''Eccentric'' is a newly-isolated mutant of Paramecium tetraurelia tha
t fails to swim backwards in response to Mg2+. In the wild type, this
backward swimming results from Mg2+ influx via a Mg2+-specific ion con
ductance (I-Mg). Voltage-clamp analysis confirmed that, as suspected,
step changes in membrane potential over a physiological range fail to
elicit I-Mg from eccentric. Further electrophysiological investigation
revealed a number of additional ion-current defects in eccentric: (i)
The Ca2+ current activated upon depolarization inactivates more slowl
y in eccentric than in the wild type, and it requires longer to recove
r from this inactivation. (ii) The Ca2+-dependent Na+ current deactiva
tes significantly faster in the mutant. (iii) The two K+ currents obse
rved upon hyperpolarization are reduced by >60% in eccentric. It is di
fficult to envision how these varied pleiotropic effects could result
from loss of a single ion current. Rather, they suggest that the eccen
tric mutation affects a global regulatory system. Two plausible hypoth
eses are discussed.