Inhibition of Mg2+ current by single-gene mutation in Paramecium.

"Eccentric" is a newly-isolated mutant of Paramecium tetraurelia that fails to swim backwards in response to Mg2+. In the wild type, this backward swimming results from Mg2+ influx via a Mg(2+)-specific ion conductance (IMg). Voltage-clamp analysis confirmed that, as suspected, step changes in membrane potential over a physiological range fail to elicit IMg from eccentric. Further electrophysiological investigation revealed a number of additional ion-current defects in eccentric: (i) The Ca2+ current activated upon depolarization inactivates more slowly in eccentric than in the wild type, and it requires longer to recover from this inactivation. (ii) The Ca(2+)-dependent Na+ current deactivates significantly faster in the mutant. (iii) The two K+ currents observed upon hyperpolarization are reduced by > 60% in eccentric. It is difficult to envision how these varied pleiotropic effects could result from loss of a single ion current. Rather, they suggest that the eccentric mutation affects a global regulatory system. Two plausible hypotheses are discussed.