Modulation of CFTR chloride channels by calyculin A and genistein

CFTR activation in human bronchial epithelial cells by novel benzoflavone and benzimidazolone compounds

Activators of the CFTR Cl- channel may be useful for therapy of cystic fibrosis. Short-circuit current (Isc) measurements were done on human bronchial epithelial cells to characterize the best flavone and benzimidazolone CFTR activators identified by lead-based combinatorial synthesis and high-throughput screening. The 7,8-benzoflavone UCcf-029 was a potent activator of Cl- transport, with activating potency (<1 microM) being much better than other flavones, such as apigenin. The benzimidazolone UCcf-853 gave similar Isc but with lower potency (5-20 microM). In combination, the effect induced by maximal UCcf-029 and UCcf-029, UCcf-853, and apigenin increased strongly with increasing basal CFTR activity: for example, Kd for activation by UCcf-029 decreased from >5 to <0.4 microM with increasing basal Isc from approximately 4 microA/cm2 to approximately 12 microA/cm2. This dependence was confirmed in permeabilized Fischer rat thyroid cells stably expressing CFTR. Our results demonstrate efficacy of novel CFTR activators in bronchial epithelia and provide evidence that activating potency depends on basal CFTR activity.

Development of cystic fibrosis and noncystic fibrosis airway cell lines

In this study, we utilized the reverse transcriptase component of telomerase, hTERT, and human papillomavirus type 16 (HPV-16) E6 and E7 genes to transform normal and cystic fibrosis (CF) human airway epithelial (HAE) cells. One cell line, designated NuLi-1 (normal lung, University of Iowa), was derived from HAE of normal genotype; three cell lines, designated CuFi (cystic fibrosis, University of Iowa)-1, CuFi-3, and CuFi-4, were derived from HAE of various CF genotypes. When grown at the air-liquid interface, the cell lines were capable of forming polarized differentiated epithelia that exhibited transepithelial resistance and maintained the ion channel physiology expected for the genotypes. The CF transmembrane conductance regulator defect in the CuFi cell lines could be corrected by infecting from the basolateral surface using adenoviral vectors. Using nuclear factor-kappaB promoter reporter constructs, we also demonstrated that the NuLi and CuFi cell lines retained nuclear factor-kappaB responses to lipopolysaccharide. These cell lines should therefore be useful as models for studying ion physiology, therapeutic intervention for CF, and innate immunity.

Oxidant regulation of gene expression and neural tube development: Insights gained from diabetic pregnancy on molecular causes of neural tube defects

AIMS/HYPOTHESIS

Maternal diabetes increases oxidative stress in embryos. Maternal diabetes also inhibits expression of embryonic genes, most notably, Pax-3, which is required for neural tube closure. Here we tested the hypothesis that oxidative stress inhibits expression of Pax-3, thereby providing a molecular basis for neural tube defects induced by diabetic pregnancy.

METHODS

Maternal diabetes-induced oxidative stress was blocked with alpha-tocopherol (vitamin E), and oxidative stress was induced with the complex III electron transport inhibitor, antimycin A, using pregnant diabetic or non-diabetic mice, primary cultures of neurulating mouse embryo tissues, or differentiating P19 embryonal carcinoma cells. Pax-3 expression was assayed by quantitative RT-PCR, and neural tube defects were scored by visual inspection. Oxidation-induced DNA fragmentation in P19 cells was assayed by electrophoretic analysis.

RESULTS

Maternal diabetes inhibited Pax-3 expression and increased neural tube defects, and alpha-tocopherol blocked these effects. In addition, induction of oxidative stress with antimycin A inhibited Pax-3 expression and increased neural tube defects. In cultured embryo tissues, high glucose-inhibited Pax-3 expression, and this effect was blocked by alpha-tocopherol and GSH-ethyl ester, and Pax-3 expression was inhibited by culture with antimycin A. In differentiating P19 cells, antimycin A inhibited Pax-3 induction but did not induce DNA strand breaks.

CONCLUSION/INTERPRETATION

Oxidative stress inhibits expression of Pax-3, a gene that is essential for neural tube closure. Impaired expression of essential developmental control genes could be the central mechanism by which neural tube defects occur during diabetic pregnancy, as well as other sources of oxidative stress.

Context sensitivity of activity-dependent increases in cerebral blood flow

Functional neuroimaging in humans is used widely to study brain function in relation to human disease and cognition. The neural basis of neuroimaging signals is probably synaptic activity, but the effect of context, defined as the interaction between synaptic inhibition, excitation, and the electroresponsive properties of the targeted neurons, is not well understood. We examined here the effect of interaction of synaptic excitation and net inhibition on the relationship between electrical activity and vascular signals in the cerebellar cortex. We show that stimulation of the net inhibitory parallel fibers simultaneously with stimulation of the excitatory climbing fibers leads to a further rise in total local field potentials (LFP) and cerebral blood flow (CBF) amplitudes, not a decrease, as predicted from theoretical studies. However, the combined stimulation of the parallel and climbing fiber systems produced changes in CBF and LFP that were smaller than their algebraic sum evoked by separate stimulation of either system. This finding was independent of the starting condition, i.e., whether inhibition was superimposed on a state of excitation or vice versa. The attenuation of the increases in LFP and CBF amplitudes was similar, suggesting that synaptic activity and CBF were coupled under these conditions. The result might be explained by a relative neuronal refractoriness that relates to the intrinsic membrane properties of Purkinje cells, which determine the recovery time of these cells. Our work implies that neuronal and vascular signals are context-sensitive and that their amplitudes are modulated by the electroresponsive properties of the targeted neurons.

The cystic fibrosis mutation G551D alters the non-Michaelis-Menten behavior of the cystic fibrosis transmembrane conductance regulator (CFTR) channel and abolishes the inhibitory Genistein binding site

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) channel activity explains most of the manifestations of the cystic fibrosis (CF) disease. To understand the consequences of CF mutations on CFTR channel activity, we compared the pharmacological properties of wild-type (wt) and G551D-CFTR. Dose-dependent relationships of wt-CFTR activated by genistein follows a non-Michaelis-Menten behavior consistent with the presence of two binding sites. With phosphorylated CFTR, a high affinity site for genistein is the activator (K(s) approximately 3 microm), whereas a second site of low affinity (K(i) approximately 75 microm) is the inhibitor. With non-phosphorylated CFTR, K(s) was increased (K(s) approximately 12 microm), but K(i) was not affected (K(i) approximately 70 microm). In G551D-CFTR cells, channel activity was recovered by co-application of forskolin and genistein in a dose-dependent manner. A further stimulation of G551D-CFTR channel activity was measured at concentrations from 30 microm to 1 mm. The dose response is described by a classical Michaelis-Menten kinetics with only a single apparent site (K(m) approximately 11 microm). Our results suggest glycine 551 in NBD1 as an important location within the low affinity inhibitory site for genistein and offers new evidence for pharmacological alteration caused by an NBD1 mutation of CFTR. This study also reveals how a mutation of an ion channel converts a non-Michaelis-Menten behavior (two binding sites) into a classical Michaelis-Menten model (one binding site).

Effect of hypothyroidism induced by propylthiouracil on ovarian function and structure in offspring from treated mothers (Rats)

The aim of the present study was to investigate the effects of hypothyroidism induced during the pre- and postnatal periods of life on ovarian function and structure in offspring (pups) 120 days of age. Three groups were used. In the prenatal group, treatment was given from conception to parturition. In the postnatal group, treatment was given from parturition to 25 days postpartum. Hypothyroidism was induced by administration of 0.1% 6-n-propyl-2-thiouracil (PTU) in the drinking water of mothers. Body weights of the offspring were measured weekly. In each group, ten offspring were sacrificed at 120 days of age. Postnatal PTU treated pups showed delay in eye opening, teething, fur development, and weaning (35-37 days) compared to control animals (28-30 days). Body weight of offspring in the postnatal PTU treatment group was significantly decreased (P < 0.001), while the prenatal PTU treatment group showed a significant increase (P < 0.0001) compared to control animals. There was a significant (P < 0.05) reduction in paired ovarian weight of offspring in the postnatal PTU treatment group compared to control animals. Diameter of the ovaries was not affected by any treatment. Regarding the morphometery, only offspring in the prenatal PTU treatment group showed a significant (P < 0.001) increase in the diameter of graafian follicles. No significant difference was observed in morphometery of the granulosa layer, primary, and developing follicles of control and all treated groups. Number of primary, developing, and graafian follicles of all the treated groups was similar to that of the control group. The corpora lutea of the postnatal PTU treated group contained a population of large numbers of luteal cells compared to the control group. The prenatal PTU treated group did not exhibit a profound effect on ovarian morphology, histology, and morphometery. No difference was found in the serum estradiol concentration of control and PTU treated groups. J. Exp. Zool. 293:407-413, 2002.

Epidermal expression of the full-length extracellular calcium-sensing receptor is required for normal keratinocyte differentiation

The importance of the extracellular calcium-sensing receptor (CaR) in the stringent control of extracellular Ca(2+) concentration is well established. However, the presence of CaR in tissues not directly involved in regulating mineral ion homeostasis such as the epidermis suggests a role for CaR in other cellular functions. Although extracellular Ca(2+) regulates the differentiation of epidermal keratinocytes, the role of CaR in this process in the epidermis is not fully understood. In this study we showed using in situ hybridization and immunohistochemistry that CaR is expressed in suprabasal keratinocytes of the mammalian epidermis. We then evaluated the changes in epidermal keratinocyte morphology and differentiation in Casr(-/-) mice lacking the full-length CaR. These mice show increased expression of an alternatively spliced form of CaR which lacks acute Ca(2+)-signaling properties. The absence of the full-length CaR in the epidermis resulted in ultrastructural changes (abnormal keratohyalin granule formation and precocious lamellar body secretion) in the terminally differentiated granular keratinocytes. Furthermore, the expression of both mRNA and protein for the calcium inducible keratinocyte differentiation markers, filaggrin and loricrin, were down-regulated in the epidermis of Casr(-/-) mice, whereas the number of proliferating cells were increased even though the calcium gradient within the epidermis was enhanced. Our results demonstrate that the epidermal expression of the full-length CaR is required for the normal terminal differentiation of keratinocytes.

Phloxine B interacts with the cystic fibrosis transmembrane conductance regulator at multiple sites to modulate channel activity

The fluorescein derivative phloxine B is a potent modulator of the cystic fibrosis transmembrane conductance regulator (CFTR). Low micromolar concentrations of phloxine B stimulate CFTR Cl(-) currents, whereas higher concentrations of the drug inhibit CFTR. In this study, we investigated the mechanism of action of phloxine B. Phloxine B (1 microm) stimulated wild-type CFTR and the most common cystic fibrosis mutation, DeltaF508, by increasing the open probability of phosphorylated CFTR Cl(-) channels. At each concentration of ATP tested, the drug slowed the rate of channel closure without altering the opening rate. Based on the effects of fluorescein derivatives on transport ATPases, these data suggest that phloxine B might stimulate CFTR by binding to the ATP-binding site of the second nucleotide-binding domain (NBD2) to slow the dissociation of ATP from NBD1. Channel block by phloxine B (40 microm) was voltage-dependent, enhanced when external Cl(-) concentration was reduced and unaffected by ATP (5 mm), suggesting that phloxine B inhibits CFTR by occluding the pore. We conclude that phloxine B interacts directly with CFTR at multiple sites to modulate channel activity. It or related agents might be of value in the development of new treatments for diseases caused by the malfunction of CFTR.

Relationship of spikes, synaptic activity, and local changes of cerebral blood flow

The coupling of electrical activity in the brain to changes in cerebral blood flow (CBF) is of interest because hemodynamic changes are used to track brain function. Recent studies, especially those investigating the cerebellar cortex, have shown that the spike rate in the principal target cell of a brain region (i.e. the efferent cell) does not affect vascular response amplitude. Subthreshold integrative synaptic processes trigger changes in the local microcirculation and local glucose consumption. The spatial specificity of the vascular response on the brain surface is limited because of the functional anatomy of the pial vessels. Within the cortex there is a characteristic laminar flow distribution, the largest changes of which are observed at the depth of maximal synaptic activity (i.e. layer IV) for an afferent input system. Under most conditions, increases in CBF are explained by activity in postsynaptic neurons, but presynaptic elements can contribute. Neurotransmitters do not mediate increases in CBF that are triggered by the concerted action of several second messenger molecules. It is important to distinguish between effective synaptic inhibition and deactivation that increase and decrease CBF and glucose consumption, respectively. In summary, hemodynamic changes evoked by neuronal activity depend on the afferent input function (i.e. all aspects of presynaptic and postsynaptic processing), but are totally independent of the efferent function (i.e., the spike rate of the same region). Thus, it is not possible to conclude whether the output level of activity of a region is increased based on brain maps that use blood-flow changes as markers.

Alterations in the sensing and transport of phosphate and calcium by differentiating chondrocytes

During endochondral bone formation and fracture healing, cells committed to chondrogenesis undergo a temporally restricted program of differentiation that is characterized by sequential changes in their phenotype and gene expression. This results in the manufacture, remodeling, and mineralization of a cartilage template on which bone is laid down. Articular chondrocytes undergo a similar but restricted differentiation program that does not proceed to mineralization, except in pathologic conditions such as osteoarthritis. The pathogenesis of disorders of cartilage development and metabolism, including osteochondrodysplasia, fracture non-union, and osteoarthritis remain poorly defined. We used the CFK2 model to examine the potential roles of phosphate and calcium ions in the regulatory pathways that mediate chondrogenesis and cartilage maturation. Differentiation was monitored over a 4-week period using a combination of morphological, biochemical, and molecular markers that have been characterized in vivo and in vitro. CFK2 cells expressed the type III sodium-dependent phosphate transporters Glvr-1 and Ram-1, as well as a calcium-sensing mechanism. Regulated expression and activity of Glvr-1 by extracellular phosphate and parathyroid hormone-related protein was restricted to an early stage of CFK2 differentiation, as evidenced by expression of type II collagen, proteoglycan, and Ihh. On the other hand, regulated expression and activity of a calcium-sensing receptor by extracellular calcium was most evident after 2 weeks of differentiation, concomitant with an increase in type X collagen expression, alkaline phosphatase activity and parathyroid hormone/parathyroid hormone-related protein receptor expression. On the basis of these temporally restricted changes in the sensing and transport of phosphate and calcium, we predict that extracellular phosphate plays a role in the commitment of chondrogenic cells to differentiation, whereas extracellular calcium plays a role at a later stage in their differentiation program.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

Thyroid hormone stimulates myoglobin expression in soleus and extensorum digitalis longus muscles of rats: concomitant alterations in the activities of Krebs cycle oxidative enzymes

Myoglobin (Mb) gene expression, Citrate Synthase (CS) and Succinate Dehydrogenase (SDH) activities of Soleus (S) and Extensorum Digitalis Longus (EDL) muscles were studied in intact, thyroidectomized and T3-treated (25 microg/100g, BW, ip, 15 days) rats. The fiber type composition of S muscle was also evaluated and used as control of the T3-induced effects. In the S muscle, the T3 treatment increased the Mb mRNA and protein expression, as well as the CS and SDH activity. These changes occurred parallel to the expected increase in type II (fast) and decrease in type I (slow)-fibers in S muscle. In the hypothyroid state, the Mb mRNA was decreased, while the Mb expression and CS activity tended to decrease. In contrast the SDH activity was increased, probably due to the enhanced motor activity that occurs as a short-term response to the hypothermia induced by hypothyroidism. In the EDL, the alterations were milder than those in S muscle in both thyroid states. These findings show that Mb gene expression is induced by T3. This is concomitant with the enhancement of Krebs Cycle enzyme activities and provides additional evidence that thyroid hormone increases the aerobic potential of skeletal muscles, as well as the speed of muscle contraction.

Type I, II, III, IV, and V cystic fibrosis transmembrane conductance regulator defects and opportunities for therapy

Recent advances in cellular and molecular biology have furthered the understanding of several genetic diseases, including cystic fibrosis. Mutations that cause cystic fibrosis are now understood in terms of the specific molecular consequences to the cystic fibrosis transmembrane conductance regulator (CFTR) protein expression and function. This knowledge has spawned interest in the development of therapies aimed directly at correcting the defective CFTR itself. In this article, we review the molecular defect underlying each recognized class of CFTR mutation and the potential therapies currently under investigation. Opportunities for protein-repair therapy appear to be vast and range from naturally occurring compounds, such as isoflavonoids, to pharmaceuticals already in clinical use, including aminoglycoside antibiotics, butyrate analogues, phosphodiesterase inhibitors, and adenosine nucleotides. Future therapies may resemble designer compounds like benzo[c]quinoliziniums or take the form of small peptide replacements. Given the heterogeneity and progressive nature of cystic fibrosis, however, optimal benefit from protein-repair therapy will most likely require the initiation of combined therapies early in the course of disease to avoid irreparable organ damage.

Potent stimulation and inhibition of the CFTR Cl(-) current by phloxine B

The effects of the fluoresceine derivative, phloxine B, on the Cl(-) current through the cystic fibrosis transmembrane conductance regulator (CFTR) were examined in Xenopus oocytes expressing human CFTR. In whole oocytes, the CFTR Cl(-) current (I(CFTR)) was activated by superfusion with isobutylmethylxanthine and forskolin. I(CFTR) was stable during activation and deactivated rapidly upon washout of the activation solution. Phloxine B slowed deactivation and, at high concentrations, inhibited I(CFTR) weakly. In excised inside-out macropatches, I(CFTR) was activated by the catalytic subunit of protein kinase A (cPKA) and MgATP. Phloxine B (0.01 - 3 microM), applied after activation, increased I(CFTR) within 30 s followed by a slow decrease which became dominant at high concentrations. Slowing of deactivation of the CFTR was observed at all concentrations. The effect of phloxine B after 30 s had a bell-shaped concentration-dependence with midpoints at 45 and 1600 nM for the stimulatory and the inhibitory limb, respectively; maximum stimulation was about 1.8 times. The slow inhibitory component, measured after 6 min, occurred with an IC(50) value of approximately 1 microM. In the absence of cPKA, phloxine B did not stimulate I(CFTR). In the presence of cPKA and MgATP, the effects of phloxine B were more prominent at low (0.02 mM) than at high ATP (2 mM). The data show that phloxine B modulates I(CFTR) by increasing channel activity and slowing channel deactivation; at high concentrations inhibition dominates. The effects may be mediated by direct interactions with CFTR from the inside of the cell.

The calcium-sensing receptor stimulates JNK in MDCK cells

The calcium-sensing receptor (CaR) stimulates ERK1 in rat fibroblasts, but its effect on other MAP kinases is not known. We used a model of renal distal tubule, the MDCK cell, to determine the effects of CaR stimulation on Jun kinase (JNK) activity. Stimulation of the CaR with 5 mM Ca(2+) resulted in a time-dependent increase in JNK activity. Activation of JNK occurred preferentially with stimulation on the basal surface relative to the apical surface. Basal administration of the CaR agonist gadolinium (30 microm) also stimulated JNK activity. Pertussis toxin blocked the ability of both CaR agonists to stimulate JNK, indicating that the effect was mediated through G(ialpha) class G proteins. Finally, we used confocal microscopy to determine that the CaR was located predominantly on the basal surface. These studies demonstrate for the first time that the CaR stimulates JNK activity.

Genistein activates CFTR-mediated Cl(-) secretion in the murine trachea and colon

The action of the isoflavone genistein on the cystic fibrosis transmembrane conductance regulator (CFTR) has been studied in many cell systems but not in intact murine tissues. We have investigated the action of genistein on murine tissues from normal and cystic fibrosis (CF) mice. Genistein increased the short-circuit current (I(sc)) in tracheal (16.4 +/- 2.8 microA/cm(2)) and colonic (40.0 +/- 4.4 microA/cm(2)) epithelia of wild-type mice. This increase was inhibited by furosemide, diphenylamine-2-carboxylate, and glibenclamide, but not by DIDS. In contrast, genistein produced no significant change in the I(sc) of the tracheal epithelium (0.9 +/- 1.1 microA/cm(2)) and decreased the I(sc) of colons from CF null (-13.1 +/- 2.3 microA/cm(2)) and DeltaF508 mice (-10.3 +/- 1.3 microA/cm(2)). Delivery of a human CFTR cDNA-liposome complex to the airways of CF null mice restored the genistein response in the tracheas to wild-type levels. Tracheas from DeltaF508 mice were also studied: 46% of trachea showed no response to genistein, whereas 54% gave an increase in I(sc) similar to that in wild type. We conclude that genistein activates CFTR-mediated Cl(-) secretion in the murine trachea and distal colon.

Role of protein phosphatases in the activation of CFTR (ABCC7) by genistein and bromotetramisole

Genistein and bromotetramisole (Br-t) strongly activate cystic fibrosis transmembrane conductance regulator (CFTR; ABCC7) chloride channels on Chinese hamster ovary cells and human airway epithelial cells. We have examined the possible role of phosphatases in stimulation by these drugs using patch-clamp and biochemical methods. Genistein inhibited the spontaneous rundown of channel activity that occurs after membrane patches are excised from cAMP-stimulated cells but had no effect on purified protein phosphatase type 1 (PP1), PP2A, PP2B, PP2C, or endogenous phosphatases when assayed as [(32)P]PO(4) release from prelabeled casein, recombinant GST-R domain fusion protein, or immunoprecipitated full-length CFTR. Br-t also slowed rundown of CFTR channels, but, in marked contrast to genistein, it did inhibit all four protein phosphatases tested. Half-maximal inhibition of PP2A and PP2C was observed with 0.5 and 1.5 mM Br-t, respectively. Protein phosphatases were also sensitive to (+)-p-Br-t, a stereoisomer of Br-t that does not inhibit alkaline phosphatases. Br-t appeared to act exclusively through phosphatases since it did not affect CFTR channels in patches that had low apparent endogenous phosphatase activity (i.e., those lacking spontaneous rundown). We conclude that genistein and Br-t act through different mechanisms. Genistein stimulates CFTR without inhibiting phosphatases, whereas Br-t acts by inhibiting a membrane-associated protein phosphatase (probably PP2C) that presumably allows basal phosphorylation to accumulate.

Malformations in offspring of diabetic rats: morphometric analysis of neural crest-derived organs and effects of maternal vitamin E treatment

BACKGROUND

We have previously reported on a malformation-prone Sprague-Dawley rat substrain (U), which presents a high frequency of micrognathia in the offspring of diabetic mothers. This malformation is related to impaired development of the cranial neural crest cells (NCC); the defect may be prevented by antioxidative treatment of the mother.

METHODS

We have therefore investigated whether fetuses of diabetic rats display other malformations associated with altered cranial NCC development and whether maternal vitamin E supplementation may affect such malformations.

RESULTS

Fetuses of diabetic rats showed low-set external ears, severely malformed Meckel's cartilage, small thyroid and thymus, and absence of parathyroid glands. Cardiac anomalies were frequently observed, including rightward displacement of the aorta, double outlet right ventricle (DORV), persistent truncus arteriosus (PTA) combined with ventricular septal defects due to a malaligned outlet septum. The malformations in the outflow tract included abnormalities of the great arteries; right-sided aortic arch/descending aorta, and double aortic arches. These defects tended to occur together within individual fetuses. Maternal dietary treatment with 2% vitamin E markedly reduced the severity of the malformations.

CONCLUSIONS

The phenotypic appearance of these defects is strikingly similar to the DiGeorge anomaly in humans, which has been found in children of diabetic mothers together with an overrepresentation of PTA and DORV. The malformations associated with defective NCC development in the offspring of diabetic U rats show several morphological similarities to those in humans; hence the teratogenic mechanisms may be similar and accessible for study.

Two mechanisms of genistein inhibition of cystic fibrosis transmembrane conductance regulator Cl- channels expressed in murine cell line

1. The isoflavone genistein may either stimulate or inhibit cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels. To investigate how genistein inhibits CFTR, we studied CFTR Cl- channels in excised inside-out membrane patches from cells expressing wild-type human CFTR. 2. Addition of genistein (100 microM) to the intracellular solution caused a small decrease in single-channel current amplitude (i), but a large reduction in open probability (Po). 3. Single-channel analysis of channel block suggested that genistein (100 microM) may inhibit CFTR by two mechanisms: first, it may slow the rate of channel opening and second, it may block open channels. 4. Acidification of the intracellular solution relieved channel block, suggesting that the anionic form of genistein may inhibit CFTR. 5. Genistein inhibition of CFTR Cl- currents was weakly voltage dependent and unaffected by changes in the extracellular Cl- concentration. 6. Channel block was relieved by pyrophosphate (5 mM) and ATP (5 mM), two agents that interact with the nucleotide-binding domains (NBDs) of CFTR to greatly stimulate channel activity. 7. ATP (5 mM) prevented the genistein-induced decrease in Po, but was without effect on the genistein-induced decrease in i. 8. The genistein-induced decrease in i was voltage dependent, whereas the genistein-induced decrease in Po was voltage independent. 9. The data suggest that genistein may inhibit CFTR by two mechanisms. First, it may interact with NBD1 to potently inhibit channel opening. Second, it may bind within the CFTR pore to weakly block Cl- permeation.

Effects of serine/threonine protein phosphatases on ion channels in excitable membranes

This review deals with the influence of serine/threonine-specific protein phosphatases on the function of ion channels in the plasma membrane of excitable tissues. Particular focus is given to developments of the past decade. Most of the electrophysiological experiments have been performed with protein phosphatase inhibitors. Therefore, a synopsis is required incorporating issues from biochemistry, pharmacology, and electrophysiology. First, we summarize the structural and biochemical properties of protein phosphatase (types 1, 2A, 2B, 2C, and 3-7) catalytic subunits and their regulatory subunits. Then the available pharmacological tools (protein inhibitors, nonprotein inhibitors, and activators) are introduced. The use of these inhibitors is discussed based on their biochemical selectivity and a number of methodological caveats. The next section reviews the effects of these tools on various classes of ion channels (i.e., voltage-gated Ca(2+) and Na(+) channels, various K(+) channels, ligand-gated channels, and anion channels). We delineate in which cases a direct interaction between a protein phosphatase and a given channel has been proven and where a more complex regulation is likely involved. Finally, we present ideas for future research and possible pathophysiological implications.

Pathogenesis of diabetes-induced congenital malformations

The increased rate of fetal malformation in diabetic pregnancy represents both a clinical problem and a research challenge. In recent years, experimental and clinical studies have given insight into the teratological mechanisms and generated suggestions for improved future treatment regimens. The teratological role of disturbances in the metabolism of inositol, prostaglandins, and reactive oxygen species has been particularly highlighted, and the beneficial effect of dietary addition of inositol, arachidonic acid and antioxidants has been elucidated in experimental work. Changes in gene expression and induction of apoptosis in embryos exposed to a diabetic environment have been investigated and assigned roles in the teratogenic processes. The diabetic environment appears to simultaneously induce alterations in several interrelated teratological pathways. The complex pathogenesis of diabetic embryopathy has started to unravel, and future research efforts will utilize both clinical intervention studies and experimental work that aim to characterize the human applicability and the cell biological components of the discovered teratological mechanisms.

Activation of cystic fibrosis transmembrane conductance regulator in rat epididymal epithelium by genistein

The effect of genistein on anion secretion via cystic fibrosis transmembrane conductance regulator (CFTR) in cultured rat cauda epididymal epithelia was studied by short-circuit current (Isc) technique. Genistein added apically stimulated a concentration-dependent rise in Isc due to Cl(-) and HCO(3)(-) secretion. The genistein-induced Isc was observed in basolaterally permeabilized monolayers, suggesting that the Isc response was mediated by the apical anion channel. The response could be blocked by the nonspecific Cl(-) channel blocker, diphenylamine-2-carboxylate (DPC), but not by the Ca(2+)-activated Cl(-) channel blocker, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). Genistein did not increase intracellular cAMP, but H-89, a protein kinase A inhibitor, completely abolished the Isc response to genistein. Moreover, pretreatment of the tissues with MDL-12330A, an adenylate cyclase inhibitor, markedly attenuated the Isc response to genistein, but the response was restored upon the addition of exogenous cAMP. Ca(2+), protein kinase C, tyrosine kinase, and protein phosphatase signalling pathways were not involved in the action of genistein. It is speculated that genistein stimulates anion secretion by direct interaction with CFTR. This requires a low level of phosphorylation of CFTR by basal protein kinase A activity. It is suggested that genistein may provide therapeutic benefit to male infertility associated with cystic fibrosis.

Expression of 25-hydroxyvitamin D3-1alpha-hydroxylase along the nephron: new insights into renal vitamin D metabolism

Renal synthesis of the active form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], is a pivotal step in calcium and phosphate homeostasis. Production of 1,25(OH)2D3 is catalyzed by the mitchondrial cytochrome P450, 25-hydroxyvitamin D3-1alpha-hydroxylase (1alpha-HYD). As a consequence of the tight regulation of vitamin D metabolism during normal physiology, studies of the expression and regulation of 1alpha-HYD have proved remarkably difficult. However, the recent cloning of the gene for 1alpha-HYD has enabled a more comprehensive analysis of the tissue distribution of 1alpha-HYD, as well as the mechanisms involved in controlling 1,25(OH)2D3 production. In particular, an understanding of site-specific expression and regulation of 1alpha-HYD along the nephron might help to elucidate a more versatile role for 1,25(OH)2D3 in renal physiology.

Molecular pharmacology of the CFTR Cl- channel

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is associated with a wide spectrum of disease. In the search for modulators of CFTR, pharmacological agents that interact directly with the CFTR Cl- channel have been identified. Some agents stimulate CFTR by interacting with the nucleotide-binding domains that control channel gating, whereas others inhibit CFTR by binding within the channel pore and preventing Cl- permeation. Knowledge of the molecular pharmacology of CFTR might lead to new treatments for diseases caused by the dysfunction of CFTR.

Development of substituted Benzo[c]quinolizinium compounds as novel activators of the cystic fibrosis chloride channel

Chloride channels play an important role in the physiology and pathophysiology of epithelia, but their pharmacology is still poorly developed. We have chemically synthesized a series of substituted benzo[c]quinolizinium (MPB) compounds. Among them, 6-hydroxy-7-chlorobenzo[c]quinolizinium (MPB-27) and 6-hydroxy-10-chlorobenzo[c]quinolizinium (MPB-07), which we show to be potent and selective activators of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. We examined the effect of MPB compounds on the activity of CFTR channels in a variety of established epithelial and nonepithelial cell systems. Using the iodide efflux technique, we show that MPB compounds activate CFTR chloride channels in Chinese hamster ovary (CHO) cells stably expressing CFTR but not in CHO cells lacking CFTR. Single and whole cell patch clamp recordings from CHO cells confirm that CFTR is the only channel activated by the drugs. Ussing chamber experiments reveal that the apical addition of MPB to human nasal epithelial cells produces a large increase of the short circuit current. This current can be totally inhibited by glibenclamide. Whole cell experiments performed on native respiratory cells isolated from wild type and CF null mice also show that MPB compounds specifically activate CFTR channels. The activation of CFTR by MPB compounds was glibenclamide-sensitive and 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid-insensitive. In the human tracheal gland cell line MM39, MPB drugs activate CFTR channels and stimulate the secretion of the antibacterial secretory leukoproteinase inhibitor. In submandibular acinar cells, MPB compounds slightly stimulate CFTR-mediated submandibular mucin secretion without changing intracellular cAMP and ATP levels. Similarly, in CHO cells MPB compounds have no effect on the intracellular levels of cAMP and ATP or on the activity of various protein phosphatases (PP1, PP2A, PP2C, or alkaline phosphatase). Our results provide evidence that substituted benzo[c]quinolizinium compounds are a novel family of activators of CFTR and of CFTR-mediated protein secretion and therefore represent a new tool to study CFTR-mediated chloride and secretory functions in epithelial tissues.

Developmental and hormonal regulation of thermosensitive neuron potential activity in rat brain

To understand the involvement of thyroid hormone on the postnatal development of hypothalamic thermosensitive neurons, we focused on the analysis of thermosensitive neuronal activity in the preoptic and anterior hypothalamic (PO/AH) regions of developing rats with and without hypothyroidism. In euthyroid rats, the distribution of thermosensitive neurons in PO/AH showed that in 3-week-old rats (46 neurons tested), 19.5% were warm-sensitive and 80.5% were nonsensitive. In 5- to 12-week-old euthyroid rats (122 neurons), 33.6% were warm-sensitive and 66.4% were nonsensitive. In 5- to 12-week-old hypothyroid rats (108 neurons), however, 18.5% were warm-sensitive and 81.5% were nonsensitive. Temperature thresholds of warm-sensitive neurons were lower in 12-week-old euthyroid rats (36.4+/-0.2 degrees C, n = 15, p<0.01,) than in 3-week-old and in 5-week-old euthyroid rats (38.5+/-0.5 degrees C, n = 9 and 38.0+/-0.3 degrees C, n = 15, respectively). The temperature thresholds of warm-sensitive neurons in 12-week-old hypothyroid rats (39.5+/-0.3 degrees C, n = 8) were similar to that of warm-sensitive neurons of 3-week-old raats (euthyroid and hypothyroid). In contrast, there was no difference in the thresholds of warm-sensitive neurons between hypothyroid and euthyroid rats at the age of 3-5 weeks. In conclusion, monitoring the thermosensitive neuronal tissue activity demonstrated the evidence that thyroid hormone regulates the maturation of warm-sensitive hypothalamic neurons in developing rat brain by electrophysiological analysis.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

The regulation of epithelial cell cAMP- and calcium-dependent chloride channels

This chapter has focused on two types of chloride conductance found in epithelial cells. The leap from the Ussing chamber to patch-clamp studies has identified yet other conductances present which have also been electrophysiologically characterized. In the case of the swelling activated wholecell chloride current, a physiological function is apparent and a single-channel basis found, but its genetic identity remains unknown (see reviews by Frizzell and Morris, 1994; and Strange et al., 1996). The outwardly rectified chloride channel has been the subject of considerable electrophysiological interest over the past 10 years and is well characterized at the single-channel level, but its physiological function remains controversial (reviewed by Frizzell and Morris, 1994; Devidas and Guggino, 1997). Yet other conductances related to the CLC gene family also appear to be present in epithelial cells of the kidney (reviewed by Jentsch, 1996; Jentsch and Gunter, 1997) where physiological functions for some isoforms are emerging. Clearly, there remain many unknowns. Chief among these is the molecular basis of GCa2+Cl and many of other the conductances. As sequences become available it is expected that the wealth of information gained by investigation into CFTR function will provide a conceptual blueprint for similar studies in these later channel clones.

Energy metabolism and body temperature of barn owls fasting in the cold

Energetic adaptation to fasting in the cold has been investigated in a nocturnal raptor, the barn owl (Tyto alba), during winter. Metabolic rate and body temperature (Tb) were monitored in captive birds, (1) after acute exposure to different ambient temperatures (Ta), and (2) during a prolonged fast in the cold (4 degrees C), to take into account the three characteristic phases of body fuel utilization that occur during a long-term but reversible fast. In postabsorptive birds, metabolic rate in the thermoneutral zone was 4. 1+/-0.1 W kg-1 and increased linearly below a lower critical temperature of 23 degrees C. Metabolic rate was 70% above basal at +4 degrees C Ta. Wet thermal conductance was 0.22 W kg-1 degrees C-1. During fasting in the cold, the mass-specific resting metabolic rate decreased by 16% during the first day (phase I) and remained constant thereafter. The amplitude of the daily rhythm in Tb was only moderately increased during phase II, with a slight lowering (0. 6 degrees C) in minimal diurnal Tb, but rose markedly in phase III with a larger drop (1.4 degrees C) in minimal diurnal Tb. Refeeding the birds ended phase III and reversed the observed changes. These results indicate that diurnal hypothermia may be used in long-term fasting barn owls and could be triggered by a threshold of body lipid depletion, according to the shift from lipid to protein fuel metabolism occurring at the phase II/phase III transition. The high cost of regulatory thermogenesis and the limited use of hypothermia during fasting may contribute to the high mortality of barn owls during winter.

Control of CFTR channel gating by phosphorylation and nucleotide hydrolysis

Control of CTFR Channel Gating by Phosphorylation and Nucleotide Hydrolysis. Physiol. Rev. 79, Suppl.: S77-S107, 1999. - The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is the protein product of the gene defective in cystic fibrosis, the most common lethal genetic disease among Caucasians. Unlike any other known ion channel, CFTR belongs to the ATP-binding cassette superfamily of transporters and, like all other family members, CFTR includes two cytoplasmic nucleotide-binding domains (NBDs), both of which bind and hydrolyze ATP. It appears that in a single open-close gating cycle, an individual CFTR channel hydrolyzes one ATP molecule at the NH2-terminal NBD to open the channel, and then binds and hydrolyzes a second ATP molecule at the COOH-terminal NBD to close the channel. This complex coordinated behavior of the two NBDs is orchestrated by multiple protein kinase A-dependent phosphorylation events, at least some of which occur within the third large cytoplasmic domain, called the regulatory domain. Two or more kinds of protein phosphatases selectively dephosphorylate distinct sites. Under appropriately controlled conditions of progressive phosphorylation or dephosphorylation, three functionally different phosphoforms of a single CFTR channel can be distinguished on the basis of channel opening and closing kinetics. Recording single CFTR channel currents affords an unprecedented opportunity to reproducibly examine, and manipulate, individual ATP hydrolysis cycles in a single molecule, in its natural environment, in real time.

Pharmacology of CFTR chloride channel activity

Pharmacology of CFTR Chloride Channel Activity. Physiol. Rev. 79, Suppl.: S109-S144, 1999. - The pharmacology of cystic fibrosis transmembrane conductance regulator (CFTR) is at an early stage of development. Here we attempt to review the status of those compounds that modulate the Cl- channel activity of CFTR. Three classes of compounds, the sulfonylureas, the disulfonic stilbenes, and the arylaminobenzoates, have been shown to directly interact with CFTR to cause channel blockade. Kinetic analysis has revealed the sulfonylureas and arylaminobenzoates interact with the open state of CFTR to cause blockade. Suggestive evidence indicates the disulfonic stilbenes act by a similar mechanism but only from the intracellular side of CFTR. Site-directed mutagenesis studies indicate the involvement of specific amino acid residues in the proposed transmembrane segment 6 for disulfonic stilbene blockade and segments 6 and 12 for arylaminobenzoate blockade. Unfortunately, these compounds (sulfonylureas, disulfonic stilbenes, arylaminobenzoate) also act at a number of other cellular sites that can indirectly alter the activity of CFTR or the transepithelial secretion of Cl-. The nonspecificity of these compounds has complicated the interpretation of results from cellular-based experiments. Compounds that increase the activity of CFTR include the alkylxanthines, phosphodiesterase inhibitors, phosphatase inhibitors, isoflavones and flavones, benzimidazolones, and psoralens. Channel activation can arise from the stimulation of the cAMP signal transduction cascade, the inhibition of inactivating enzymes (phosphodiesterases, phosphatases), as well as the direct binding to CFTR. However, in contrast to the compounds that block CFTR, a detailed understanding of how the above compounds increase the activity of CFTR has not yet emerged.

Rescue of angiotensinogen-knockout mice

Angiotensinogen, the precursor of angiotensins I and II, is a critical component of the renin-angiotensin system that plays an important role in regulating blood pressure and electrolyte homeostasis. Genetically altered mice lacking angiotensinogen (Agt-KO) showed an expected phenotype, such as marked hypotension, but unexpected ones including abnormal kidney morphology, reduced survival rates of newborns, and impaired blood-brain barrier function after cold injury. To examine whether disruption of the angiotensinogen gene is responsible for the observed phenotypes, we generated a line of mice heterozygous for the mouse angiotensinogen gene under the control of a mouse metallothionein-I promoter (MT-Agt) and crossmated transgenic mice with Agt-KO mice. The resulting mice (MT-Agt(+/-)/Agt(-/-):MT-Agt/KO) produced the plasma level of angiotensin I comparable to that of wild-type mice, and the mutant phenotypes were rescued. These results indicated that the resultant phenotypes due to the genetic deficiency of mouse angiotensinogen can be corrected by restoring angiotensinogen and angiotensin I in the circulation.

[Mucoviscidosis: therapeutic strategies are multiplying]

Since the cloning of the defective gene in cystic fibrosis, much has been learned on the function of CFTR and on the mechanisms regulating its expression. Based on the current understanding of the processes involved in lung disease progression, a number of approaches have been developed using gene therapy and pharmacological agents. Several of these agents have been reported to restitute a function to CFTR with specific mutations. Other molecules act on channels other than CFTR, and may be effective by bypassing CFTR itself. In the present review the various therapeutical strategies currently investigated are discussed.

Carbachol stimulates transactivation of epidermal growth factor receptor and mitogen-activated protein kinase in T84 cells. Implications for carbachol-stimulated chloride secretion

We have examined the role of tyrosine phosphorylation in regulation of calcium-dependent chloride secretion across T84 colonic epithelial cells. The calcium-mediated agonist carbachol (CCh, 100 microM) stimulated a time-dependent increase in tyrosine phosphorylation of a range of proteins (with molecular masses ranging up to 180 kDa) in T84 cells. The tyrosine kinase inhibitor, genistein (5 microM), significantly potentiated chloride secretory responses to CCh, indicating a role for CCh-stimulated tyrosine phosphorylation in negative regulation of CCh-stimulated secretory responses. Further studies revealed that CCh stimulated an increase in both phosphorylation and activity of the extracellular signal-regulated kinase (ERK) isoforms of mitogen-activated protein kinase. Chloride secretory responses to CCh were also potentiated by the mitogen-activated protein kinase inhibitor, PD98059 (20 microM). Phosphorylation of ERK in response to CCh was mimicked by the protein kinase C (PKC) activator, phorbol myristate acetate (100 nM), but was not altered by the PKC inhibitor GF 109203X (1 microM). ERK phosphorylation was also induced by epidermal growth factor (EGF) (100 ng/ml). Immunoprecipitation/Western blot studies revealed that CCh stimulated tyrosine phosphorylation of the EGF receptor (EGFr) and increased co-immunoprecipitation of the adapter proteins, Shc and Grb2, with the EGFr. An inhibitor of EGFr phosphorylation, tyrphostin AG1478 (1 microM), reversed CCh-stimulated phosphorylation of both EGFr and ERK. Tyrphostin AG1478 also potentiated chloride secretory responses to CCh. We conclude that CCh activates ERK in T84 cells via a mechanism involving transactivation of the EGFr, and that this pathway constitutes an inhibitory signaling pathway by which chloride secretory responses to CCh may be negatively regulated.

Cystic fibrosis transmembrane conductance regulator activation by cAMP-independent mechanisms

Recent studies have demonstrated that several compounds with diverse structures can activate wild-type cystic fibrosis transmembrane conductance regulator (CFTR) by non-receptor-mediated mechanisms. Some of these compounds have been shown to enhance cAMP-dependent activation of DeltaF508-CFTR. This study was undertaken to compare the mechanisms by which genistein, IBMX, milrinone, 8-cyclopentyl-1, 3-dipropylxanthine (CPX), the benzimidazolone NS004, and calyculin A increase CFTR activity. Our studies demonstrate that, in transfected NIH-3T3 cells, maximal enhancements of forskolin-dependent DeltaF508-CFTR activity are greatest with genistein, IBMX, and NS004. Milrinone, genistein, CPX, NS004, and calyculin A do not increase cellular cAMP. Because forskolin and calyculin A increase in vivo phosphorylation of cAMP binding response element (CREB), the inability of milrinone, genistein, CPX, and NS004 to increase CREB phosphorylation suggests that they do not stimulate protein kinase A or inhibit phosphatase activity. Our data suggest that the mechanisms by which genistein and NS004 activate CFTR differ. We also demonstrate that, in NIH-3T3 cells, IBMX-dependent enhancement of cAMP-dependent CFTR activity is not due to an increase in cellular cAMP and may involve a mechanism like that of genistein.

Therapies directed at the basic defect in cystic fibrosis

There are over 600 unique mutations in the cystic fibrosis (CF) gene that can be classified in five general categories with respect to specific defect. Through basic research into the genetic and physiologic consequences of these mutations, it has become possible to design genotype-specific therapeutic strategies. New pharmaceutical agents are under development for the rescue of defective cystic fibrosis transmembrane conductance regulator mRNA or protein. Some of these compounds are undergoing study in CF patients in Phase I clinical trials. This article evaluates the current research directed at translating a basic molecular understanding of the disease into innovative new treatments.

Bradykinin B2-receptor-mediated modulation of membrane currents in guinea-pig cardiomyocytes

1. In order to define the electrophysiological mechanism(s) responsible for bradykinin (BK)-induced positive inotropic and chronotropic responses in isolated guinea-pig atria, effects of BK on the membrane currents were examined in isolated atrial cells using patch clamp techniques. 2. BK (0.1-1000 nM) increased the L-type Ca2+ current (I(Ca)), which was recorded from enzymatically-dissociated atrial myocytes by the nystatin-perforated patch method, in a concentration-dependent fashion, and the calculated EC50 value for increasing I(Ca) was 5.2 nM. In conventional ruptured patch experiments, BK inhibited the muscarinic acetylcholine receptor-operated K+ current (I(K.ACh)) that was activated by the muscarinic agonist carbachol (1 microM) with an EC50 value of 0.57 nM. Both the increase in I(Ca) and the decrease in I(K.ACh) were blocked by HOE140, a selective bradykinin B2 receptor antagonist. 3. The BK-induced inhibition of I(K.ACh) was significantly attenuated by staurosporine and calphostin C, protein kinase C inhibitors. In addition, the I(K.ACh) inhibition by BK was also attenuated by the tyrosine kinase inhibitor genistein or tyrphostin but not by daidzein, an inactive analogue of genistein. However, neither protein kinase C inhibitor nor tyrosine kinase inhibitor affected the BK-induced increase in I(Ca). 4. In the presence and absence of muscarinic stimulation, BK prolonged the action potential recorded from the atrial cells in the current clamp mode. 5. We conclude that BK increases I(Ca) and decreases I(K.ACh) in atrial cells, resulting in positive inotropic and chronotropic responses in atrial preparations. Protein kinase C activation, and possibly tyrosine kinase activation, may be involved in the B2-receptor-mediated I(K.ACh) inhibition.

Genistein increases the sensitivity of cardiac ion channels to beta-adrenergic receptor stimulation

The whole-cell patch-clamp technique was used to monitor the effects of genistein, a tyrosine kinase inhibitor, on membrane currents recorded from isolated guinea pig ventricular myocytes. Under control conditions, genistein (50 micromol/L) did not activate the latent cAMP-regulated Cl- current (ICl). However, in the presence of a subthreshold concentration (1 nmol/L) of the beta-adrenergic agonist isoproterenol (Iso), genistein caused a near-maximal activation of this current. In the absence of genistein, Iso activated ICl with an EC50 of 5 nmol/L. In the presence of genistein, Iso activated ICl with an EC50 of 0.3 nmol/L. This facilitatory effect was not observed in the presence of daidzein (50 micromol/L), an analogue of genistein that only weakly inhibits tyrosine kinase activity. Furthermore, peroxovanadate, a potent inhibitor of phosphotyrosine phosphatase activity, inhibited ICl activated by Iso alone, and it blocked the stimulatory effect of genistein in the presence of Iso. To determine whether the stimulatory effect of genistein was specific for ICl, we also studied its action on the cAMP-regulated delayed rectifier K+ current (IK) and L-type Ca2+ current (ICa-L) present in these cells. Basal IK and ICa-L were partially (approximately 30% to 40%) inhibited by genistein. However, this inhibitory effect was mimicked by daidzein, suggesting that inhibition of tyrosine kinase activity is not involved. In addition to the nonspecific inhibitory effect, genistein also caused a significant increase in the beta-adrenergic sensitivity of the unblocked cationic currents. In the absence of genistein, 1 nmol/L Iso had no effect on either IK or ICa-L. However, in the presence of genistein, 1 nmol/L Iso significantly increased the magnitude of both currents. These results suggest that tyrosine kinase activity may play an important role in regulating beta-adrenergic responsiveness of the heart.

Role of tyrosine phosphorylation in leptin activation of ATP-sensitive K+ channels in the rat insulinoma cell line CRI-G1

1. Using whole-cell and cell-attached recording configurations, the role of phosphorylation in leptin activation of ATP-sensitive K+ (KATP) channels was examined in the rat CRI-G1 insulinoma cell line. 2. Whole-cell current clamp recordings demonstrated that, following dialysis with the non-hydrolysable ATP analogue 5'-adenylylimidodiphosphate (AMP-PNP; 3-5 mM), the leptin-induced hyperpolarization and increase in K+ conductance were completely inhibited. 3. Under current clamp conditions, application of the broad-spectrum protein kinase inhibitor H-7 (10 microM) had no effect on the resting membrane potential or slope conductance of CRI-G1 insulinoma cells and did not occlude the actions of leptin. 4. Application of the tyrosine kinase inhibitors genistein (10 microM), tyrphostin B42 (10 microM) and herbimycin A (500 nM) all resulted in activation of KATP channels. In cell-attached recordings, the presence of tyrphostin B42 (10 microM) in the pipette solution activated tolbutamide-sensitive KATP channels in CRI-G1 cells. In contrast, the inactive analogues of genistein and tyrphostin B42 were without effect. 5. The serine/threonine-specific protein phosphatase inhibitors okadaic acid (50 nM) and cyclosporin A (1 microM) did not prevent or reverse leptin activation of KATP channels. In contrast, whole-cell dialysis with the tyrosine phosphatase inhibitor orthovanadate (500 microM) prevented the actions of both leptin and tyrphostin B42. 6. In conclusion, leptin activation of KATP channels appears to require inhibition of tyrosine kinases and subsequent dephosphorylation. This process is likely to occur prior to activation of phosphoinositide 3-kinase (PI 3-kinase) as wortmannin prevented activation of KATP channels by tyrphostin B42.

Partial restoration of cAMP-stimulated CFTR chloride channel activity in DeltaF508 cells by deoxyspergualin

Deletion of the codon encoding phenylalanine 508 (DeltaF508) is the most common mutation in cystic fibrosis (CF) and results in a trafficking defect. Mutant DeltaF508-CF transmembrane conductance regulator (CFTR) protein retains functional activity, but the nascent protein is recognized as abnormal and, in consequence, is retained in the endoplasmic reticulum (ER) and degraded. It has been proposed that this retention in the ER is mediated, at least in part, by the cellular chaperones heat shock protein (HSP) 70 and calnexin. We have investigated the ability of deoxyspergualin (DSG), a compound known to compete effectively for binding with HSP70 and HSP90, to promote trafficking of DeltaF508-CFTR to the cell membrane. We show that DSG treatment of immortalized human CF epithelial cells (DeltaF508) and cells expressing recombinant DeltaF508-CFTR partially restored cAMP-stimulated CFTR Cl- channel activity at the plasma membrane. Although there are several possible explanations for these results, one simple interpretation is that DSG may have altered the interaction between DeltaF508-CFTR and its associated chaperones. If this is correct, agents capable of altering the normal functioning of cellular chaperones may provide yet another means of restoring CFTR Cl- channel activity to CF subjects harboring this class of mutations.

Maternal antioxidant treatments prevent diabetes-induced alterations of mitochondrial morphology in rat embryos

Previous studies have suggested that production of reactive oxygen species by embryonic mitochondria may have a role in the induction of both high-amplitude mitochondrial swelling and embryonic dysmorphogenesis in diabetic pregnancy. The present study analyzed the relationships between a putative metabolite-induced production of free oxygen radicals, mitochondrial lipid peroxidation, and high-amplitude mitochondrial swelling in embryos during organogenesis. For studies in vitro, day 9 embryos of normal rats were cultured for 48 h with a high concentration of glucose in the absence or presence of alpha-cyano-4-hydroxycinnamic acid (CHC), a mitochondrial pyruvate transport inhibitor. The morphology of mitochondria in the neuroepithelium of the embryos was studied with the aid of transmission electron microscopy. For studies in vivo, normal and diabetic pregnant rats were fed a diet supplemented with the antioxidants alpha-tocopherol (vitamin E) or 2,6-di-tert-butyl-4-methylphenol (BHT), and the ultrastructure of mitochondria in the embryonic neuroepithelium and in the visceral yolk sac was investigated on gestational day 11. Exposure to a high concentration of glucose in vitro or to maternal diabetes in vivo induced high-amplitude swelling of mitochondria in the neuroepithelium of the embryos. The swelling of mitochondria was prevented by addition of CHC to the culture media or by maternal ingestion of antioxidant-supplemented food. In diabetic pregnancy, embryonic mitochondria during organogenesis produce free oxygen radicals that cause mitochondrial lipid peroxidation and swelling and furthermore embryonic dysmorphogenesis. Dietary supplementation with antioxidants to the mother may prevent embryonic malformations in diabetic pregnancy by inhibition of mitochondrial dysfunction.

Co-expression of a Ca2+-inhibitable adenylyl cyclase and of a Ca2+-sensing receptor in the cortical thick ascending limb cell of the rat kidney. Inhibition of hormone-dependent cAMP accumulation by extracellular Ca2+

The Ca2+-sensing receptor protein and the Ca2+-inhibitable type 6 adenylyl cyclase mRNA are present in a defined segment of the rat renal tubule leading to the hypothesis of their possible functional co-expression in a same cell and thus to a possible inhibition of cAMP content by extracellular Ca2+. By using microdissected segments, we compared the properties of regulation of extracellular Ca2+-mediated activation of Ca2+ receptor to those elicited by prostaglandin E2 and angiotensin II. The three agents inhibited a common pool of hormone-stimulated cAMP content by different mechanisms as follows. (i) Extracellular Ca2+, coupled to phospholipase C activation via a pertussis toxin-insensitive G protein, induced a dose-dependent inhibition of cAMP content (1.25 mM Ca2+ eliciting 50% inhibition) resulting from both stimulation of cAMP hydrolysis and inhibition of cAMP synthesis; this latter effect was mediated by capacitive Ca2+ influx as well as release of intracellular Ca2+. (ii) Angiotensin II, coupled to the same transduction pathway, also decreased cAMP content; however, its inhibitory effect on cAMP was mainly accounted for by an increase of cAMP hydrolysis, although angiotensin II and extracellular Ca2+ can induce comparable release of intracellular Ca2+. (iii) Prostaglandin E2, coupled to pertussis toxin-sensitive G protein, inhibited the same pool of adenylyl cyclase units as extracellular Ca2+ but by a different mechanism. The functional properties of the adenylyl cyclase were similar to those described for type 6. The results establish that the co-expression of a Ca2+-inhibitable adenylyl cyclase and of a Ca2+-sensing receptor in a same cell allows an inhibition of cAMP accumulation by physiological concentrations of extracellular Ca2+.

Differential regulation of single CFTR channels by PP2C, PP2A, and other phosphatases

Cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel activity declines rapidly when excised from transfected Chinese hamster ovary (CHO) or human airway cells because of membrane-associated phosphatase activity. In the present study, we found that CFTR channels usually remained active in patches excised from baby hamster kidney (BHK) cells overexpressing CFTR. Those patches with stable channel activity were used to investigate the regulation of CFTR by exogenous protein phosphatases (PP). Adding PP2A, PP2C, or alkaline phosphatase to excised patches reduced CFTR channel activity by > 90% but did not abolish it completely. PP2B caused weak deactivation, whereas PP1 had no detectable effect on open probability (Po). Interestingly, the time course of deactivation by PP2C was identical to that of the spontaneous rundown observed in some patches after excision. PP2C and PP2A had distinct effects on channel gating Po declined during exposure to exogenous PP2C (and during spontaneous rundown, when it was observed) without any change in mean burst duration. By contrast, deactivation by exogenous PP2A was associated with a dramatic shortening of burst duration similar to that reported previously in patches from cardiac cells during deactivation of CFTR by endogenous phosphatases. Rundown of CFTR-mediated current across intact T84 epithelial cell monolayers was insensitive to toxic levels of the PP2A inhibitor calyculin A. These results demonstrate that exogenous PP2C is a potent regulator of CFTR activity, that its effects on single-channel gating are distinct from those of PP2A but similar to those of endogenous phosphatases in CHO, BHK, and T84 epithelial cells, and that multiple protein phosphatases may be required for complete deactivation of CFTR channels.

ClC and CFTR chloride channel gating

Chloride channels are widely expressed and play important roles in cell volume regulation, transepithelial transport, intracellular pH regulation, and membrane excitability. Most chloride channels have yet to be identified at a molecular level. The ClC gene family and the cystic fibrosis transmembrane conductance regulator (CFTR) are distinct chloride channels expressed in many cell types, and mutations in their genes are the cause of several diseases including myotonias, cystic fibrosis, and kidney stones. Because of their molecular definition and roles in disease, these channels have been studied intensively over the past several years. The focus of this review is on recent studies that have provided new insights into the mechanisms governing the opening and closing, i.e. gating, of the ClC and CFTR chloride channels.

Actions of genistein on cystic fibrosis transmembrane conductance regulator channel gating. Evidence for two binding sites with opposite effects

Previous studies have shown that genistein increased cystic fibrosis transmembrane conductance regulator (CFTR) channel activity in the presence of saturating concentrations of forskolin and calyculin A in intact cells. Possible molecular mechanisms for genistein's action include inhibition of tyrosine kinases, inhibition of serine/threonine protein phosphatases, or direct binding of genistein to CFTR. Since genistein inhibits several enzymes that hydrolyze ATP, and ATP hydrolysis is an intrinsic property of CFTR, we examined the effect of genistein on CFTR gating in excised inside-out patches from Hi-5 insect cells and NIH3T3 cells expressing recombinant CFTR. Genistein (50 microM) did not open phosphorylated CFTR channels by itself, but increased the ATP- induced CFTR channel current by approximately twofold. A similar magnitude of enhancement was observed when genistein was applied with PKI, a specific inhibitor of protein kinase A, or vanadate, a tyrosine phosphatase inhibitor, suggesting that inhibition of protein phosphatases or tyrosine kinases does not account for genistein's effects. The enhancement of channel current increased with increasing concentrations of genistein and reached a maximum at 35 microM genistein. At higher concentrations of genistein concentration, CFTR channel current decreased, resulting in a bell-shaped dose-response relationship. In the absence of genistein, both open- and closed-time histograms could be fitted with a single exponential function, yielding a mean open time (tauO) of 0.302 +/- 0.002 s, and a mean closed time (tauC) of 0.406 +/- 0.003 s. In the presence of 50 microM genistein, the open time histogram could be fitted with a double exponential function with tauO1 = 0.429 +/- 0. 003 s and tauO2 = 2.033 +/- 0.173 s. Thus, genistein induced a prolonged open state, an effect that mimics that of nonhydrolyzable ATP analogs. Closed time analysis showed that 50 microM genistein caused a prolonged closed state with a time constant of 2.410 +/- 0.035 s. We thus conclude that (a) the effects of genistein are likely caused by a direct binding of the drug to the CFTR protein, and (b) at least two binding sites are required to explain the effects of genistein: a high affinity site that decreases the closing rate and a low affinity site that reduces the opening rate.

Metabolic alteration in neural tissue of rat embryos exposed to beta-hydroxybutyrate during organogenesis

Hyperketonemia has been identified as an important factor in diabetic pregnancy affecting growth and development of the offspring. In order to assess the immediate metabolic alterations in embryos caused by excess ketone bodies, we studied rat embryonic neural tissue exposed to a high concentration of beta-hydroxybutyrate in vitro. Beta-hydroxybutyrate inhibited oxygen uptake of the neural tissue of day 9 and day 10 embryos by 12.8% and 1 1.2%, but did not affect that of day 11 and day 12 tissue. In contrast, glucose utilization of the neural tissue of day 9 and day 10 embryos was not altered. However, a 30% decrease in glucose utilization was observed in the neural tissue of day 11 and day 12 embryos exposed to beta-hydroxybutyrate. Thus, beta-hydroxybutyrate induced different metabolic alterations in the embryonic neural tissue during early and late organogenesis, which suggests different modes of teratogenic action of ketone bodies in different parts of gestation.