Specific inhibition of the Na(+)-driven flagellar motors of alkalophilic Bacillus strains by the amiloride analog phenamil.

Amiloride, a specific inhibitor for the Na(+)-driven flagellar motors of alkalophilic Bacillus strains, was found to cause growth inhibition; therefore, the use of amiloride for the isolation of motility mutants was difficult. On the other hand, phenamil, an amiloride analog, inhibited motor rotation without affecting cell growth. A concentration of 50 microM phenamil completely inhibited the motility of strain RA-1 but showed no effect on the membrane potential, the intracellular pH, or Na(+)-coupled amino acid transport, which was consistent with the fact that there was no effect on cell growth. Kinetic analysis of the inhibition of motility by phenamil indicated that the inhibition was noncompetitive with Na+ in the medium. A motility mutant was isolated as a swarmer on a swarm agar plate containing 50 microM phenamil. The motility of the mutant showed an increased resistance to phenamil but normal sensitivity to amiloride. These results suggest that phenamil and amiloride interact at different sites on the motor. By examining various bacterial species, phenamil was found to be a specific and potent inhibitor for the Na(+)-driven flaggellar motors not only in various strains of alkalophilic Bacillus spp. but also in a marine Vibrio sp.     

Thrombin signal transduction mechanisms in rat vascular smooth muscle cells. Calcium and protein kinase C-dependent and -independent pathways.

Sustained generation of alpha-thrombin and its breakdown forms at sites of thromboses has focused attention on the roles thrombin may play in vascular responses to thrombosis and injury. We have previously shown that alpha-thrombin stimulates many growth signals in cultured rat aortic smooth muscle cells (VSMC). To characterize thrombin growth mechanisms, we studied the effects on cultured VSMC of gamma-thrombin (catalytically active with obstructed anion-binding site required for clotting activity) and D-phenylalanyl-L-prolyl-L-arginine chloromethylketone-alpha-thrombin (catalytically inactive with intact anion-binding exosite) on cultured VSMC. Either derivative alone failed to increase growth, but in combination at 130 nM each, they caused a 75 +/- 5% increase in protein synthesis, similar to that observed with alpha-thrombin. This increase in protein synthesis was related to activation of protein kinase C (PKC) and Na+/H+ exchange, because only in combination could the derivatives increase phosphorylation of a 76,000-dalton PKC substrate and alkalinize the cells. Activation of PKC was correlated with a synergistic effect of the derivatives on diacylglycerol formation at 2 min (maximum, 55 +/- 1% combined increase vs. 24 +/- 9% and 4 +/- 4% individual increases with gamma- and D-phenylalanyl-L-prolyl-L-arginine chloromethylketone-alpha-thrombin alone, respectively, p less than 0.05). The derivatives stimulated PKC without increasing inositol trisphosphate, intracellular Ca2+, or expression of the protooncogene, c-fos. Thus, thrombin stimulation of Na+/H+ exchange, diacylglycerol formation, and growth of VSMC can be distinguished from thrombin mobilization of [Ca2+]i and induction of c-fos mRNA. These data indicate the presence of more than one mechanism for thrombin-mediated signaling events in cultured VSMC. Our results also suggest that various thrombin forms retained in clots may have significant effects on VSMC growth and function.     

Refined estimation of kinetic parameters of the Na+/H+ antiport in human fibroblasts and platelets

A technique is presented to estimate the initial rates of Na(+)-dependent alkalinization of acidified human fibroblasts and platelets and assess the kinetics of the Na+/H+ antiport in these cells. Cytosolic pH (pHi) exhibits an exponential recovery following cellular acidification. Thus, the length of the time interval selected to monitor changes in pHi (delta pHi) is critical to estimating the kinetics of the Na+/H+ antiport. We compared kinetic parameters of the Na+/H+ antiport, using computed and observed changes in delta pHi, for arbitrarily selected time intervals following Na(+)-dependent activation. In both cells, significant increases in both the [Na+] for half-maximal activation (K0.5) and maximal velocities (Vmax) were observed as delta pHi was decreased. We conclude that kinetic parameters derived from initial rate determinations enable a more accurate characterization of the Na+/H+ antiport.     

3H]phenamil, a radiolabelled diuretic for the analysis of the amiloride-sensitive Na+ channels in kidney membranes

The interaction of amiloride and amiloride derivatives with the Na+ channels of pig kidney membranes was studied from 22Na+ uptake experiments. The order of potency of the different molecules tested is: phenamil greater than benzamil greater than amiloride, ethylisopropylamiloride. [3H]labelled phenamil was prepared and used to titrate Na+ channels in pig kidney membranes. Kinetics experiments, equilibrium binding studies and competition experiments between [3H]phenamil and unlabelled phenamil indicate that phenamil recognizes a single family of binding sites with a Kd value of 20 nM and a maximum binding capacity of 11.5 pmol/mg of protein. The order of potency of different amiloride analogs tested in [3H]phenamil competition experiments is identical to that found for the inhibition of 22Na+ uptake by apical Na+ channels.     

Angiotensin II effect on cytosolic pH in cultured rat vascular smooth muscle cells

This study investigated fluctuations of cytosolic pH (pHi) of cultured rat vascular smooth muscle cells (VSMCs) in reaction to metabolic alterations induced by angiotensin II (AII). Serially passed VSMCs from Wistar rat aortae were grown on coverslips and loaded with the pH-sensitive fluorescent indicator 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein. A biphasic reaction was seen after exposure of these cells to AII (1 nM to 1 microM); an initial and relatively brief phase of acidification was followed by sustained alkalinization. The rate of acidification and magnitude of alkalinization were dose-dependent. This biphasic effect of AII was also demonstrated in Ca2+-free medium and was mimicked by subjecting VSMCs to the calcium ionophore A23187 (5 microM) in Ca2+-containing medium but not in Ca2+-free medium. Verapamil (10 microM) almost entirely eliminated the AII-induced acidification, whereas amiloride analogues 5-(N-methyl-N-isobutyl)amiloride and 5-(N-ethyl-N-isopropyl)amiloride (100 microM) as well as Na+-deficient medium abolished the subsequent (alkalinization) phase produced by the hormone. Activation of the Na+/H+ antiport by subjecting VSMCs to phorbol 12-myristate 13-acetate (100 nM) prevented a subsequent effect of AII on the pHi profile. This resistance to a further action of the hormone was not mediated via cytoplasmic alkalinization. AII produced a dramatic redistribution in the cellular compartments of 45Ca2+ associated with accelerated 45Ca2+ washout. These findings suggest that the AII-induced acidification phase may relate to activation of the Ca2+ pump (Ca2+/H+ exchange) and that this process can take place in the presence and absence of extracellular Ca2+. The alkalinization phase is the consequence of stimulation of the Na+/H+ antiport, which in cultured VSMCs can be activated by a rise in cytosolic free Ca2+ as well as other mechanisms.     

Intracellular accumulation of potent amiloride analogues by human neutrophils

The mechanism of uptake of a series of amiloride derivatives by human neutrophils was investigated using [14C]amiloride and the 14C-labeled 5-(1-hexahydroazepinyl)-6-bromo analogue (BrMM) which is approximately 500-fold more potent than the parent compound at inhibiting Na+/H+ exchange. At an external concentration of 2 microM, the influx of BrMM at 37 degrees C was rapid, reaching a steady state by approximately 20 min. The rate of BrMM uptake (approximately 25 mumol/liter.min) was approximately 90-fold faster than for the same concentration of amiloride, a finding which correlates with differences in lipid partitioning of the two compounds. Uptake was unrelated to specific binding to Na+/H+ exchange transport sites: influx of either drug was nonsaturable whereas amiloride- and BrMM-mediated inhibition of Na+/H+ countertransport obeyed Michaelis-Menten kinetics with apparent Ki values of approximately 75 and approximately 0.2 microM. Entry occurred exclusively via the neutral (uncharged) forms (pK'a 8.40-8.55). Influx was markedly pH-dependent: it was enhanced by extracellular alkalinization and reduced by acidification. Influx was, however, insensitive to large changes in membrane voltage, thereby implying the protonated (charged) species to be impermeant. About 75% of the total intracellular pool of amiloride, but only approximately 25% of BrMM, is contained within the lysosomes, an expected consequence of the partitioning and subsequent trapping of a weak base within this strongly acidic subcellular compartment. With BrMM, there was a relative approximately 60-fold enrichment in the internal/external water concentration ratio of the drug; the value for amiloride was much less, approximately 4. This disparity is consistent with substantial binding of BrMM to internal constituents, presumably to proteins and/or nucleic acids. Thus, it is important to recognize that potentially large intracellular accumulations of potent analogues can occur that are not directly involved in inhibition of Na+/H+ exchange. These findings sound a cautionary note in the interpretation of results using these drugs in all cells, especially those of small size with high surface-to-volume ratios.     

Recombinant human interleukin 1-stimulated Na+/H+ exchange is not required for differentiation in pre-B lymphocyte cell line, 70Z/3

Interleukin-1 a polypeptide hormone produced by activated macrophages is a mixture of at least two proteins, interleukin-1 alpha (IL-1 alpha) and interleukin-1 beta (IL-1 beta). We have previously shown that macrophage-derived interleukin-1 induced new kappa light chain synthesis for surface IgM expression in a murine pre-B like cell line 70Z/3, a finding associated with an early amiloride-sensitive rise in the total intracellular sodium concentration. Because IL-1 alpha and IL-1 beta are structurally quite different, in this study their effect on 70Z/3 was examined separately. The results show that both human rIL-1 alpha and rIL-1 beta induce the differentiation of 70Z/3, but a higher concentration of rIL-1 beta compared to rIL-1 alpha is needed for a maximal response. At saturating concentrations, both rIL-1 alpha and rIL-1 beta induce a simultaneous rise in intracellular pH and sodium concentration. Because rIL-1 mediated intracellular alkalinization and sodium rise are amiloride sensitive, they likely occur through stimulation of the Na+/H+ exchanger across the cell membrane. Inhibition of the Na+/H+ antiport with an amiloride analog did not have an effect on rIL-1 induced surface IgM expression or the rIL-1-mediated increase in kappa light chain specific mRNA level. Therefore, these results indicate that an increase in pHi or [Na]i is not required for IL-1 induced 70Z/3 differentiation.     

Evidence that Na+/H+ exchange regulates receptor-mediated phospholipase A2 activation in human platelets

Data in the previous paper suggest that epinephrine can mobilize a small pool of arachidonic acid via an enzymatic pathway distinct from phospholipase C and that this pathway is blocked by perturbations that block Na+/H+ exchange. The present studies demonstrate that epinephrine and ADP stimulate a phosphatidylinositol-hydrolyzing phospholipase A2 activity in human platelets. This occurs even when measurable phospholipase C activation, platelet secretion, and secondary aggregation are blocked with the thromboxane A2 receptor antagonist SQ29548. Furthermore, perturbants of Na+/H+ exchange diminish lysophosphatidylinositol production in response to epinephrine, ADP, and thrombin, but not to the Ca2+ ionophore A23187. Artificial alkalinization of the platelet interior with methylamine reverses the effect of the Na+/H+ antiporter inhibitor, ethylisopropylamiloride, on thrombin-stimulated lysolipid production, suggesting that the alkalinization of the platelet interior which would occur secondary to activation of Na+/H+ exchange might play an important role in phospholipase A2 activation. In addition, treatment of platelets with methylamine increases the sensitivity of phospholipase A2 to activation by the Ca2+ ionophore A23187, suggesting that changes in pH and Ca2+ may regulate phospholipase A2 activity synergistically. Finally, epinephrine causes a prompt decrease in platelet-chlortetracyclin fluorescence even in the presence of cyclooxygenase inhibitors, suggesting that epinephrine is able to mobilize membrane-bound Ca2+ independent of phospholipase C activation. Taken together, the data suggest that epinephrine-provoked stimulation of phospholipase A2 activity may occur as a result of Ca2+ mobilization and a concomitant intraplatelet alkalinization resulting from accelerated Na+/H+ exchange.     

Blastocoel expansion in the preimplantation mouse embryo: role of extracellular sodium and chloride and possible apical routes of their entry

The trophectoderm of the mouse blastocyst is a fluid transporting epithelium that is responsible for generating a fluid-filled cavity called the blastocoel. Vectorial transport of ions from the medium into the blastocoel generates an osmotic gradient that drives fluid across this epithelium. We report here that substitution of Na+ or Cl-, but not K+, in the medium halves the rate of blastocoel expansion in the mouse blastocyst. Entrance of Na+ into the trophectoderm may involve several routes, since both blastocoel expansion and 22Na+ uptake are decreased in the presence of the highly specific Na+/H+ exchanger inhibitor, 5-(N-ethyl-N-isopropyl)amiloride, and to a lesser extent with the amiloride-sensitive Na+-channel blocker, benzamil. Uptake of 22Na+ manifests saturation kinetics as a function of extracellular Na+ concentration, whereas uptake of 36Cl- is linear. Furthermore, neither 4,4-diisothiocyanostilbene-2,2-disulfonic acid, which is an inhibitor of the Cl-/HCO3- exchanger, nor 2-(3,4-dichlorobenzyl)-5-nitrobenzoic acid, which is a Cl- -channel blocker, affect either blastocoel expansion or 36Cl- uptake. These results suggest that Na+ entry into the mouse blastocyst is carrier-mediated and probably involves several routes that include the Na+/H+ exchanger and possibly the Na+-channel. Chloride entry, however, may not be carrier-mediated and may occur through a paracellular route, i.e., between the trophectodermal cells.