title: Amiloride-sensitive sodium transport in lamprey red blood cells: evidence for two distinct transport pathways.
abstract: To determine Na+/H+ exchange in lamprey erythrocyte membranes, the cells were acidified to pH(i) 6.0 using the K+/H+ ionophore nigericin. Incubation of acidified erythrocytes in a NaCl medium at pH 8.0 caused a considerable rise in 22Na+ influx and H+ efflux during the first 1 min of exposure. In addition, exposure of acidified red cells to NaCl medium was associated with rapid elevation of intracellular Na+ content. The acid-induced changes in Na+ influx and H+ efflux were almost completely inhibited by amiloride and dimethylamiloride. In native lamprey erythrocytes, amiloride-sensitive Na+ influx progressively increased as the osmolality of incubation medium was increased by addition of 100, 200, or 300 mmol/l sucrose. Unexpectedly, the hypertonic stress induced a small, yet statistically significant decrease in intracellular Na+ content in these cells. The reduction in the cellular Na+ content increased with hypertonicity of the medium. The acid- and shrinkage-induced Na+ influxes were inhibited by both amiloride and 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) in a dose-dependent manner. For both blockers, the half-maximal inhibitory values (IC50) were much greater for the shrinkage-induced (44 and 15 micromol/l for amiloride and EIPA, respectively) than for the acid-induced Na+ influx (5.1 and 3.3 micromol/l, respectively). The data obtained are the first demonstration of the presence of a Na+/H+ exchanger with high activity in acidified (pH(i) 6.0) lamprey red blood cells (on average, 512 +/- 56 mmol/l cells/h, n = 13). The amiloride-sensitive Na+ influxes produced by hypertonic cell shrinkage and acid load are likely to be mediated by distinct ion transporters in these cells.
authors.affiliation: Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, Thorez pr. 44, 194223 St. Petersburg, Russia. gusev@iephb.ru
authors.forename: G P
authors.initials: GP
authors.lastname: Gusev
authors.forename: T I
authors.initials: TI
authors.lastname: Ivanova
mesh_terms: D000584:Amiloride / Q000494:pharmacology*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D048429:Cell Size / Q000187:drug effects; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004912:Erythrocytes / Q000737:chemistry / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D007798:Lampreys / Q000097:blood* / Q000502:physiology*; D009550:Nigericin / Q000494:pharmacology; D011522:Protons; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000502:physiology*; D014882:Water-Electrolyte Balance / Q000187:drug effects / Q000502:physiology
chemical_list: D011522:Protons; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D012964:Sodium; D009550:Nigericin