Regulation of NaCl entry into Necturus gallbladder epithelium by protein kinase C

Effect of massive sympathetic nervous system activation on coronary blood flow and myocardial energy pool

Our previous work indicates that myocardial ischemia could be the mechanism responsible for the left ventricular (LV) dysfunction that frequently develops after massive sympathetic nervous system (SNS) activation. In this study, coronary blood flow (CBF) and myocardial ATP, creatine phosphate, and lactate concentrations were measured after massively activating the SNS of anesthetized rabbits with an intracisternal injection of veratrine. CBF was measured at time 0 (baseline), and at 2, 10, and 20 min after SNS activation in one group, and at 0, 45, 90, and 150 min in a second group. Myocardial ATP, creatine phosphate, and lactate were measured at 0, 2, 10, 20, 90, and 150 min in separate groups of rabbits. SNS activation caused LV dysfunction in approximately 60% of the rabbits. SNS-related increases in CBF kept pace with the increases in myocardial energy demand as determined from the systolic pressure-heart rate product. The subendocardial-to-subepicardial blood flow ratio did not change significantly. Myocardial creatine phosphate concentration was depressed 2 min after SNS activation and remained depressed for at least 20 min. ATP fell continuously and was significantly lower than baseline by 20 min. Tissue lactate concentration was elevated at this time. By 90 min, the concentrations of all three metabolites had recovered. These results indicate that myocardial high-energy phosphate compounds fall after massive SNS activation, but ischemia does not appear to be the underlying mechanism.

A primary culture of guinea pig gallbladder epithelial cells that is responsive to secretagogues

We have developed a cell culture of guinea pig gallbladder epithelial cells with which to study ion transport. When grown on permeable supports, the cultured epithelia developed a transepithelial resistance (R(t)) of approximately 500 Omega. cm(2). The epithelial cell origin of the cell culture was further confirmed by immunocytochemical localization of cytokeratin. Ionomycin and forskolin increased transepithelial voltage and short-circuit current (I(sc)) and decreased R(t). The response to ionomycin was transient, whereas that to forskolin was sustained. Both were attenuated by replacement of Cl(-) and/or HCO(3)(-). Mucosal addition of the anion transport inhibitors DIDS or diphenylamine-2-carboxylic acid (DPC) blocked the response to ionomycin. The response to forskolin was blocked by DPC but not by DIDS. Ionomycin, but not forskolin, increased intracellular Ca(2+) concentration in fura 2-loaded cells. PGE(2), histamine, vasoactive intestinal polypeptide, and secretin elicited a sustained increase in I(sc). Responses to ATP and CCK were transient. Thus cultured guinea pig gallbladder epithelia display the range of responses observed in the native tissue and are an appropriate model for studies of ion transport in gallbladder and intestinal epithelia.

Effect of HCO(3)(-) on TPA- and IBMX-induced anion conductances in Necturus gallbladder epithelial cells

Effects of HCO(3)(-) on protein kinase C (PKC)- and protein kinase A (PKA)-induced anion conductances were investigated in Necturus gallbladder epithelial cells. In HCO(3)(-)-free media, activation of PKC via 12-O-tetradecanoylphorbol 13-acetate (TPA) depolarized apical membrane potential (V(a)) and decreased fractional apical voltage ratio (F(R)). These effects were blocked by mucosal 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), a Cl(-) channel blocker. In HCO(3)(-) media, TPA induced significantly greater changes in V(a) and F(R). These effects were blocked only when NPPB was present in both mucosal and basolateral compartments. The data suggest that TPA activates NPPB-sensitive apical Cl(-) conductance (g(Cl)(a)) in the absence of HCO(3)(-); in its presence, TPA stimulated both NPPB-sensitive g(Cl)(a) and basolateral Cl(-) conductance (g(Cl)(b)). Activation of PKA via 3-isobutyl-1-methylxanthine (IBMX) also decreased V(a) and F(R); however, these changes were not affected by external HCO(3)(-). We conclude that HCO(3)(-) modulates the effects of PKC on g(Cl)(b). In HCO(3)(-) medium, TPA and IBMX also induced an initial transient hyperpolarization and increase in intracellular pH. Because these changes were independent of mucosal Na(+) and Cl(-), it is suggested that TPA and IBMX induce a transient increase in apical HCO(3)(-) conductance.

Expression of cell volume-regulated kinase h-sgk in pancreatic tissue

Transcript levels of the human serine/threonine kinase h-sgk have been found to be highest in pancreas. In the present study, localization and regulation of h-sgk transcription in pancreatic tissue were elucidated. As was apparent from radioactive in situ hybridization, most pancreatic acinar cells expressed high levels of h-sgk mRNA. h-sgk mRNA-positive cells were also found in ductal epithelia but not in pancreatic islets. In biopsy specimens from patients with pancreatitis, h-sgk mRNA levels were decreased in acinar cells but abundant in numerous mononuclear interstitial cells within areas of pancreatic necrosis and fibrosis. As shown by Northern blotting, h-sgk transcription in DAN-G pancreatic tumor cells is upregulated by osmotic cell shrinkage, serum, phorbol esters (phorbol 12,13-didecanoate), and Ca(2+) ionophore A-23187 and decreased by staurosporine and cAMP. In conclusion, h-sgk transcription is regulated not only by cell volume but also by serum, protein kinase C stimulation, cAMP, and increase of intracellular Ca(2+) activity. The kinase may participate not only in normal function of exocrine pancreas but also in fibrosing pancreatitis.

Role of angiotensin II in sympathetic nervous system induced left ventricular dysfunction

Experiments were undertaken to determine whether angiotensin (Ang) II concentration increases during massive sympathetic nervous system (SNS) activation and whether such an increase plays a role in the pathogenesis of SNS-induced left ventricular (LV) dysfunction. We also sought to determine whether excessive Ca2+ uptake through L-type channels due to intense adrenoceptor activation is responsible for the LV dysfunction. AngII concentration was measured in the plasma and myocardium before and after massively activating the SNS with an intracisternal injection of veratrine. In separate experiments, rabbits were given losartan, enalaprilat, enalaprilat plus HOE-140, nifedipine, -Bay K 4866, or saline before massively activating the SNS. LV function was evaluated 2.5 h later. The intense SNS activity caused plasma and myocardial AngII to increase by 400 and 437%, respectively. AngII receptor blockade did not prevent LV dysfunction. In contrast, enalaprilat reduced the degree of dysfunction, but its cardioprotection was abolished by HOE-140. Although nifedipine prevented SNS-induced LV dysfunction, administration of the Ca2+ channel opener, -Bay K 4866, did not increase its severity. Our results indicate that AngII is not involved in the pathogenesis of SNS-induced LV dysfunction and that the cardioprotection provided by angiotensin converting enzyme (ACE) inhibition is due to activation of a bradykinin pathway. Furthermore, the finding that the magnitude of the LV dysfunction was reduced by enalaprilat, and not increased by -Bay K 4866, suggests that intense adrenoceptor activation of L-type Ca2+ channels is not the primary pathogenetic mechanism.Key words: converting-enzyme inhibitor, calcium channel opener-blocker, myocardial contractility, catecholamines, rabbits.

Salidiuretic action by genistein in the isolated, perfused rat kidney

The urinary isoflavonoid genistein inhibits membrane Na-K-Cl cotransporters at similar concentrations as furosemide, but the significance of this action is unknown. Genistein was therefore investigated in rats for its potential salidiuretic actions. In the isolated, perfused rat kidney, genistein induced a maximal salidiuretic action similar to that of furosemide but was 3 to 5 times less potent than furosemide in terms of active doses (natriuresis EC50, 237+/-92 versus 56+/-20 micromol/L for genistein and furosemide, respectively). Genistein and furosemide had no additive salidiuretic actions. Genistein had no significant effect on glomerular filtration rate but was able to significantly reduce renal vascular resistance with respect to vehicle isolated perfused kidney. Indomethacin (10 micromol/L), a blocker of prostaglandin biosynthesis, reduced salidiuresis and renal vasorelaxation by genistein. Subcutaneous genistein (15 mg/kg) induced a statistically significant increase in diuresis and natriuresis with respect to vehicle during the first 6 hours of administration in rats. In conclusion, genistein compares well with furosemide in vitro for its salidiuretic profile and potency in the isolated perfused rat kidney and is also natriuretic by the subcutaneous route in the rat. Further studies are required to investigate potential natriuretic and perhaps hypotensive actions of dietary genistein.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Characteristics of membrane transport processes of macula densa cells

1. Macula densa (MD) cells are located within the thick ascending limb (TAL) and have their apical surface in contact with tubular fluid and their basilar region in contact with the glomerulus. These cells sense changes in luminal fluid sodium chloride concentration ([NaCl]) and transmit signals resulting in changes in vascular resistance (tubuloglomerular feedback) and renin release. 2. Current efforts have focused on understanding the cellular transport mechanisms of MD cells. Progress in this area has benefited from the use of the isolated perfused TAL-glomerular preparation, which permits direct access to MD cells. 3. Using microelectrodes to measure basolateral membrane potential (VBL) of MD cells, it was found that VBL was very sensitive to changes in luminal fluid [NaCl]. As [NaCl] was elevated from 20 to 150 mmol/L, VBL was found to depolarize by over 30 mV. 4. Basolateral membrane potential measurements were also used to identify an apical Na+:2Cl-:K+ cotransport pathway in MD cells that is the major pathway for NaCl entry into these cells. 5. Other work identified a basolateral chloride channel that is presumed to be responsible for changes in VBL during alterations in luminal [NaCl]. This channel, which is the predominant conductance across the basolateral membrane, may be regulated by intracellular Ca2+ and cAMP. 6. An apical Na+:H+ exchanger in MD cells was detected by measuring changes in intracellular pH using the fluorescent probe 2',7'-bis-(2-carboxyethyl)-5(and-6) carboxyfluorescein. 7. Using patch-clamp techniques, a high density of pH- and Ca(2+)-sensitive K+ channels was observed at the apical membrane of MD cells. 8. Other studies found that, at the normal physiological conditions prevailing at the end of the TAL (luminal [NaCl] of 20-60 mmol/L), reabsorption mediated by MD cells is very sensitive to changes in luminal [NaCl].

Cell swelling activates the K+ conductance and inhibits the Cl- conductance of the basolateral membrane of cells from a leaky epithelium

Necturus gallbladder epithelial cells bathed in 10 mM HCO3/1% CO2 display sizable basolateral membrane conductances for Cl- (GClb) and K+ (GKb). Lowering the osmolality of the apical bathing solution hyperpolarized both apical and basolateral membranes and increased the K+/Cl- selectivity of the basolateral membrane. Hyperosmotic solutions had the opposite effects. Intracellular free-calcium concentration ([Ca2+]i) increased transiently during hyposmotic swelling (peak at approximately 30 s, return to baseline within approximately 90 s), but chelation of cell Ca2+ did not prevent the membrane hyperpolarization elicited by the hyposmotic solution. Cable analysis experiments showed that the electrical resistance of the basolateral membrane decreased during hyposmotic swelling and increased during hyperosmotic shrinkage, whereas the apical membrane resistance was unchanged in hyposmotic solution and decreased in hyperosmotic solution. We assessed changes in cell volume in the epithelium by measuring changes in the intracellular concentration of an impermeant cation (tetramethylammonium), and in isolated polarized cells measuring changes in intracellular calcein fluorescence, and observed that these epithelial cells do not undergo measurable volume regulation over 10-12 min after osmotic swelling. Depolarization of the basolateral membrane voltage (Vcs) produced a significant increase in the change in Vcs elicited by lowering basolateral solution [Cl-], whereas hyperpolarization of Vcs had the opposite effect. These results suggest that: (a) Hyposmotic swelling increases GKb and decreases GClb. These two effects appear to be linked, i.e., the increase in GKb produces membrane hyperpolarization, which in turn reduces GClb. (b) Hyperosmotic shrinkage has the opposite effects on GKb and GClb. (c) Cell swelling causes a transient increase in [Ca2+]i, but this response may not be necessary for the increase in GKb during cell swelling.

Role of the NHE3 isoform of the Na+/H+ exchanger in sodium absorption by the rabbit gallbladder

The absorption of water and electrolytes by the gallbladder seems to be largely dependent upon a Na+/H+ exchange at the apical membrane of the gallbladder epithelium. To find out if the exchanger involved is the NHE3 isoform, as in other absorbing epithelia, two studies were performed using the rabbit gallbladder. First, we studied 22Na absorption in Ussing chambers with Krebs buffer as a control solution, and in the presence of amiloride (100, 200 or 1000 microM), ethyl-isopropyl-amiloride (EIPA, 1 or 5 microM), or the phorbol ester, phorbol 12-myristate 13-acetate (PMA, 1 microM). A net mucosal-to-serosal Na+ flux was observed with control buffer. No inhibition of this net flux was observed with 5 microM EIPA, and the IC50 for amiloride was found to be 200 microM. PMA induced a reduction of absorption by 30% that was prevented by incubation with calphostin C. Resistance to amiloride and EIPA, and inhibition by PMA are consistent with the involvement of the NHE3 isoform. The second study involved reverse-transcriptase polymerase chain reaction (RT-PCR) of total gallbladder RNA, with two primers designed to amplify a 645-base-pair fragment from NHE3 mRNA. A cDNA fragment of the expected size was actually obtained from gallbladder RNA, while RT-PCR of RNA from the liver, which does not contain NHE3, gave negative results. A sequence of 492 nucleotides of the amplified product was determined, which was almost superimposable onto the known sequence of the corresponding fragment of rabbit NHE3. It is concluded that, in rabbit gallbladder, neutral NaCl absorption is, at least in part, dependent on the NHE3 isoform of the Na+/H+ exchanger.