Cell uncoupling and protein kinase C: correlation in a cell line but not in a differentiated tissue

Quantitation of neonicotinoid insecticides, plus qualitative screening for other xenobiotics, in small-mass avian tissue samples using UHPLC high-resolution mass spectrometry

We developed and validated a liquid chromatography-high-resolution mass spectrometry (LC-HRMS) analytical method for quantitatively measuring pesticide concentrations in small-body avian tissue samples using homogenized 1-2-d-old chicken carcasses as the test matrix. We quantified the following key insecticides: sulfoxaflor (sulfoximine class) and the neonicotinoids dinotefuran, nitenpyram, thiamethoxam, acetamiprid, thiacloprid, clothianidin, and imidacloprid. We used fortified chick carcass samples to validate method accuracy (80-125% recoveries), precision (<20% relative standard deviation), and sensitivity (≤1.2 ppb) for these targeted analytes. This method also uses full-scan, high-resolution MS to screen for the presence of a wide variety of other xenobiotics in bird carcasses. The utility of our screening process was demonstrated by the detection of carbaryl in some samples. This sensitive LC-HRMS analytical method for insecticide detection in a matrix of homogenized carcass is ideal for evaluating small birds for insecticide exposure. This novel whole-carcass method may allow for research studies of small-bodied, free-ranging avian species, and could provide insight regarding their exposure to multiple classes of environmental contaminants.

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

Reorganization of cytoskeletal proteins by Escherichia coli heat-stable enterotoxin (STa)-mediated signaling cascade

BACKGROUND

IP(3)-mediated calcium mobilization from intracellular stores activates and translocates PKC-alpha from cytosol to membrane fraction in response to STa in COLO-205 cell line. The present study was undertaken to determine the involvement of cytoskeleton proteins in translocation of PKC-alpha to membrane from cytosol in the Escherichiacoli STa-mediated signaling cascade in a human colonic carcinoma cell line COLO-205.

METHODS

Western blots and consequent densitometric analysis were used to assess time-dependent redistribution of cytoskeletal proteins. This redistribution was further confirmed by using confocal microscopy. Pharmacological reagents were applied to colonic carcinoma cells to disrupt the microfilaments (cytochalasin D) and microtubules (nocodazole).

RESULTS

STa treatment in COLO-205 cells showed dynamic redistribution and an increase in actin content in the Triton-insoluble fraction, which corresponds to an increase in polymerization within 1min. Moreover, pharmacological disruption of actin-based cytoskeleton greatly disturbed PKC-alpha translocation to the membrane.

CONCLUSIONS

These results suggested that the organization of actin cytoskeleton is rapidly rearranged following E. coli STa treatment and the integrity of the actin cytoskeleton played a crucial role in PKC-alpha movement in colonic cells. Depolymerization of tubulin had no effect on the ability of the kinase to be translocated to the membrane.

GENERAL SIGNIFICANCE

In the present study, we have shown for the first time that in colonic carcinoma cells, STa-mediated rapid changes of actin cytoskeleton arrangement might be involved in the translocation of PKC-alpha to membrane.

Modulatory effects of cAMP and PKC activation on gap junctional intercellular communication among thymic epithelial cells

BACKGROUND

We investigated the effects of the signaling molecules, cyclic AMP (cAMP) and protein-kinase C (PKC), on gap junctional intercellular communication (GJIC) between thymic epithelial cells (TEC).

RESULTS

Treatment with 8-Br-cAMP, a cAMP analog; or forskolin, which stimulates cAMP production, resulted in an increase in dye transfer between adjacent TEC, inducing a three-fold enhancement in the mean fluorescence of coupled cells, ascertained by flow cytometry after calcein transfer. These treatments also increased Cx43 mRNA expression, and stimulated Cx43 protein accumulation in regions of intercellular contacts. VIP, adenosine, and epinephrine which may also signal through cyclic nucleotides were tested. The first two molecules did not mimic the effects of 8-Br-cAMP, however epinephrine was able to increase GJIC suggesting that this molecule functions as an endogenous inter-TEC GJIC modulators. Stimulation of PKC by phorbol-myristate-acetate inhibited inter-TEC GJIC. Importantly, both the enhancing and the decreasing effects, respectively induced by cAMP and PKC, were observed in both mouse and human TEC preparations. Lastly, experiments using mouse thymocyte/TEC heterocellular co-cultures suggested that the presence of thymocytes does not affect the degree of inter-TEC GJIC.

CONCLUSIONS

Overall, our data indicate that cAMP and PKC intracellular pathways are involved in the homeostatic control of the gap junction-mediated communication in the thymic epithelium, exerting respectively a positive and negative role upon cell coupling. This control is phylogenetically conserved in the thymus, since it was seen in both mouse and human TEC preparations. Lastly, our work provides new clues for a better understanding of how the thymic epithelial network can work as a physiological syncytium.

Localization of ATP-sensitive K+ channel subunits in rat submandibular gland

ATP-sensitive K(+) (K(ATP)) channel subunits were investigated in rat submandibular gland (SMG). RT-PCR detected the presence of mRNA transcripts of the Kir6.1, Kir6.2, SUR2A, and SUR2B in the SMG, whereas SUR1 mRNA was barely detected. Western blot analysis provided the evidence that these four K(ATP) channel subunits are expressed in rat SMG. Immunostaining detected that these four K(ATP) channel subunits are widely distributed, with different intensities, in myoepithelial cells, epithelial cells of intercalated ducts, granular convoluted tubules, striated ducts, and excretory ducts. Immunofluorescence double staining showed that Kir6.1 and Kir6.2 colocalized with SUR2A in the myoepithelial cells, granular convoluted tubules, striated ducts, and excretory ducts. Kir6.1 and Kir6.2 also colocalized with SUR2B, mainly in the duct system, e.g., the granular convoluted tubules, striated ducts, and excretory ducts. Taken together, these results indicate that the K(ATP) channels in SMG may consist of Kir6.1, Kir6.2, SUR2A, and SUR2B, with various combinations of colocalization with each other, and may play important roles in rat SMG during salivary secretion.

Oxalate-induced activation of PKC-alpha and -delta regulates NADPH oxidase-mediated oxidative injury in renal tubular epithelial cells

Oxalate-induced oxidative stress contributes to cell injury and promotes renal deposition of calcium oxalate crystals. However, we do not know how oxalate stimulates reactive oxygen species (ROS) in renal tubular epithelial cells. We investigated the signaling mechanism of oxalate-induced ROS formation in these cells and found that oxalate significantly increased membrane-associated protein kinase C (PKC) activity while at the same time lowering cytosolic PKC activity. Oxalate markedly translocated PKC-alpha and -delta from the cytosol to the cell membrane. Pretreatment of LLC-PK1 cells with specific inhibitors of PKC-alpha or -delta significantly blocked oxalate-induced generation of superoxide and hydrogen peroxide along with NADPH oxidase activity, LDH release, lipid hydroperoxide formation, and apoptosis. The PKC activator PMA mimicked oxalate's effect on oxidative stress in LLC-PK1 cells as well as cytosol-to-membrane translocation of PKC-alpha and -delta. Silencing of PKC-alpha expression by PKC-alpha-specific small interfering RNA significantly attenuated oxalate-induced cell injury by decreasing hydrogen peroxide generation and LDH release. We believe this is the first demonstration that PKC-alpha- and -delta-dependent activation of NADPH oxidase is one of the mechanisms responsible for oxalate-induced oxidative injury in renal tubular epithelial cells. The study suggests that the therapeutic approach might be considered toward attenuating oxalate-induced PKC signaling-mediated oxidative injury in recurrent stone formers.

The effects of connexin phosphorylation on gap junctional communication

Gap junctions are specialized membrane domains composed of collections of channels that directly connect neighboring cells providing for the cell-to-cell diffusion of small molecules, including ions, amino acids, nucleotides, and second messengers. Vertebrate gap junctions are composed of proteins encoded by the "connexin" gene family. In most cases examined, connexins are modified post-translationally by phosphorylation. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the connexin "lifecycle", such as the trafficking, assembly/disassembly, degradation, as well as, the gating of gap junction channels. Since connexin43 (Cx43) is widely expressed in tissues and cell lines, we understand the most about how it is regulated, and thus, connexin43 phosphorylation is a major focus of this review. Recent reports utilizing new methodologies combined with the latest genome information have shown that activation of several kinases including protein kinase A, protein kinase C, p34(cdc2)/cyclin B kinase, casein kinase 1, mitogen-activated protein (MAP) kinase and pp60(src) kinase can lead to phosphorylation at 12 of the 21 serine and two of the six tyrosine residues in the C-terminal region of connexin43. In several cases, use of site-directed mutants of these sites have shown that these specific phosphorylation events can be linked to changes in gap junctional communication.

Pharmacological evidence for system-dependent involvement of protein kinase C isoenzymes in phorbol ester-suppressed gap junctional communication

Several phorbol esters are potent activators of protein kinase C. They down-regulate gap junctional intercellular communication and induce phosphorylation of connexin43, but the sensitivity and extent of responses vary much between systems. We asked whether the total protein kinase C enzyme activity or the protein kinase C isoenzyme constitution was of importance for such variations. Some fibroblastic culture systems were compared. It was concluded that the total protein kinase C enzyme activity did not determine the sensitivity to phorbol esters. Furthermore, the use of isotype-specific inhibitors of protein kinase C indicated that protein kinase C alpha, delta, and epsilon may be involved to different extents in different fibroblastic systems in the response to phorbol esters.

Phosphorylation of connexin43 on serine368 by protein kinase C regulates gap junctional communication

Phorbol esters (e.g., TPA) activate protein kinase C (PKC), increase connexin43 (Cx43) phosphorylation, and decrease cell-cell communication via gap junctions in many cell types. We asked whether PKC directly phosphorylates and regulates Cx43. Rat epithelial T51B cells metabolically labeled with (32)P(i) yielded two-dimensional phosphotryptic maps of Cx43 with several phosphopeptides that increased in intensity upon TPA treatment. One of these peptides comigrated with the major phosphopeptide observed after PKC phosphorylation of immunoaffinity-purified Cx43. Purification of this comigrating peptide and subsequent sequencing indicated that the phosphorylated serine was residue 368. To pursue the functional importance of phosphorylation at this site, fibroblasts from Cx43(-/-) mice were transfected with either wild-type (Cx43wt) or mutant Cx43 (Cx43-S368A). Intercellular dye transfer studies revealed different responses to TPA and were followed by single channel analyses. TPA stimulation of T51B cells or Cx43wt-transfected fibroblasts caused a large increase in the relative frequency of approximately 50-pS channel events and a concomitant loss of approximately 100-pS channel events. This change to approximately 50-pS events was absent when cells transfected with Cx43-S368A were treated with TPA. These data strongly suggest that PKC directly phosphorylates Cx43 on S368 in vivo, which results in a change in single channel behavior that contributes to a decrease in intercellular communication.

Does gut function limit hummingbird food intake?

Many nectar-feeding bird species decrease food intake when sugar concentration in food is increased. This feeding response can be explained by two alternative hypotheses: compensatory feeding and physiological constraint. The compensatory feeding hypothesis predicts that if birds vary intake to maintain a constant energy intake to match energy expenditures, then they should increase intake when expenditures are increased. Broad-tailed hummingbirds were presented with sucrose solutions at four concentrations (292, 584, 876, and 1,168 mmol L(-1)) and exposed to two environmental temperatures (10 degrees and 22 degrees C). Birds decreased volumetric food intake in response to sugar concentration. However, when they were exposed to a relatively sudden drop in environmental temperature and, hence, to an acute increase in thermoregulatory energy expenditures, they did not increase their rate of energy consumption and lost mass. These results support the existence of a physiological constraint on feeding intake. A simple chemical reactor model based on intestinal morphology and in vitro measurements of sucrose hydrolysis predicted observed intake rates closely. This model suggests that intestinal sucrose hydrolysis rates were near maximal and, thus, may have imposed limits to sugar assimilation. Although sugar assimilation was high (95%), the proportions of excreted sucrose, glucose, and fructose found in excreta differed significantly. The monosaccharides glucose and fructose were about eight and three times more abundant than sucrose, respectively. Broad-tailed hummingbirds are small high-altitude endotherms that face unpredictable weather and the energetic expense of premigratory fattening. Digestive processes have the potential to impose severe challenges to their energy budgets.

Short term regulation of cell-cell communication in TM3 Leydig cells. A perforated patch study

Determination of the junctional conductance (g(j)) in TM3 Leydig cells by the dual whole cell patch clamp technique (DWCPC) shows that coupling undergoes a rapid and irreversible run down. Addition of ATP or cAMP derivatives to the pipette solution has been shown to prevent this phenomenon in several tissues, but this same treatment is unable to inhibit run down in Leydig cells. Because the run down in junctional conductance may pose serious problems to the interpretation of results, we also measured g(j) by using the double perforated patch clamp technique (DPPT). Access to the cell interior was achieved by adding 200 microgram/ml of nystatin to the pipette solution. With this method, run down in g(j) was greatly reduced, amounting to no more than 5% of the initial value. Exposure of the cells, under DWCPC or DPPT, to dibutyryl cAMP or to tumor promoting agent (TPA) led to a decrease in cell to cell communication. Staurosporine, a PKC inhibitor, increased g(j) and was able to prevent and reverse the uncoupling action of cAMP or TPA. Our results indicate that cell-cell communication in Leydig cells is down regulated by both protein kinases A and C, interacting in a complex manner.

Energetics of hummingbird foraging at low ambient temperature

Because of their small size and the high energetic costs of hovering and forward flight, hummingbirds achieve the highest mass-specific metabolic rates known among vertebrates. Rufous hummingbirds (Selasphorus rufus) stop to refuel on floral nectar in subalpine meadows as they migrate south from British Columbia to Mexico. In such habitats they face the challenge of achieving daily net energy gain despite the high energetic costs of flight and thermoregulation at near-freezing morning temperatures. Hummingbirds provided with 15 or 20% sucrose while subjected to these conditions for 4 h in the laboratory did not remain in energy balance and lost mass. However, they achieved energy balance or net energy gain on 30% sucrose. Because these sucrose concentrations are within the range observed in the nectar of hummingbird-visited flowers, the results suggest that the energetic cost of thermoregulation may influence the coevolution of hummingbirds and flowers. Hummingbirds maintaining energy balance at low ambient temperature via high foraging frequencies and high rates of energy intake can sustain average metabolic rates of about 250 W/kg over a 4-h period. These are the highest metabolic rates known among vertebrates at which rates of dietary energy intake equal rates of energy expenditure.

Misregulation of connexin43 gap junction channels and congenital heart defects

Although there is general agreement that gap junction channels formed by the connexin43 (Cx43; alpha 1) protein most likely have important roles during heart development, evidence to support this view has been equivocal. Lacking this information, it is difficult to understand the basis of heart malformations found in the Cx43 knockout mice and in children with a severe form of visceroatrial heterotaxia that coincides with missense mutations of the Cx43 gene. To address this issue we used a combination of western blots to follow the emergence of Cx43 in heart, and in vitro and in vivo phosphorylation to assess the effect of mutation on Cx43 phosphorylation. We evaluated the activity ratios of cAMP-dependent protein kinase and protein kinase C in hearts of 8.5-day-old mouse embryos through to birth. The results demonstrate that Cx43 is present in the native phosphorylated species in day 8.5 hearts and thereafter. Further, the activities of cAMP-dependent protein kinase and protein kinase C are mirror images of each other during the 8.5-10.5 days of early heart development. From these results we conclude that Cx43 gap junction channels are present and capable of being regulated by day 8.5 of embryonic heart development.

Modulation of pancreatic acinar cell to cell coupling during ACh-evoked changes in cytosolic Ca2+

The temporal changes in cytosolic free Ca2+ ([Ca2+]i), Ca2+-dependent membrane currents (Im), and gap junctional current (Ij) elicited by acetylcholine (ACh) were measured in rat pancreatic acinar cells using digital imaging and dual perforated patch-clamp recording. ACh (50 nM-5 microM) increased [Ca2+]i and evoked Im currents without altering Ij in 19 of 37 acinar cell pairs. Although [Ca2+]i rose asynchronously in cells comprising a cluster, the delay of the [Ca2+]i responses decreased with increasing ACh concentrations. Perfusion of inositol 1,4,5-trisphosphate (IP3) into one cell of a cluster resulted in [Ca2+]i responses in neighboring cells that were not necessarily in direct contact with the stimulated one. This suggests that extensive coupling between acinar cells provides a pathway for cell-to-cell diffusion of Ca2+-releasing signals. Strikingly, maximal (1-5 microM) ACh concentrations reduced Ij by 69 +/- 15% (n = 9) in 25% of the cell pairs subjected to dual patch-clamping. This decrease occurred shortly after the Im peak and was prevented by incubating acinar cells in a Ca2+-free medium, suggesting that uncoupling was subsequent to the initiation of the Ca2+-mobilizing responses. Depletion of Ca2+-sequestering stores by thapsigargin resulted in a reduction of intercellular communication similar to that observed with ACh. In addition, ACh-induced uncoupling was prevented by blocking nitric oxide production with L-nitro-arginine and restored by exposing acinar cells to dibutyryl cGMP. The results suggest that ACh-induced uncoupling and capacitative Ca2+ entry are regulated concurrently. Closure of gap junction channels may occur to functionally isolate nearby cells differing in their intrinsic sensitivity to ACh and thereby to allow for sustained activity of groups of secreting cells.

Regulation of gap junctions by protein phosphorylation

Gap junctions are constituted by intercellular channels and provide a pathway for transfer of ions and small molecules between adjacent cells of most tissues. The degree of intercellular coupling mediated by gap junctions depends on the number of gap junction channels and their activity may be a function of the state of phosphorylation of connexins, the structural subunit of gap junction channels. Protein phosphorylation has been proposed to control intercellular gap junctional communication at several steps from gene expression to protein degradation, including translational and post-translational modification of connexins (i.e., phosphorylation of the assembled channel acting as a gating mechanism) and assembly into and removal from the plasma membrane. Several connexins contain sites for phosphorylation for more than one protein kinase. These consensus sites vary between connexins and have been preferentially identified in the C-terminus. Changes in intercellular communication mediated by protein phosphorylation are believed to control various physiological tissue and cell functions as well as to be altered under pathological conditions.

The differential effects of 12-O-tetradecanoylphorbol-13-acetate on the gap junctions and connexins of the developing mammalian lens

Epithelial cells in primary ovine lens cultures express the gap junction proteins connexin43 (Cx43) and connexin49 (Cx49; a.k.a. MP70), a homologue of mouse connexin50. In contrast, lens cultures of differentiated, fiber-like cells (termed lentoid cells) express Cx49 and connexin46 (Cx46), but not Cx43. To investigate the regulation of lens cell gap junctions by protein kinase C (PKC), differentiating lens cultures were treated with the PKC activator 12-O-tetradecanoylphorbol-13-acetate (beta-TPA). Within 10 min, beta-TPA significantly inhibited the transfer of Lucifer Yellow dye between epithelial, but not lentoid, cells. This inhibition was correlated with the phosphorylation of Cx43 and was followed by the gradual disappearance of Cx43 from cell interfaces. The protein kinase inhibitor staurosporine prevented Cx43 phosphorylation and the loss of Cx43 from intercellular junctions. Following treatment of cultures with beta-TPA for 2-6 hr, Cx49 disappeared from epithelial cell interfaces, and by 24 hr of beta-TPA treatment, levels of Cx49 detected on immunoblots of purified epithelial membrane fractions had also diminished significantly. The beta-TPA-induced loss of Cx49 both from regions of epithelial cell contact and from isolated membranes was correlated with the disappearance of Cx49 mRNA. In contrast to the epithelial connexins, the lentoid connexins Cx49 and Cx46 were unaffected by even extended beta-TPA treatment. In spite of lentoid dye transfer being refractory to beta-TPA, significant levels of PKC-alpha (a beta-TPA-sensitive isoform) were detected in the lentoid cell. The response of lens gap junctions to beta-TPA depends upon the stage of differentiation and the complement of connexins expressed. The contrasting effects of beta-TPA on Cx43 and Cx49 in lens epithelial cells indicate a fundamental difference in the regulation of these connexin proteins in the developing mammalian lens.

Connections with connexins: the molecular basis of direct intercellular signaling

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Rapid activation of calcium-sensitive Na+/H+ exchange induced by 20-hydroxyecdysone in salivary gland cells of Drosophila melanogaster

Ecdysteroids play an important role in the larval moulting process of insects. 20-Hydroxyecdysone (20E) causes the induction of specific 'puffs' in polytene chromosomes of Drosophila melanogaster salivary gland cells. Although it is known that inorganic ions control pretranscriptional processes in the cell nucleus, the intracellular mechanisms of gene activation are still unclear. Therefore, we examined the effects of 20E on plasma membrane ion transport of Drosophila melanogaster salivary gland cells. Isolated glands of the third larval stage were superfused with a solution mimicking the haemolymph. The relative K+ conductance of the cell membrane (tK+) was measured with microelectrodes by performing ion substitution experiments. Under control conditions tK+ averaged to 0.16 + 0.02 (n = 15). Addition of 5 x 10(-6) M 20E increased tK+ within 2 min by 19.1 +/- 4.2% (n = 15). This rapid response to 20E was elicited only in the presence of calcium. Moreover, starting from a steady-state intracellular pH of 7.20-7.60, 20E induced a rise in cytoplasmic pH by 0.27 +/- 0.06 (n = 6) within minutes. Amiloride (10(-3) M), a blocker of plasma membrane Na+/H+ exchange, prevented the 20E-induced intracellular alkalinization. We conclude that 20E activates a calcium-sensitive plasma membrane Na+/H+ exchange leading to a rise of plasma membrane K+ conductance and intracellular alkalinization both being prerequisites for steroid hormone induced gene activation.

Effect of NH4+/NH3 on cytosolic pH and the K+ channels of freshly isolated cells from the thick ascending limb of Henle's loop

The conductance properties of the luminal membrane of cells from the thick ascending limb of Henle's loop of rat kidney (TAL) are dominated by K+. In excised membrane patches the luminal K+ channel is regulated by pH changes on the cytosolic side. To examine this pH regulation in intact cells of freshly isolated TAL segments we measured the membrane voltage (Vm) in slow-whole-cell (SWC) recordings and the open probability (Po) of K+ channels in the cell-attached nystatin (CAN) configuration, where channel activity and part of Vm can be recorded. The pipette solution contained K+ 125 mmol/l and Cl- 32 mmol/l. Intracellular pH was determined by 2',7'bis(2-carboxyethyl)-5,(6)-carboxyfluorescein (BCECF) fluorescence. pH changes were induced by the addition of 10 mmol/l NH4+/NH3 to the bath. In the presence of NH4+/NH3 intracellular pH acidified by 0.53 +/- 0.11 units (n = 7). Inhibition of the Na+2Cl-K+ cotransporter by furosemide (0.1 mmol/l) reversed this effect and led to a transient alkalinisation by 0.62 +/- 0.14 units (n = 7). In SWC experiments Vm of TAL cells was -72 +/- 1 mV (n = 70). NH4+/NH3 depolarised Vm by 22 +/- 2 mV (n = 25). In 11 SWC experiments furosemide (0.1 mmol/l) attenuated the depolarising effect of NH4+ from 24 +/- 3 mV to 7 +/- 3 mV. Under control conditions the single-channel conductance of TAL K+ channels in CAN experiments was 66 +/- 5 pS and the reversal voltage for K+ currents was 70 +/- 2 mV (n = 35). The Po of K+ channels in CAN patches was reduced by NH4+/NH3 from 0.45 +/- 0.15 to 0.09 +/- 0.07 (n = 7). NH4+/NH3 exposure depolarised the zero current voltage of the permeabilised patches by -9.7 +/- 3.6 mV (n = 5). The results show that TAL K+ channels are regulated by cytosolic pH in the intact cell. The cytosolic pH is acidified by NH4+/NH3 exposure at concentrations which are physiologically relevant because Na+2Cl-K+(NH4+) cotransporter-mediated import of NH4+ exceeds the rate of NH3 diffusion into the TAL. K+ channels are inhibited by this acidification and the cells depolarise. In the presence of furosemide TAL cells alkalinise proving that NH4+ uptake occurs by the Na+2Cl-K+ cotransporter. The findings that, in the presence of NH4+/NH3 and furosemide, Vm is not completely repolarised and that K+ channels are not activated suggest that the respective K+ channels may in addition to their pH regulation be inhibited directly by NH4+/NH3.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

Electrophysiological investigation of microdissected gastric glands of bullfrog. I. Basolateral membrane properties in the resting state

In the present experiments we have made a new attempt to characterize the ion transport properties of H(+)-secreting cells of the gastric mucosa using electrophysiological techniques. Individual gastric glands of bullfrog fundus mucosa were manually dissected, mounted in holding pipettes and superfused with various test solutions while individual cells were punctured with conventional or H(+)-sensitive double-barrelled microelectrodes. All measurements were performed in the resting state (0.1 mmol/l cimetidine). In HCO3(-)-containing control Ringer solution the cell membrane potential (Vb) averaged -45.6 +/- 0.9 mV (+/- SEM, n = 54). From the fast initial Vb responses to changing bath K+, Na+, Cl- or HCO3- concentrations we deduced that the basolateral cell membrane contains conductances for K+, Na+, and Cl- but not for HCO3-, and that a Na(+)-HCO3- cotransporter is not present. The K+ conductance was inhibited by Ba2+ (3 mmol/l), but the Cl- conductance was not inhibited by 4,4' diisothiocyanato-stilbene-2,2' disulphonic acid (DIDS, 0.3 mmol/l), nor selectively inhibited by 5-nitro-2-(3)- phenylpropyl-aminobenzoate (NPPB, 10 mumol/l). In a great number of cells the Vb response to Cl- substitution revealed two components: an initial spiking depolarization which reflected conductive Cl- efflux and a secondary slow hyperpolarization, the origin of which was not immediately evident. Since the latter response could be mimicked by CO2-free perfusion, strongly depressed by Ba2+ and eliminated by DIDS, we conclude that it reflects HCO3- uptake into the cells via a DIDS sensitive Cl-/HCO3- exchanger which alkalinizes the cells and stimulates the basolateral K+ conductance. Our results confirm, revise and extend the results of previous, less direct, investigations of gastric cell ion transport.

Evidence for two types of potassium current in rat choroid plexus epithelial cells

The whole-cell patch-clamp technique was applied to rat choroid plexus epithelial cells. The resting membrane potential was -53 mV. The whole-cell conductance was mainly K+ selective, and the K+ current observed appeared to contain two distinct components. Depolarizing voltage pulses (more positive than 0 mV) evoked time-dependent outward currents which resembled delayed-rectifying K+ currents in other tissues. The current exhibited time-dependent activation and, at potentials more positive than 40 mV, slower time-dependent inactivation. The reversal potential measured by tail current analysis showed a shift of 43 mV for a tenfold increase in extracellular K+ concentration ([K+]o). The current was reduced by extracellular 5 mM Ba2+, 5 mM tetraethylammonium (TEA+), 5 mM Cs+ and 1 mM 4-aminopyridine (4-AP). In contrast, hyperpolarizing voltage pulses evoked time-independent, inward-rectifying currents. The reversal potential measured by voltage-ramp commands showed a shift of 42 mV for a tenfold increase in [K+]o. The chord conductance did not appear to increase with increasing [K+]o. The current was reduced by extracellular 5 mM Ba2+ and 0.5 mM Cs+, but not by 5 mM TEA+ or 1 mM 4-AP. These data suggest that two populations of K+ channel contribute to the conductance of choroid plexus epithelial cells.

Regulation of gap junctional coupling in isolated pancreatic acinar cell pairs by cholecystokinin-octapeptide, vasoactive intestinal peptide (VIP) and a VIP-antagonist

Cholecystokinin-octapeptide (CCK-OP) induces a time- and dose-dependent decrease of gap junctional conductance in isolated pairs of pancreatic acinar cells. In double whole-cell experiments, the time course could be described by the latency and the half-life time (t1/2) of cell-to-cell uncoupling. The latency shows a biphasic dependence on [CCK-OP] with a minimum of about 50 sec at 10(-9) M CCK-OP. In the presence of vasoactive intestinal peptide (VIP), the biphasic relationship is shifted to lower CCK-OP concentrations. The increase of latency at high concentrations of CCK-OP (> 10(-9) M) was blocked by addition of a VIP-antagonist. t1/2 decreases monophasically with increasing [CCK-OP]. Addition of GTP gamma S to the pipette solution suppresses the [CCK-OP] dependence of the latency and potentiates the uncoupling phase. The kinetic data are discussed in terms of CCK binding to receptors of high and low affinity. Evidence is presented that secretion and cell-to-cell coupling are not related by an all-or-none process, but that for physiological CCK-OP concentrations, gap junctional uncoupling follows secretion.

In situ regulation of cell-cell communication by the cAMP-dependent protein kinase and protein kinase C

The effects of cAMP-dependent protein kinase A and protein kinase C on cell-cell communication have been examined in primary ovarian granulosa cells microinjected with purified components of these two regulatory cascades. These cells possess connexin43 (alpha 1)-type gap junctions, and are well-coupled electrotonically and as judged by the cell-to-cell transfer of fluorescent dye. Within 2-3 min after injection of the protein kinase A inhibitor (PKI) communication was sharply reduced or ceased, but resumed in about 3 min with the injection of the protein kinase A catalytic subunit. A similar resumption also occurred in PKI-injected cells after exposure to follicle stimulating hormone. Microinjection of the protein kinase C inhibitor protein caused a transient cessation of communication that spontaneously returned within 15-20 min. Treatment of cells with activators of protein kinase C, TPA or OAG for 60 min caused a significant reduction in communication that could be restored within 2-5 min by the subsequent injection of either the protein kinase C inhibitor or the protein kinase A catalytic subunit. With a longer exposure to either protein kinase C activator communication could not be restored and this appeared to be related to the absence of aggregates of connexin43 in membrane as detected immunologically. In cells injected with alkaline phosphatase communication stopped but returned either spontaneously within 20 min or within 2-3 min of injecting the cell with either the protein kinase A catalytic subunit or with protein kinase C. When untreated cells were injected with protein kinase C communication diminished or ceased within 5 min. Collectively these results demonstrate that cell-cell communication is regulated by both protein kinase A and C, but in a complex interrelated manner, quite likely by multiple phosphorylation of proteins within or regulating connexin-43 containing gap junctions.

Effects of phorbol ester on gap junctions of neonatal rat heart cells

Myocytes were isolated from the ventricles of neonatal rat hearts and cultured for 1-3 days. Newly formed cell pairs were used to examine the conductance of gap junctions, gj. Measurements were performed using a dual voltage-clamp method in conjunction with a whole-cell, tight-seal recording. Exposure to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA, 100-160 nM) led to a decrease in gj. Single-channel events recorded immediately before complete uncoupling yielded a single-channel conductance, gamma j, of 40.5 pS, implying that TPA affects the channel kinetics rather than gamma j. TPA-induced uncoupling was observed at subphysiological levels of cytosolic Ca2+ (pipette solution = 18 nM), not at physiological levels (pipette solution = 170 nM). The effects of TPA could not be mimicked by 250 microM 1-oleoyl-2-acetyl-glycerol (OAG). Preincubation with TPA (up to 24 h) revealed no changes in gj attributable to down-regulation of protein kinase C, PKC. Pretreatment with PKC inhibitors, staurosporine or PKCI, prevented the TPA-dependent decrease in gj. TPA-dependent uncoupling was not impaired by 4-bromophenacyl bromide, an inhibitor of phospholipase A2, PLA2; conversely, an arachidonic acid-dependent decrease in gj was not prevented by PKCI. This suggests that gj regulation does not involve an interaction between PLA2 and PKC.

Gap-junctional properties of electrically coupled skate horizontal cells in culture

Whole-cell voltage-clamp recordings were used to examine the unusual pharmacological properties of the electrical coupling between rod-driven horizontal cells in skate retina as revealed previously by receptive-field measurements (Qian & Ripps, 1992). The junctional resistance was measured in electrically coupled cell pairs that had been enzymatically isolated and maintained in culture; the typical value was about 19.92 M omega (n = 45), more than an order of magnitude lower than the nonjunctional membrane resistance. These data and the intercellular spread of the fluorescent dye Lucifer Yellow provide a good indication that skate horizontal cells are well coupled. The junctional conductance between cells was not modulated by the neurotransmitters dopamine (200 microM) or GABA (1 mM), nor was it affected by the membrane-permeable analogues of cAMP or cGMP, or the adenylate cyclase activator, forskolin. Although resistant to agents that have been reported to alter horizontal-cell coupling in cone-driven horizontal cells, the junctional conductance between paired horizontal cells of skate was greatly reduced by the application of 20 mM acetate, which is known to effectively reduce intracellular pH. Together with the results obtained in situ on the receptive-field properties of skate horizontal cells, these findings indicate that the gap-junctional properties of rod-driven horizontal cells of the skate are fundamentally different from those of cone-driven horizontal cells in other species. This raises the possibility that there is more than one class of electrical synapse on vertebrate horizontal cells.

An inwardly rectifying potassium channel in the basolateral membrane of sheep parotid secretory cells

Using whole-cell patch-clamp techniques, we demonstrate that sheep parotid secretory cells have both inwardly and outwardly rectifying currents. The outwardly rectifying current, which is blocked by 10 mmol/liter tetraethylammonium (TEA) applied extracellularly, is probably carried by the 250 pS Ca(2+)- and voltage-activated K+ (BK) channel which has been described in previous studies. In contrast, the inwardly rectifying current, which is also carried by K+ ions, is not sensitive to TEA. It is similar to the inwardly rectifying currents observed in many excitable tissues in that (i) its conductance is dependent on the square root of the extracellular K+, (ii) the voltage range over which it is activated is influenced by the extracellular K+ concentration and (iii) it is blocked by the addition of Cs+ ions (670 mumol/liter) to the bathing solution. Our previously published cell-attached patch studies have shown that the channel type most commonly observed in the basolateral membrane of unstimulated sheep parotid secretory cells is a K+ channel with a conductance of 30 pS and, in this study, we find that its conductance also depends on the square root of the extracellular K+ concentration. It thus seems likely that it carries the inwardly rectifying K+ current seen in the whole-cell studies.

Limitations of the dual voltage clamp method in assaying conductance and kinetics of gap junction channels

The electrical properties of gap junctions in cell pairs are usually studied by means of the dual voltage clamp method. The voltage across the junctional channels, however, cannot be controlled adequately due to an artificial resistance and a natural resistance, both connected in series with the gap junction. The access resistances to the cell interior of the recording pipettes make up the artificial resistance. The natural resistance consists of the cytoplasmic access resistances to the tightly packed gap junction channels in both cells. A mathematical model was constructed to calculate the actual voltage across each gap junction channel. The stochastic open-close kinetics of the individual channels were incorporated into this model. It is concluded that even in the ideal case of complete compensation of pipette series resistance, the number of channels comprised in the gap junction may be largely underestimated. Furthermore, normalized steady-state junctional conductance may be largely overestimated, so that transjunctional voltage dependence is easily masked. The model is used to discuss conclusions drawn from dual voltage clamp experiments and offers alternative explanations for various experimental observations.

The effect of hyperosmotic challenge upon ion transport in cultured renal epithelial layers (MDCK)

Exposure of the basal-lateral surfaces of MDCK epithelia, mounted in Ussing chambers, to medium made hyperosmotic by the non-electrolyte mannitol, resulted in a marked inhibition of the adrenaline-stimulated inward short-circuit current (Cl- secretion). This inhibition was unaccompanied by a reversal of the adrenaline-stimulated increment in tissue conductance, indicating that the inhibition was due to modulation of ion transport at the basal-lateral membranes. Loop-diuretic-sensitive 86Rb(K+) efflux mediated by the Na+ - K+ -2 Cl- cotransporter at the basal-lateral membranes was markedly stimulated by hypertonic exposure. A diuretic-sensitive K+ (Cl-) loss was observed in shrunken cells upon prolonged exposure (20 min), showing that the net direction of "cotransport" flux was outward. 86Rb(K+) efflux stimulated by adrenaline (100 microM), exogenous ATP (100 microM) and A23187 (10 microM) was attenuated in shrunken cells, suggesting that basal-lateral K+ conductance is reduced in hyperosmotic media. "Cotransport" stimulation by hyperosmotic medium was asymmetric, apical bathing hypertonicity being ineffective. These data are consistent with a low hydraulic permeability of the apical membranes.

Hummingbird flight: sustaining the highest mass-specific metabolic rates among vertebrates

Resting and maximal mass-specific metabolic rates scale inversely with body mass. Small hummingbirds achieve the highest known mass-specific metabolic rates among vertebrate homeotherms. Maximal capacities for O2 and substrate delivery to muscle mitochondria, as well as mitochondrial oxidative capacities in these animals may be at the upper limits of what are structurally and functionally possible given the constraints inherent in vertebrate design. Such constraints on the evolutionary design of functional capacities may play an important role in determining the lower limits to vertebrate homeotherm size and the upper limits to mass-specific metabolic rate.

Signal propagation via gap junctions, a key step in the regulation of liver metabolism by the sympathetic hepatic nerves

Cell-to-cell communication via gap junctions has been proposed to be involved in the metabolic actions of sympathetic liver nerves in the rat. The effects of hepatic nerve stimulation and noradrenaline-, PGF2 alpha- and glucagon infusion on glucose metabolism and perfusion flow were studied in perfused rat liver in the absence and presence of the gap junctional inhibitors, heptanol, carbenoxolone and (4 beta)phorbol 12-myristate 13-acetate (4 beta PMA). (i) Stimulation of the hepatic nerve plexus increased glucose output, decreased flow and caused an overflow of noradrenaline into the hepatic vein. (ii) Heptanol completely inhibited not only the nerve stimulation-dependent metabolic and hemodynamic alterations but also the noradrenaline overflow. Thus the heptanol-dependent inhibitions were caused primarily by a strong impairment of transmitter release. (iii) Carbenoxolone inhibited the effects of neurostimulation on glucose metabolism partially by about 50%, whereas it left perfusion flow and noradrenaline overflow essentially unaltered. (iv) 4 beta PMA reduced the nerve stimulation-dependent enhancement of glucose release by about 80% but the noradrenaline-dependent increase in glucose output only by about 30%; the increase in glucose release by PGF2 alpha and by glucagon remained essentially unaltered. 4 beta PMA reduced the nerve stimulation-dependent decrease in portal flow by about 35% but did not affect the noradrenaline-and PGF2 alpha-elicited alterations, nor did it alter noradrenaline overflow. The results allow the conclusion that gap junctional communication plays a major role in the regulation of hepatic carbohydrate metabolism by sympathetic liver nerves, but not by circulating noradrenaline, PGF2 alpha or glucagon.

Potassium conductance of smooth muscle cells from rabbit aorta in primary culture

Vascular smooth muscle cells were obtained from rabbit aorta and were studied in primary culture on days 1-7 after seeding with electrophysiological techniques. In impalement experiments a mean membrane potential difference (PD) of -50 +/- 0.3 mV (n = 387) was obtained with Ringer-type solution in the bath. PD was depolarized by 6 +/- 0.3 mV (n = 45) and 16 +/- 2 mV (n = 5) when the bath K+ concentration was increased from the control value of 3.6 mmol/l to 13.6 and 23.6 mmol/l, respectively. Ba2+ (0.1-1 mmol/l) depolarized PD. Tetraethylammonium (TEA, 10 mmol/l) depolarized PD only slightly but significantly. Verapamil (0.1 mmol/l) and charybdotoxin (10 nmol/l) had no effect on PD. The conductance properties of these cells were further examined with the patch-clamp technique. K+ channels were spontaneously present in cell-attached patches. When the pipette was filled with 145 mmol/l KCl, a mean conductance (gK) of 209.6 +/- 4.6 mV (n = 17) was read from the current/voltage curves at a clamp voltage (Vc) of 0 mV. After excision K+ channels were found in 129 patches with inside-out and in 50 with outside-out configuration. With KCl on one and NaCl on the other side the mean gK at a Vc of 0 mV was 134.6 +/- 3.9 pS (n = 179). The mean permeability was 0.89 +/- 0.03 x 10(-12) cm3/s. With symmetrical KCl solution the mean gK was 227 +/- 6 pS (n = 17). The conductance sequence was gK much greater than gRb = gCs = gNa = 0. TEA blocked dose-dependently only from the outside (1-10 mmol/l). Lidocaine (5 mmol/l) quinidine (0.01-1 mmol/l) and quinine (0.01-1 mmol/l) blocked from both sides. Charybdotoxin (0.5-5 nmol/l) blocked only from the extracellular side. Ba2+ blocked from the cytosolic side and the inhibition was increased by depolarization and reduced by hyperpolarization. At a Vc of 0 mV a half-maximal inhibition (IC50) of 2 mumol/l was obtained. Verapamil and diltiazem blocked from both sides, verapamil with an IC50 of 2 mumol/l and diltiazem with an IC50 of 10 mumol/l. The open probability of this channel was increased by CA2+ on the cytosolic side at activities greater than 0.1 mumol/l. Half-maximal activation occurred at Ca2+ activities exceeding 1 mumol/l. The present data indicate that the vascular smooth muscle cells of rabbit aorta in primary culture possess a K+ conductance. In excised patches only a maxi K+ channel was detected. This channel has properties different from the macroscopic K+ conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Biophysical properties of gap junctions between freshly dispersed pairs of mouse pancreatic beta cells

Coupling between beta cells through gap junctions has been postulated as a principal mechanism of electrical synchronization of glucose-induced activity throughout the islet of Langerhans. We characterized junctional conductance between isolated pairs of mouse pancreatic beta cells by whole-cell recording with two independent patch-clamp circuits. Most pairs were coupled (67%, n = 155), although the mean junctional conductance (gj) (215 +/- 110 pS) was lower than reported in other tissues. Coupling could be recorded for long periods, up to 40 min. Voltage imposed across the junctional or nonjunctional membranes had no effect on gj. Up to several hours of treatment to increase intracellular cAMP levels did not affect gj. Electrically coupled pairs did not show transfer of the dye Lucifer yellow. Octanol (2 mM) reversibly decreased gj. Lower concentrations of octanol (0.5 mM) and heptanol (0.5 mM) than required to uncouple beta cells decreased voltage-dependent K+ and Ca2+ currents in nonjunctional membranes. Although gj recorded in these experiments would be expected to be provided by current flowing through only a few channels of the unitary conductance previously reported for other gap junctions, no unitary junctional currents were observed even during reversible suppression of gj by octanol. This result suggests either that the single channel conductance of gap junction channels between beta cells is smaller than in other tissues (less than 20 pS) or that the small mean conductance is due to transitions between open and closed states that are too rapid or too slow to be resolved.

Gap junctional permeability is affected by cell volume changes and modulates volume regulation

Isolated pancreatic acinar cell pairs became electrically uncoupled by exposure to a mild hypotonic shock. Reduction of bath osmolarity caused a delayed closure of gap junctional channels in the minute range. Dialysis of cell pairs by GTP[S] in the double whole-cell patch-clamp mode shortened the latency and shifted the hypotonically induced electrical uncoupling to lower osmolarity changes. Cellular treatment with cytochalasin B catalyzed electrical uncoupling by a hypotonic shock. In all cases, electrical uncoupling could be blocked completely by the protein kinase C (PKC) inhibitor polymyxin B. These results provide the first evidence suggesting that changes of cell volume and gap junctional permeability are correlated and that a G-protein dependent mechanism is involved. Evidence is presented that gap junctional coupling modulates volume regulation.

The molecular basis of enzyme secretion

Acinar cells are one of the best studied models of exocytotic secretion. A number of different hormones and neurotransmitters interact with specific membrane receptors, and it is commonly held that pancreatic secretagogues stimulate enzyme release via the elevation of either cytosolic free Ca2+ or cellular cyclic adenosine monophosphate. The discovery of the pivotal role played by phospholipid metabolism in the chain of events leading to secretion, together with the introduction of sensitive techniques to monitor cytosolic free Ca2+, has generated a series of studies that have challenged this classical model. Thus, several observations in pancreatic acini as well as other cell types have argued against the notion that a generalized increase in cytosolic free Ca2+ represents a sufficient and necessary stimulus for exocytosis in nonexcitable cells. Furthermore, the demonstration that a single agonist activates multiple transduction pathways has served to refute the schematic view that receptor agonists activate only one second messenger system. The aim of this article is to review the recent advances in understanding the molecular and cellular mechanisms of signal transduction, with particular emphasis on the inositol lipid pathway, and to integrate this information into a new working model of enzyme secretion from acinar cells.

Phosphorylation of connexin 32, a hepatocyte gap-junction protein, by cAMP-dependent protein kinase, protein kinase C and Ca2+/calmodulin-dependent protein kinase II

Phosphorylation of connexin 32, the major liver gap-junction protein, was studied in purified liver gap junctions and in hepatocytes. In isolated gap junctions, connexin 32 was phosphorylated by cAMP-dependent protein kinase (cAMP-PK), by protein kinase C (PKC) and by Ca2+/calmodulin-dependent protein kinase II (Ca2+/CaM-PK II). Connexin 26 was not phosphorylated by these three protein kinases. Phosphopeptide mapping of connexin 32 demonstrated that cAMP-PK and PKC primarily phosphorylated a seryl residue in a peptide termed peptide 1. PKC also phosphorylated seryl residues in additional peptides. CA2+/CaM-PK II phosphorylated serine and to a lesser extent, threonine, at sites different from those phosphorylated by the other two protein kinases. A synthetic peptide PSRKGSGFGHRL-amine (residues 228-239 based on the deduced amino acid sequence of rat connexin 32) was phosphorylated by cAMP-PK and by PKC, with kinetic properties being similar to those for other physiological substrates phosphorylated by these enzymes. Ca2+/CaM-PK II did not phosphorylate the peptide. Phosphopeptide mapping and amino acid sequencing of the phosphorylated synthetic peptide indicated that Ser233 of connexin 32 was present in peptide 1 and was phosphorylated by cAMP-PK or by PKC. In hepatocytes labeled with [32P]orthophosphoric acid, treatment with forskolin or 20-deoxy-20-oxophorbol 12,13-dibutyrate (PDBt) resulted in increased 32P-incorporation into connexin 32. Phosphopeptide mapping and phosphoamino acid analysis showed that a seryl residue in peptide 1 was most prominently phosphorylated under basal conditions. Treatment with forskolin or PDBt stimulated the phosphorylation of peptide 1. PDBt treatment also increased the phosphorylation of seryl residues in several other peptides. PDBt did not affect the cAMP-PK activity in hepatocytes. It has previously been shown that phorbol ester reduces dye coupling in several cell types, however in rat hepatocytes, dye coupling was not reduced by treatment with PDBt. Thus, activation of PKC may have differential effects on junctional permeability in different cell types; one source of this variability may be differences in the sites of phosphorylation in different gap-junction proteins.

Electrophysiology of cultured human lens epithelial cells

The lens epithelial K+ conductance plays a key role in maintaining the lens ionic steady state. The specific channels responsible for this conductance are unknown. We used cultured lens epithelia and patch-clamp technology to address this problem. Human lens epithelial explants were cultured and after 1-4 passages were dissociated and used in this study. The cells from which we measured had a mean diameter of 31 +/- 1 microns (SEM, n = 26). The resting voltage was -19 +/- 4 mV (SEM, n = 10) and the input resistance was 2.5 +/- 0.5 G omega (SEM, n = 17) at -60 mV. Two currents were prominent in whole-cell recordings. An outwardly rectifying current was seen in nearly every cell. The magnitude of this current was a function of K+ concentration and was blocked by 3 mM tetraethylammonium. The instantaneous current-voltage relationship was linear in symmetric K+, implying that the outward rectification was due to gating. The current showed complex activation and inactivation kinetics. The second current seen was a transient inward current. This current had kinetics very similar to the traditional Na+ current of excitable cells and was blocked by 0.1 microM tetrodotoxin. In single-channel recordings, a 150-pS K+ channel and a 35-pS nonselective cation channel were seen but neither account for the macroscopic currents measured.

Regulation of inwardly rectifying K+ channels by intracellular pH in opossum kidney cells

The effects of intracellular pH on an inwardly rectifying K+ channel ("Kin channel") in opossum kidney (OK) cells were examined using the patch-clamp technique. Experiments with inside-out patches were first carried out in Mg2(+)- and adenosine triphosphate (ATP)-free conditions, where Mg2(+)-induced inactivation and ATP-induced reactivation of Kin channels were suppressed. When the bath (cytoplasmic side) pH was decreased from 7.3 to either 6.8 or 6.3, Kin channels were markedly inhibited. The effect of acid pH was not fully reversible. When the bath pH was increased from 7.3 to 7.8, 8.3 or 8.8, the channels were activated reversibly. The channel activity exhibited a sigmoidal pH dependence with a maximum sensitivity at pH 7.5. Inside-out experiments were also carried out with a solution containing 3 mM Mg-ATP and a similar pH sensitivity was observed. However, in contrast with the results obtained in the absence of Mg2+ and ATP, the effect of acid pH was fully reversible. Experiments with cell-attached patches demonstrated that changes in intracellular pH, which were induced by changing extracellular pH in the presence of an H+ ionophore, could influence the channel activity reversibly. It is concluded that the activity of Kin channels can be controlled by the intracellular pH under physiological conditions.

The luminal K+ channel of the thick ascending limb of Henle's loop

In vitro perfused rat thick ascending limbs of Henle's loop (TAL) were used (n = 260) to analyse the conductance properties of the luminal membrane applying the patch-clamp technique. Medullary (mTAL) and cortical (cTAL) tubule segments were dissected and perfused in vitro. The free end of the tubule was held and immobilized at one edge by a holding pipette kept under continuous suction. A micropositioner was used to insert a patch pipette into the lumen, and a gigaohm seal with the luminal membrane was achieved in 455 instances out of considerably more trials. In approximately 20% of all gigaohm seals recordings of single ionic channels were obtained. We have identified only one single type of K+ channel in these cell-attached and cell-excised recordings. In the cell-attached configuration with KCl or NaCl in the pipette, the channel had a conductance of 60 +/- 6 pS (n = 24) and 31 +/- 7 pS (n = 4) respectively. In cell-free patches with KCl either in the patch pipette or in the bath and with a Ringer-type solution (NaCl) on the opposite side the conductance was 72 +/- 4 pS (n = 37) at a clamp voltage of 0 mV. The permeability was 0.33 +/- 0.02 . 10(-12) cm3/s. The selectivity sequence of this channel was: K+ = Rb+ = NH4+ = Cs+ greater than Li+ much greater than Na+ = 0; the conductance sequence was K+ much greater than Li+ much greater than Rb+ = Cs+ = NH4+ = Na+ = 0. In excised patches Rb+, Cs+ and NH4+ when present in the bath at 145 mmol/l all inhibited K+ currents out of the pipette. The channel kinetics were described by one open (9.5 +/- 1.5 ms, n = 18) and by two closed (1.4 +/- 0.1 and 14 +/- 2 ms) time constants. The open probability of this channel was increased by depolarization. The channel open probability was reduced voltage dependently by Ba2+ (half maximal inhibition at 0 mV: 0.07 mmol/l) from the cytosolic side. Verapamil, diltiazem, quinine and quinidine inhibited at approximately 1 mumol/l -0.1 mmol/l from either side. Similarly, the amino cations lidocaine, tetraethylammonium and choline inhibited at 10-100 mmol/l. The channel was downregulated in its open probability by cytosolic Ca2+ activities greater than 10(-7) mol/l and by adenosine triphosphate greater than or equal to 10(-4) mol/l. The open probability was downregulated by decreasing cytosolic pH (2-fold by a decrease in pH by less than or equal to 0.2 units).(ABSTRACT TRUNCATED AT 400 WORDS)

A G-protein mediates secretagogue-induced gap junctional channel closure in pancreatic acinar cells

Using the double whole-cell patch-clamp technique, we determined that dialysis of cell pairs by GTP[S] potentiated electrical uncoupling induced by extracellular addition of carbamylcholine (CCh). An inhibitor of diglyceride lipase, RHC 80267, further potentiated CCh/GTP[S]-induced junctional channel closure, probably by accumulation of diacylglycerol. Moreover, the protein kinase C inhibitor polymyxin B completely blocked uncoupling elicited by CCh/GTP[S]. These results provide the first evidence suggesting that gap junction channel closure by cholinergic stimulation is mediated by a G-protein, which acts by increasing phosphatidylinositol biphosphate breakdown and protein kinase C activity.

Mechanism of aldosterone-induced increase of K+ conductance in early distal renal tubule cells of the frog

Isolated early distal tubule cells (EDC) of frog kidney were incubated for 20-28 hr in the presence of aldosterone and then whole-cell K+ currents were measured at constant intracellular pH by the whole-cell voltage-clamp technique. Aldosterone increased barium-inhibitable whole-cell K+ conductance (gK+) threefold. This effect was reduced by amiloride and totally abolished by ouabain. However, aldosterone could still raise gK- in ouabain-treated cells in the presence of furosemide. We tested whether changes in intracellular pH (pHi) could be a signal for cells to regulate gK+. After removal of aldosterone, the increase in gK+ was preserved by subsequent incubation for 8 hr at pH 7.6 but abolished at pH 6.6. In the complete absence of aldosterone, incubation of cells at pH 8.0 for 20-28 hr raised pHi and doubled gK+. Using the patch-clamp technique, three types of K+-selective channels were identified, which had conductances of 24, 45 and 59 pS. Aldosterone had no effect on the conductance or open probability (Po) of any of the three types of channels. However, the incidence of observing type II channels was increased from 4 to 22%. Type II channels were also found to be pH sensitive, Po was increased by raising pH. These results indicate that prolonged aldosterone treatment raises pHi and increases gK+ by promoting insertion of K+ channels into the cell membrane. Channel insertion is itself triggered by raising both pHi and increasing the activity of the Na+/K+ pump in early distal cells of frog kidney.

Apical K+ channels of frog diluting segment: inhibition by acidification

The apical potassium conductance of the amphibian diluting segment is regulated by the intracellular pH. Alkalinisation of the cytosol, whether directly by bathing the cell in an alkaline medium, or following activation of an apical Na/H exchanger by aldosterone, results in an increase in the K conductance. Early distal tubules were isolated from slices of Frog kidneys and the apical membranes exposed by everting the tubule with the aid of microperfusion pipettes. Single channels in the apical membrane were studied in the cell-attached configuration while the tubules were bathed in Ringer with a pH of either 7.4 (control) or 6.6 (acid). Under control conditions single channel currents were readily seen at the resting membrane potential. Upon acidification of the superfusion solution the open probability of the channels was decreased from 0.76 to 0.15. Thus the reduction in apical conductance is brought about, at least in part, by a reduction in the open probability of the channels upon cellular acidification.