular injection of Ca⁺⁺-binding agents (K₂SO₄ and K salt of EDTA solution, K₃ citrate solution, etc.) renders the fiber capable of initiating all-or-none spikes. The overshoot of such a spike potential increases with increasing external Ca concentration, the increment for a tenfold increase in Ca concentration being about 29 mv. The threshold membrane potential for the spike and also for the K conductance increase shifts to more positive membrane potentials with increasing [Ca⁺⁺]_out. The removal of Na ions from the external medium does not change the configuration of the spike potential. In the absence of Ca⁺⁺ in the external medium, the spike potential is restored by Ba⁺⁺ and Sr⁺⁺ but not by Mg⁺⁺. The overshoot of the spike potential increases with increasing [Ba⁺⁺]_out or [Sr⁺⁺]_out. The Ca influx through the membrane of the fiber treated with K₂SO₄ and EDTA was examined with Ca⁴⁵. The influx was 14 pmol per sec. per cm² for the resting membrane and 35 to 85 pmol per cm² for one spike. From these results it is concluded that the spike potential of the barnacle muscle fiber results from the permeability increase of the membrane to Ca⁺⁺ (Ba⁺⁺ or Sr⁺⁺).     

Ultracytochemical study of Ca++-ATPase and K+-NPPase activities in retinal photoreceptors of the guinea pig

Summary Ca⁺⁺-ATPase activity was demonstrated histochemically at light- and electron-microscopic levels in inner and outer segments of retinal photoreceptor cells of the guinea pig with the use of a newly developed one-step lead-citrate method (Ando et al. 1981). The localization of ouabain-sensitive, K⁺-dependent p-nitrophenylphosphatase (K⁺-NPPase) activity, which represents the second dephosphorylative step of the Na⁺-K⁺-ATPase system, was studied by use of the one-step method newly adapted for ultracytochemistry (Mayahara et al. 1980). In retinal photoreceptor cells fixed for 15 min in 2% paraformaldehyde the electron-dense Ca⁺⁺-ATPase reaction product accumulated significantly on the inner membranes of the mitochondria but not on the plasmalemma or other cytoplasmic elements of the inner segments. The membranes of the outer segments remained unstained except the membrane arrays in close apposition to the retinal pigment epithelium. The cytochemical reaction was Ca⁺⁺- and substrate-dependent and showed sensitivity to oligomycin. When Mg⁺⁺-ions were used instead of Ca⁺⁺-ions, a distinct reaction was also found on mitochondrial inner membranes.In contrast to the localization of the Ca⁺⁺ -ATPase activity, the K⁺-NPPase activity was demonstrated only on the plasmalemma of the inner segments, but not on the mitochondria, other cytoplasmic elements or the outer segment membranes. This reaction was almost completely abolished by ouabain or by elimination of K⁺ from the incubation medium.Fellow of the Alexander von Humboldt Foundation, Bonn, Federal Republic of Germany