Correlation between loss of a Mg2+ conductance and an adaptation defect in a mutant of Paramecium tetraurelia.
Paramecium tetraurelia responds to chronic KCl-induced depolarization by swimming backward, but the ciliate recovers within seconds and then undergoes a prolonged adaptation period during which sensitivity to external stimuli is altered radically. We examined the role of Mg2+ in this phenomenon, prompted by finding that mutations in the eccentric-A gene both suppressed a Mg(2+)-specific conductance and prevented adaptation. Adaptation of the wild type proceeded normally when extracellular Mg2+ was varied from 0-20 mM, however, suggesting that channel-mediated Mg2+ fluxes were not involved. In seeking alternative explanations for the eccentric mutant phenotype, we ascertained that there was an osmotic component to adaptation but that K(+)-induced depolarization was the primary stimulus. We also noted that wild-type and eccentric mutant cells depolarized by equivalent amounts in KCl, suggesting that the genetic lesion must lie downstream of membrane-potential change. We also examined whether the adaptation-induced behavioral changes and, indeed, the defect in eccentric might be explained in terms of Mg2+ and Na+ efflux during behavioral testing, but experimental observations failed to support this notion. Finally, we consider the possibility that eccentric gene mutation prevents adaptation by interfering with intracellular free Mg2+ homeostasis in Paramecium.
Preston
R R
RR
Department of Physiology, MCP Hahnemann University, Philadelphia, Pennsylvania 19129, USA. preston@mcphu.edu
Hammond
J A
JA
D000222:Adaptation, Physiological / Q000235:genetics*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004553:Electric Conductivity; D017125:Genes, Protozoan; D007473:Ion Channels / Q000502:physiology; D008274:Magnesium / Q000378:metabolism*; D008564:Membrane Potentials; D009154:Mutation; D009994:Osmolar Concentration; D010247:Paramecium / Q000235:genetics / Q000502:physiology*; D011189:Potassium Chloride / Q000494:pharmacology
D007473:Ion Channels; D011189:Potassium Chloride; D008274:Magnesium; D002118:Calcium