Effects of amiloride analogues on adult Notophthalmus viridescens limb stump currents

We have previously investigated the relevance to limb regeneration of epidermally driven, Na+-dependent limb stump currents by blocking epidermal Na+ channels with amiloride, 3,5-diamino-6-chloro-N-(diaminomethylene)pyrazinecarboxamide. In order to reduce Notophthalmus viridescens stump currents more effectively than with amiloride, we have examined six amiloride analogues. Of these, only benzamil, 3,5-diamino-6-chloro-N-[(benzylamino)aminomethylene]pyrazinecarboxamide, was more effective than amiloride. The concentration of benzamil that reduced stump currents to half their initial value was 0.034 microM, while this concentration for amiloride was 0.42 microM. We also found a contribution of calcium ions to these currents. When immersed in water whose Ca2+ concentration decreased stepwise from 1 to 0 mM, stump currents decreased significantly, although to a variable extent, depending on the batch of newts. With 30 microM benzamil and 0.5 mM calcium (in water that also contained 1.5 mM NaCl and 0.06 mM KCl) stump currents could be reduced to very low levels and, in many cases, changed to incurrents.     

Dependence of myocardial damage on the anion composition and osmotic pressure in the extracellular fluid during "calcium paradox" in rats]

The data obtained reveal that elevation of extracellular osmolarity with sucrose during reintroduction of Ca-containing medium after 10 minutes of Ca2+ removal prevents loss of haemoglobin in a concentration-dependent mode. Reducing the extracellular osmolarity of the reperfusion medium by means of decreasing the concentration of sodium chloride and calcium chloride exacerbates the loss of haemoglobin from the cardiomyocytes. There is a close correlation between the water contents in tissues and the loss of haemoglobin during the "calcium paradox". The findings suggest dependence of the heart damage during the "calcium paradox" on anionic composition of extracellular space and activity of anionic transporters.     

Ethylisopropyl-amiloride: a new and highly potent derivative of amiloride for the inhibition of the Na+/H+ exchange system in various cell types

Ethylisopropyl-amiloride is 100 times more potent than amiloride for inhibiting the Na+/H+ exchanger of 3T3 fibroblasts, chick skeletal muscle cells and chick cardiac cells. Half-maximum effects, measured at 3 mM external Na+ are observed at 20-100 nM and 5 microM for ethylisopropyl-amiloride and amiloride respectively. As previously observed for amiloride, the effect of ethylisopropyl-amiloride is antagonized by external Na+ ions.     

IP3-and cAMP-induced responses in isolated olfactory receptor neurons from the channel catfish

Summary Olfactory receptor neurons enzymatically dissociated from channel catfish olfactory epithelium were depolarized transiently following dialysis of IP₃ or cAMP (added to the patch pipette) into the cytoplasm. Voltage and current responses to IP₃ were blocked by ruthenium red, a blocker of an IP₃-gated Ca²⁺-release channel in sarcoplasmic reticulum. In contrast, the responses to cAMP were not blocked by extracellularly applied ruthenium red, nor by l-cis-diltiazem or amiloride and two of its derivatives. The current elicited by cytoplasmic IP₃ in neurons under voltage clamp displayed a voltage dependence different from that of the cAMP response which showed marked outward rectification. A sustained depolarization was caused by increased cytoplasmic IP₃ or cAMP when the buffering capacity for Ca²⁺ of the pipette solution was increased, when extracellular Ca²⁺ was removed or after addition of 20–200 nm charibdotoxin to the bathing solution, indicating that the repolarization was caused by an increase in [Ca _i ] that opened Ca²⁺-activated K⁺ channels. The results suggest that different conductances modulated by either IP₃ or cAMP are involved in mediating olfactory transduction in catfish olfactory receptor neurons and that Ca²⁺-activated K⁺ channels contribute to the termination of the IP₃ and cAMP responses.Abbreviations ATP adenosine 5-triphosphate - BAPTA (bis-(o-aminophenoxy)-ethane-N-N-N-N)-tetraacetic acid - cAMP adenosine cyclic 3,5-monophosphate - cGMP guanosine cyclic 3,5-monophosphate - CTX charybdotoxin - DCB 3,4-dichlorobenzamil - EDTA ethylenediaminetetraacetic acid - EGTA ethylenglycol-bis-(b-aminoethyl)-N-N-N-N-tetraacetic acid - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - IP₃ inositol-1,4,5-triphosphate - NMDG N-methyl-d-glucamine We would like to thank the Tanabe Seiyaku Co., Ltd., for their gift of l-cis-diltiazem. This work was supported by National Institutes of Health grants DC00566 and BRSG S07RR05825.     

Regulation of cytoplasmic pH in rat sublingual mucous acini at rest and during muscarinic stimulation

The regulation of intracellular pH (pHi) in rat sublingual mucous acini was monitored using dual-wavelength microfluorometry of the pH-sensitive dye BCECF (2',7'-biscarboxyethyl-5(6)-carboxyfluorescein). Acini attached to coverslips and continuously superfused with HCO3(-)-containing medium (25 mM NaHCO3/5% CO2; pH 7.4) have a steady-state pHi of 7.25 +/- 0.02. Acid loading of acinar cells using the NH4+/NH3 prepulse technique resulted in a Na(+)-dependent, MIBA-inhibitable (5-(N-methyl-N-isobutyl) amiloride, Ki approximately 0.42 microM) pHi recovery, the kinetics of which were not influenced by the absence of extracellular Cl-. The rate and magnitude of the pHi recovery were dependent on the extracellular Na+ concentration, indicating that Na+/H+ exchange plays a critical role in maintaining pHi above the pH predicted for electrochemical equilibrium. When the NH4+/NH3 concentration was varied, the rate of pHi recovery was enhanced as the extent of the intracellular acidification increased, demonstrating that the activity of the Na+/H+ exchanger is regulated by the concentration of intracellular protons. Switching BCECF-loaded acini to a Cl(-)-free medium did not significantly alter resting pHi, suggesting the absence of Cl-/HCO3- exchange activity. Muscarinic stimulation resulted in a rapid and sustained cytosolic acidification (t 1/2 < 30 sec; 0.16 +/- 0.02 pH unit), the magnitude of which was amplified greater than two-fold in the presence of MIBA (0.37 +/- 0.05 pH unit) or in the absence of extracellular Na+ (0.34 +/- 0.03 pH unit). The agonist-induced intracellular acidification was blunted in HCO3(-)-free media and was inhibited by DPC (diphenylamine-2-carboxylate), an anion channel blocker. In contrast, the acidification was not influenced by removal of extracellular Cl-. The Ca2+ ionophore, ionomycin, mimicked the effects of stimulation, whereas preloading acini with BAPTA (bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetra-acetic acid) to chelate intracellular Ca2+ blocked the agonist-induced cytoplasmic acidification. The above results indicate that during muscarinic stimulation an intracellular acidification occurs which: (i) is partially buffered by increased Na+/H+ exchange activity; (ii) is most likely mediated by HCO3- efflux via an anion channel; and (iii) requires an increase in cytosolic free [Ca2+].     

Extracellular ATP stimulates an amiloride-sensitive sodium influx in human lymphocytes

Extracellular ATP has been shown to increase the Na+ permeability of human lymphocytes by 3 to 12-fold. The kinetics of this ATP-induced response were studied by measuring 22Na+ influx into chronic lymphocytic leukemic lymphocytes incubated in low-sodium media without divalent cations. ATP-stimulated uptake of 22Na-ions was linear over 4 min incubation and this influx component showed a sigmoid dependence on ATP concentration. Hill analysis yielded a K1/2 of 160 microM and a n value of 2.5. The nucleotide ATP-gamma-S (1-2 mM) gave 30% of the permeability increase produced by ATP, but UTP (2 mM) and dTTP (2 mM) had no effect on 22Na influx. The amiloride analogs 5-(N-ethyl-N-isopropyl) amiloride and 5-(N,N-hexamethylene) amiloride, which are potent inhibitors of Na(+)-H+ countertransport, abolished 72-95% of the ATP-stimulated 22Na+ influx. However, the involvement of Na(+)-H+ countertransport in the ATP-stimulated Na+ influx was excluded by three lines of evidence. Sodium influx was stimulated 7-fold by extracellular ATP but only 2.4-fold by hypertonic conditions which are known to activate Na(+)-H+ countertransport. Addition of ATP to lymphocytes produced no change in intracellular pH when these cells were suspended in isotonic NaCl media. Finally ATP caused a membrane depolarization of lymphocytes which is inconsistent with stimulation of electroneutral Na(+)-H+ exchange. These data suggest that ATP acts cooperatively to induce the formation of membrane channels which allow increased Na+ influx by a pathway which is partially inhibited by amiloride and its analogs.     

Stimulation of Ca2+ uptake into epididymal bull spermatozoa by analogues of amiloride

Certain amiloride analogues 3',4'-dichlorobenzamil 2',4'-dimethylbenzamil and alpha',2'-benzobenzamil hydrochloride (ATBB) stimulate calcium accumulation and motility by epididymal bovine spermatozoa. This stimulation can be seen at a range of 0.1-0.4 mM, while at higher concentration there is inhibition of calcium uptake by these amiloride analogues. The amiloride derivative 5-(4-chlorobenzyl)-2',4'-dimethylbenzamil (CBDMB), which bears a 4-chlorobenzyl substituent on the 5-amino nitrogen atom, did not stimulate calcium uptake. The amiloride analogue 3',4'-dichlorobenzamil inhibits the Na+/Ca2(+)-exchange activity in isolated plasma membrane vesicles, and the stimulatory effect of 3',4'-dichlorobenzamil on calcium uptake into epididymal sperm could be seen in Na(+)-free medium. Thus, the stimulation of Ca2+ accumulation in the cells caused by 3',4'-dichlorobenzamil is not a result of inhibiting the Na(+)-dependent Ca2+ clearance. There is no stimulation of Ca2+ uptake into ejaculated cells by adding 3',4'-dichlorobenzamil, which is not due to the presence of the calcium-transport inhibitor (caltrin) in these cells [Rufo, G.A., Schoff, P.K. & Lardy, H.A. (1984) J. Biol. Chem. 259, 2547-2552]. The stimulatory effect of 3',4'-dichlorobenzamil on Ca2+ uptake is inhibited by the voltage-dependent Ca2(+)-channel blockers nifedipin and diltiazem. This indicates that the stimulation of Ca2+ uptake by the amiloride analogues is due to the activation of a voltage-dependent Ca2+ channel of the plasma membrane.     

Nerve growth factor does not activate Na+/H+ exchange in PC12 pheochromocytoma cells

We have reexamined the possible role of the Na+/H+ antiport in the cellular response by PC12 pheochromocytoma cells to nerve growth factor (NGF). In contrast to previous reports, we observe no activation of Na+/H+ exchange in these cells, using a very sensitive assay based on the measurement of cytoplasmic pH with dimethylfluorescein dextran (Rothenberg et al., J. Biol. Chem., 258:4883-4809, 1983). Our measurements indicate that the PC12 pheochromocytoma cells, under all conditions tested, show a high rate of Na+/H+ exchange. The discrepancy between these observations and previous experiments could be due to differences in cells in different laboratories, but also to changes in cell adhesion induced by NGF. We describe conditions where intracellular pH and rates of Na+ uptake can be measured reliably in PC12 cells with adequate controls for cell adhesion. We conclude that activation of Na+/H+ exchange is neither sufficient nor required for the differentiation of PC12 cells induced by NGF.