Elevation of cyclic AMP activates an actin-dependent contraction in teleost retinal rods

Cone myoid elongation involves unidirectional microtubule movement mediated by dynein-1

Using structured illumination microscopy and photoconvertible tubulin in zebrafish photoreceptors, it is shown that microtubules move together during myoid elongation, a dark adaptive process in cone photoreceptors. Additionally, cytoplasmic dynein-1, localized at the base of the elongating myoid, mediates this unidirectional movement of microtubules.

Teleosts and amphibians exhibit retinomotor movements, morphological changes in photoreceptors regulated by light and circadian rhythms. Cone myoid elongation occurs during dark adaptation, leading to the positioning of the cone outer segment closer to the retinal pigment epithelium. Although it has been shown that microtubules are essential for cone myoid elongation, the underlying mechanism has not been established. In this work, we generated a transgenic line of zebrafish expressing a photoconvertible form of α-tubulin (tdEOS-tubulin) specifically in cone photoreceptors. Using superresolution structured illumination microscopy in conjunction with both pharmacological and genetic manipulation, we show that cytoplasmic dynein-1, which localizes to the junction between the ellipsoid and myoid, functions to shuttle microtubules from the ellipsoid into the myoid during the course of myoid elongation. We propose a novel model by which stationary complexes of cytoplasmic dynein-1 are responsible for the shuttling of microtubules between the ellipsoid and myoid is the underlying force for the morphological change of myoid elongation.

What drives cell morphogenesis: a look inside the vertebrate photoreceptor

Vision mediating photoreceptor cells are specialized light-sensitive neurons in the outer layer of the vertebrate retina. The human retina contains approximately 130 million of such photoreceptors, which enable images of the external environment to be captured at high resolution and high sensitivity. Rod and cone photoreceptor subtypes are further specialized for sensing light in low and high illumination, respectively. To enable visual function, these photoreceptors have developed elaborate morphological domains for the detection of light (outer segments), for changing cell shape (inner segments), and for communication with neighboring retinal neurons (synaptic terminals). Furthermore, rod and cone subtypes feature unique morphological variations of these specialized characteristics. Here, we review the major aspects of vertebrate photoreceptor morphology and key genetic mechanisms that drive their formation. These mechanisms are necessary for cell differentiation as well as function. Their defects lead to cell death.

Photoreceptor-specific expression, light-dependent localization, and transcriptional targets of the zinc-finger protein Yin Yang 1 in the chicken retina

The zinc-finger transcription factor Yin Yang 1 (YY1) is a multifunctional protein that plays a critical role in embryonic development. Although it has been shown to play a role in eye development, its expression in the retina was not previously described. Here, we investigated YY1 expression in chicken tissues and we identified the neural retina as one of the tissues with highest YY1 protein levels. Immunohistochemical detection of YY1 in the retina revealed a clear-cut photoreceptor specificity and day/night differences in the cytoplasmic localization of the protein. YY1 was also present at high concentration in the nuclei of some photoreceptors. Gel-shift assays indicated YY1 bound to regulatory regions of several genes specifically expressed in photoreceptors. One of these genes, hydroxyindole-O-methyltransferase (EC 2.1.1.4), encodes the last enzyme of the melatonin synthesis pathway. Although over-expression of chicken YY1 was not sufficient to activate the chicken hydroxyindole-O-methyltransferase promoter in HEK293 cells, the YY1-binding site contained in this promoter was clearly required for full transcriptional activity in chicken embryonic retinal cells. These results suggest a role of YY1 in regulating the melatoninergic function of retinal photoreceptors.

PKCalpha translocation is microtubule-dependent in passaged smooth muscle cells

The translocation of protein kinase C (PKC) isozymes from their inactive cell locus to a variety of cytoskeletal, organelle, and plasmalemmal sites is thought to play an important role in their activation and substrate specificity. We have utilized confocal microscopy to compare phorbol 12, 13 dibutyrate (PDB) - stimulated translocation of PKCalpha in cultured cells derived from rat vascular smooth muscle. In enzymatically dispersed, passaged smooth muscle cells, PKCalpha was uniformly distributed throughout the unstimulated cell. PDB stimulation resulted in extensive association of the PKCalpha into filamentous strands with subsequent accumulation of the isoform in the peri-nuclear region of the cell. Dual immunostaining indicated that PKCalpha was extensively colocalized with microtubules in the interval immediately following PDB stimulation but was largely disassociated from microtubules at 10 min, at which time the translocation of PKCalpha to the peri-nucleus/nucleus was nearly complete. It was further found that the use of colchicine to disrupt the microtubules caused the loss of PKCalpha translocation to the peri-nuclear region. By comparison, cytochalasin B disruption of actin microfilaments had no significant effect on this parameter. The data suggest that PDB stimulation results in a transient association of PKCalpha with cell microtubules and that the microtubules play an important role in the translocation of PKCalpha from the cytosol in passaged cells derived from rat aortic smooth muscle.

Rhodopsin trafficking and its role in retinal dystrophies

We review the sorting/targeting steps involved in the delivery of rhodopsin to the outer segment compartment of highly polarized photoreceptor cells. The transport of rhodopsin includes (1) the sorting/budding of rhodopsin-containing vesicles at the trans-Golgi network, (2) the directional translocation of rhodopsin-bearing vesicles through the inner segment, and (3) the delivery of rhodopsin across the connecting cilium to the outer segment. Several independent lines of evidence suggest that the carboxyl-terminal, cytoplasmic tail of rhodopsin is involved in the post-Golgi trafficking of rhodopsin. Inappropriate subcellular targeting of naturally occurring rhodopsin mutants in vivo leads to photoreceptor cell death. Thus, the genes encoding mutations in the cellular components involved in photoreceptor protein transport are likely candidate genes for retinal dystrophies.

Interocular effect of actin depolymerization on spinule formation in teleost retina

Teleost retinas adapted to light show numerous spinules invaginated in the cone pedicles whereas darkness induces a reduction in the number of spinules. Horizontal cells show nematosomes whose size decreases as the number of spinules increases. We have investigated the involvement of actin filaments in spinule formation, by using cytochalasin D through intraocular injection into an eye. The ultrastructural analysis reveals that cytochalasin D impairs spinule formation and nematosome-size reduction in both treated and contralateral untreated retinas.

Involvement of the cytoskeleton in calcium-dependent stress relaxation of rat aortic smooth muscle

Rat aortic smooth muscle exhibits a remarkable capacity for stress relaxation, the release of tension following tissue stretch. Stress relaxation was markedly enhanced in contracted aortic rings compared with unstimulated tissue. The magnitude of stress relaxation in contracted aortic rings correlated well with the passive tension imposed on the tissue by stretching, but showed little relationship to changes in tissue length or to the level of tension developed in response to agonist stimulation prior to stretch. The enhancement of stress relaxation in precontracted tissue was not affected by intimal rubbing or treatment with L-NAME. By comparison, the removal of extracellular calcium markedly attenuated stress relaxation. In addition, the use of cytochalasin B to block actin polymerization inhibited stress relaxation, whereas colchicine, a drug used to cause microtubule disassembly, had no effect on the phenomenon. The results indicate that the enhanced stress relaxation in contracted tissue is a calcium-dependent process and is not due to passive tissue elastic properties. We suggest that stress relaxation may not involve cross-bridge formation but could be explained by the remodelling of a portion of the tension-bearing actin cytoskeleton.

Isolation of a cDNA encoding a photoreceptor cell-specific actin-bundling protein: retinal fascin

We have isolated a novel retina-specific gene, retinal fascin, encoding a new member of actin-bundling protein gene family, from a bovine retina cDNA library. The cDNA encodes a 492 amino acid protein which shows 36-57% amino acid identity with three vertebrate fascins, echinoid fascin and Drosophila singed gene. Northern blot analysis revealed that retinal fascin mRNA was exclusively expressed in the eye and not seen in other tissues examined. In situ hybridization analysis indicated that retinal fascin mRNA signals were found only in the inner segment of the photoreceptor layer and outer nuclear layer, indicating that retinal fascin was specifically expressed in photoreceptor cells. As fascins are actin-bundling proteins important for constructing several intracellular structures, retinal fascin might play a pivotal role in photoreceptor cell-specific events, such as disk morphogenesis.

Protein content and cAMP-dependent phosphorylation of fractionated white perch retina

In the retinas of teleost fish dopamine, released from interplexiform cells, modulates synaptic transmission at both the chemical and electrical synapses of retinal horizontal cells. This modulation is due to activation of adenylate cyclase and phosphorylation by protein kinase A, perhaps of the synaptic ion channel proteins themselves. In this study we have fractionated the white perch retina by Percoll density gradient centrifugation in order to identify proteins which coenrich with horizontal cells. In addition we have tested retinal fractions for phosphorylation by native cAMP-dependent kinase. Our findings indicate that there are at least 3 proteins of molecular weights 28, 43/44 and 50 kDa which coenrich with horizontal cells and 3 proteins of 30/31 kDa, 35 kDa (putative rhodopsin) and 48 kDa (putative arrestin) which coenrich with photoreceptor fractions. The 43/44 kDa phosphoprotein is a target for cAMP-dependent protein phosphorylation and thus is apparently an element of the dopaminergic modulatory pathway in perch horizontal cells.

Actin-dependent myoid elongation in teleost rod inner/outer segments occurs in the absence of net actin polymerization

In the retinas of teleost fish, rod photoreceptors elongate in response to light. Light-activated elongation is mediated by the myoid of the rod inner segment and is actin-dependent. Inner segment F-actin filaments form bundles running parallel to the cell's long axis. We examined the mechanism of rod elongation using mechanically-detached rod fragments, consisting of the motile inner segment and sensory outer segment (RIS-ROS). When RIS-ROS are isolated from dark-adapted green sunfish and cultured in the light, they elongate 15 microns at 0.3-0.6 microns/min. Elongation was inhibited 65% by 0.1 microM Cytochalasin D, suggesting a requirement for actin assembly. To determine the extent of assembly during elongation, we used three approaches to measure the F-actin content in RIS-ROS: detection of pelletable actin by SDS-PAGE after detergent-extraction of RIS-ROS; quantification of fluorescein-phalloidin binding by fluorimetry, fluorescence-activated cell sorting and image analysis; estimation of total F-actin filament length by electron microscopy. All three assays indicated that no net assembly of RIS-ROS F-actin accompanied myoid elongation. An increase in F-actin content within the elongated myoid was counterbalanced by a decrease in F-actin content within the 13 microvillus-like calycal processes located at the end of the inner segment opposite to the growing myoid. O'Connor and Burnside (Journal of Cell Biology 89:517-524, 1981) showed that minus-ends of rod F-actin filaments are oriented towards the elongating myoid while plus-ends are oriented towards the shortening calycal processes. Our observations suggest that RIS-ROS elongation entails actin polymerization at the minus-ends of filaments coupled with depolymerization at the filament plus-ends.

Posttranslational modifications of tubulin in teleost photoreceptor cytoskeletons

1. Posttranslational modifications of tubulin by acetylation and detyrosination have been correlated previously with microtubule stability in numerous cell types. 2. In this study, posttranslational modifications of tubulin and their regional distribution within teleost photoreceptor cones and rods are demonstrated immunohistochemically using antibodies specific for acetylated, detyrosinated, or tyrosinated tubulin. 3. Immunolocalization was carried out on isolated whole cones and mechanically detached rod and cone inner/outer segments. 4. Acetylated tubulin within rods and cones is found only in microtubules of the ciliary axoneme of the outer segment. Detyrosinated tubulin is also enriched in axonemes of both rod and cone outer segments. 5. Distributions of tyrosinated and detyrosinated cytoplasmic microtubules differ within cones and rods. In cones, detyrosinated and tyrosinated tubulins are both abundant throughout the cell body. In rods, the ellipsoid and myoid contain much more tyrosinated tubulin than detyrosinated tubulin. Comparisons between whole cones and cone fragments suggest that detyrosinated microtubules are more stable than tyrosinated microtubules in teleost photoreceptors. 6. Our findings provide further evidence that microtubules of teleost cones differ from rod microtubules in their stabilities and rapidity of turnover within the photoreceptor inner segment.

Alpha-actinin and actin in the outer retina: a double immunoelectron microscopic study

Actin has many diverse functions in the outer retina. To help elucidate its organization in this area, we have investigated the extent of its association with the actin cross-linking protein alpha-actinin. Ultrathin sections of chicken retina were double-immunolabelled with monospecific antibodies against actin and alpha-actinin. The highest relative amount of alpha-actinin to actin label was measured in the adherens junctions between the individual retinal pigmented epithelial (RPE) cells and between the photoreceptor and Mueller cells; in the photoreceptor myoid; and in the RPE basal microvilli. The lowest amount was in the Mueller cell microvilli, the RPE apical processes, and in the photoreceptor ellipsoid. It is likely that the areas containing the highest ratio of alpha-actinin to actin labelling are where the actin filaments are most highly cross-linked into bundles and linked to the plasma membrane by alpha-actinin. Actin filaments terminate in these areas, and, except for the myoid region, they are involved in cell-cell or cell-substrate adherens junctions.

Redistribution of membranes and cytoskeletal proteins in chicken oxyntic cells during the HCl secretory cycle: ultrastructural and immunofluorescence study

Changes in ultrastructure and cytoskeletal organization by avian oxyntic cells, at the onset of HCl secretion, were analysed. Cells in resting state, induced by fasting and cimetidine, were compared with histamine stimulated secreting cells. Ultrastructural studies were done by transmission electron microscopy; the distribution of prekeratin, myosin, and filamin-like protein, by immunofluorescence; and that of F-actin using FITC-phalloidin. Resting cells show short pericellular clefts. These are increasingly deepened in secreting cells by a reorganization of the lateral cell borders involving displacement of the junctional complexes toward the cell base and incorporation of the tubular system to the luminal plasma membrane. In secreting cells, the processes of the secretory surface are concentrated in a pericellular groove. Histamine stimulation induces a drastic redistribution of cytoskeletal proteins. In chicken oxyntic cells, in addition to the F-actin cytoskeleton associated with the membranes of the secretory surface, there is a cytoskeletal ring containing F-actin, myosin, and a filamin-like protein, located at the level of the junctional complexes. In resting cells, filaments and masses of cytoskeletal matrix are associated with the zonula adherens. In secreting cells, the junctional complexes maintain their association with the filamentous ring, while the amorphous matrix is replaced by microfilaments that support the processes of the luminal surface. Intermediate filaments form a peripheral ring probably associated with the zonula adherens, and project from the ring toward the cell cytoplasm. Thus, with the onset of HCl secretion, the apical cytoskeletal ring of resting cells displaces toward the cell base. A role for this cytoskeletal ring in the changes in shape parallel to HCl secretion is discussed.

Renewal of electrotonic synapses in teleost retinal horizontal cells

In teleost retinas, the somata of same-type cone horizontal cells are electrically coupled via extensive gap junctions, as are the axon terminals of same-type cells. This coupling persists throughout the animal's life and is modulated by dopamine and conditions of light- vs. dark-adaptation. Gap junction particle density in goldfish horizontal cell somata has also been shown to change under these conditions, indicating that these junctions are dynamic. We have used electron microscopy to examine gap junctions in bass horizontal cells with a fixation method that facilitates detection of gap junctions. Annular gap junction profiles were observed in the somatic cytoplasm of all cone horizontal cell types in both light- and dark-adapted animals. Serial sections showed that most profiles represented gap junction vesicles free within the cytoplasm; the remainder represented vesicles still attached to extensive plasma membrane gap junctions by a thin cytoplasmic neck, suggestive of an intermediate stage in endocytosis. Observations of gap junction vesicles containing fragments of gap junctional membrane and/or fused with lysosomal bodies further supported this hypothesis. Because gap junctions persist between the horizontal cells, we propose that gap junction endocytosis and lysosomal degradation are balanced by addition of new junctions. While endocytosis has been widely demonstrated to serve in programmed removal of gap junctions (without subsequent replacement), from both nonneuronal cells and developing neurons, this study indicates that it can also function in the renewal of electrical synapses in the adult teleost retina, where gap junction elimination is not the goal.

Disruption of microfilament organization and deregulation of disk membrane morphogenesis by cytochalasin D in rod and cone photoreceptors

Morphogenesis of photoreceptor outer segment disks appears to occur by an evagination of the ciliary plasma membrane (Steinberg et al., J Comp Neurol 190:501-519, '80). We tested if polymerized actin (F-actin) was necessary for the regulation of this postulated process by incubating Xenopus eyecups with 5 or 25 microM cytochalasin D for 6-28 hours. During the second hour, the incubation medium contained 3H-leucine. Both concentrations of cytochalasin resulted in: 1) dissolution of the rhodamine-phalloidin labeling pattern of photoreceptors, and 2) collapse of the calycal processes (which are normally filled with actin filaments) and disappearance of the inner segment microfilaments. In addition, the few most basal rod and cone outer segment disks appeared several times their normal diameter. These oversized disks had incorporated 3H-leucine and extended along the margin of the outer or inner segment. The nature of the overgrown disks is consistent only with a morphogenetic process involving evaginations of the ciliary plasma membrane. Deregulation by cytochalasin D was manifest by excessive growth of a few nascent disks rather than normal growth of many. Therefore, the normal network of actin filaments is apparently not necessary for continued evagination of the membrane, but it does seem to be an essential part of the mechanism that initiates the evagination of the ciliary plasma membrane and/or the mechanism that controls how far nascent disks grow.

Contraction and relaxation of cultured mesangial cells on a silicone rubber surface

Glomerular mesangial cells (MC) in culture are believed to contract or relax in response to agents such as angiotensin II and cyclic AMP. However, cells grown on glass or plastic surfaces are limited in their response to vasoactive agents because of the rigid surfaces to which they adhere; thus, interpretation of a change in shape as contraction, relaxation, or detachment is difficult. We have grown MC on a flexible silicone rubber (dimethylpolysiloxane) substrate (DMPS), and studied with sequential photographs several models of cell contraction, relaxation, and detachment. When the cells contracted, the DMPS became wrinkled; when the cells relaxed, the DMPS lost wrinkles. In contrast, if the cells detached, the sheet lost wrinkles as the cells became smaller and rounder. Angiotensin II (5 X 10(-7) M), and calcium ionophore A23187 (2 X 10(-6) M) increased wrinkles in more than 30% of cells at 22 degrees C and more than 40% of the cells at 36 degrees C. The earliest effect was visible within five to 10 minutes at 22 degrees C and within one minute at 36 degrees C and increased until 40 minutes; thereafter, the cells relaxed and wrinkles were reduced. 10(-1) M Na azide prevented the increase in wrinkles produced by angiotensin II. Seventy-two percent of the angiotensin II-treated cells whose margins could be seen in their entirety, and 78% of the calcium ionophore-treated cells showed a reduction in surface area at a time when new wrinkles were appearing or wrinkles were increasing in size. Dibutyryl cyclic AMP, a known smooth muscle relaxant, produced a decrease or loss of wrinkles in 90% of the cells, and an accompanying increase in surface area. Untreated control cells, observed in conjunction with the above series, showed little change in wrinkles. Ten percent DMSO, an actin-translocating agent, produced a reversible disappearance of wrinkles. These models of contraction and relaxation could be distinguished from cell detachment; EDTA, for example, in the presence of zero calcium, diminished both cell size and wrinkles, with an accompanying lifting of cells from the surface. Similar results were obtained with cytochalasin B and chlorpromazine. Thus, the silicone rubber system accurately reflects the contraction, relaxation and detachment of cultured mesangial cells in response to a variety of agents.

Dopaminergic regulation of cone retinomotor movement in isolated teleost retinas: I. Induction of cone contraction is mediated by D2 receptors

In the retinas of lower vertebrates, retinal photoreceptors and melanin pigment granules of the retinal pigment epithelium (RPE) undergo characteristic movements in response to changes in light intensity and to signals from an endogenous circadian clock. To identify agents responsible for mediating light and/or circadian regulation of these retinomotor movements, we investigated the effects of hormones and neurotransmitters on cone, rod, and RPE movements in the green sunfish, Lepomis cyanellus. We report here that 3,4-dihydroxyphenylethylamine (dopamine) mimics the effect of light by inducing light-adaptive retinomotor movements in all three cell types. In isolated dark-cultured retinas, dopamine induced light-adaptive cone contraction with a half-maximal effect at 10(-8) M. This effect of dopamine was inhibited by antagonists with a potency order characteristic of D2 receptor mediation. The dopamine uptake blocker benztropine also induced light-adaptive cone contraction in isolated dark-cultured retinas, suggesting that there is continuous dopamine release in the dark but that concomitant uptake normally prevents activation of cone contraction. That dopamine plays a role in light regulation of cone movement is further suggested by the observation that light-induced cone contraction was partially inhibited by sulpiride, a selective D2 dopamine antagonist, or by Co2+, a blocker of synaptic transmission. Sulpiride also promoted dark-adaptive cone elongation in isolated light-adapted retinas, suggesting that continuous dopamine action is required in the light to maintain the light-adapted cone position. Dopamine can act directly on D2 receptors located on rod and cone inner/outer segments: dopamine induced light-adaptive retinomotor movements in isolated distal fragments of dark-adapted photoreceptors cultured in the dark. Together our results indicate that dopamine induces light-adaptive retinomotor movements in cones, rods, and RPE cells by activating D2 receptors. We suggest that, in vivo, dopamine plays a role in both light and circadian regulation of retinomotor movements.

Dopamine inhibits forskolin- and 3-isobutyl-1-methylxanthine-induced dark-adaptive retinomotor movements in isolated teleost retinas

We have been investigating the mechanisms of diurnal and circadian regulation of teleost retinomotor movements. In the retinas of lower vertebrates, photoreceptors and melanin pigment granules of the retinal pigment epithelium (RPE) undergo movements at dawn and dusk. These movements continue to occur at subjective dawn and dusk in animals maintained in constant darkness. Cone myoids contract at dawn and elongate at dusk; RPE pigment disperses into the epithelial cells' long apical processes at dawn and aggregates into the cell bodies at dusk. We report here that forskolin, an adenylate cyclase activator, and 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor, each induces dark-adaptive cone and RPE retinomotor movements in isolated light-adapted green sunfish retinas cultured in constant light. Forskolin induces a 22-fold elevation in retinal cyclic AMP content. Forskolin- and IBMX-induced movements are inhibited approximately 65% and 95%, respectively, by 3,4-dihydroxyphenylethylamine (dopamine). However, dopamine does not inhibit dark-adaptive movements induced by dibutyryl cyclic AMP. Epinephrine is much less effective than dopamine in inhibiting forskolin-induced movements, while phenylephrine and clonidine are totally ineffective. These results are consistent with our previous findings that treatments that increase intracellular cyclic AMP content promote dark-adaptive retinomotor movement. They further suggest that dopamine inhibits adenylate cyclase activity in photoreceptors and RPE cells and thereby favors light-adaptive retinomotor movements.

cAMP dependent actin phosphorylation in developing rat lung and type II epithelial cells

Cyclic adenosine monophosphate (cAMP) increased in vitro phosphorylation of protein of 43,000 daltons in cytosolic fractions of rat lung and type II epithelial cells. The phosphoprotein was identified as 32P-actin by means of migration in one- and two-dimensional SDS-polyacrylamide gel electrophoresis and by phosphopeptide mapping followed by immunoperoxidase staining of peptides with anti-actin monoclonal antibody. Phosphorylation of actin in lung and type II cell cytosol was entirely cAMP dependent and the phosphorylated amino acid was identified as 32P-serine. Actin phosphorylation increased during the perinatal period of development, was barely detectable between 17 and 20 days gestation, increased prior to birth, and increased dramatically during the first week of life. Actin was the major substrate of cAMP-dependent protein kinase in lung cytosol from postnatal rats. Changes in actin phosphorylation that occur during development were not due to changes in cytosolic actin content or cAMP-dependent protein kinase activity but appeared to be related to the presence of factors inhibiting cAMP-dependent actin phosphorylation in fetal lung cytosol. Actin was also the major cAMP-dependent phosphoprotein identified in cytosolic fractions of purified type II epithelial cells. cAMP-dependent phosphorylation of pulmonary actin is developmentally regulated, occurring in association with other aspects of type II epithelial cell maturation during the perinatal period.

Desensitization of the serum effect on Na+ influx in cultured human fibroblasts

Stimulation of an amiloride-sensitive Na+ influx pathway, which mediates Na+/H+ exchange, has been postulated to be an important step in the initiation of DNA synthesis in quiescent human fibroblasts. If the elevation of intracellular Na+ or the alkalinization of intracellular pH resulting from the activation of this system is a trigger for subsequent mitogenic events, then its inactivation may also be important to cellular functions. We investigated the duration of the activation of Na+ influx by serum in human foreskin fibroblasts (HSWP). It was found that activation of Na+ influx by 10% serum was transient, declining with a t 1/2 = 15 min. Similarly, the Na+ content of the cells rose rapidly following serum addition and decreased with a t 1/2 = 15 min. In addition, both the lys-bradykinin- and the vasopressin-stimulated Na+ influx and Na+ content declined with a t 1/2 of approximately 15 min. Similar results were obtained using both Tris-buffered and Hepes-buffered, amino-acid-free EMEM. Finally, the above experiments were repeated under conditions normally used to assess the mitogenic response of cells. It was found that in cells arrested in G0 by serum deprivation in CO2-buffered EMEM, the serum activated Na+ flux was also transient with a t 1/2 of approximately 20 min. The desensitization of cells to serum could be readily (t 1/2 = 20') reversed by a subsequent incubation of cells in serum-free medium. Stimulation of Na+ influx by both the divalent cation ionophore A23187 and the phospholipase activator melittin in also desensitized rapidly, suggesting the process is independent of receptor downregulation. The desensitization during serum preincubation occurred in both low Na+ and low pH medium suggesting that the process is not due to negative feedback on the transport system via a rise in cellular Na+ concentration or a rise in intracellular pH. Although the mechanism of desensitization is at present not known, it is likely to be a physiologically important event.