Calcium current activated upon hyperpolarization of Paramecium tetraurelia.
Hyperpolarization of Paramecium tetraurelia under conditions where K+ currents are suppressed elicits an inward current that activates rapidly toward a peak at 25-80 ms and decays thereafter. This peak current (Ihyp) is not affected by removing Cl ions from the microelectrodes used to clamp membrane potential, or by changing extracellular Cl- concentration, but is lost upon removing extracellular Ca2+. Ihyp is also lost upon replacing extracellular Ca2+ with equimolar concentrations of Ba2+, Co2+, Mg2+, Mn2+, or Sr2+, suggesting that the permeability mechanism that mediates Ihyp is highly selective for Ca2+. Divalent cations also inhibit Ihyp when introduced extracellularly, in a concentration- and voltage-dependent manner. Ba2+ inhibits Ihyp with an apparent dissociation constant of 81 microM at -110 mV, and with an effective valence of 0.42. Ihyp is also inhibited reversibly by amiloride, with a dissociation constant of 0.4 mM. Ihyp is not affected significantly by changes in extracellular Na+, K+, or H+ concentration, or by EGTA injection. Also, it is unaffected by manipulations or mutations that suppress the depolarization-activated Ca2+ current or the various Ca(2+)-dependent currents of Paramecium. We suggest that Ihyp is mediated by a novel, hyperpolarization-activated calcium conductance that is distinct from the one activated by depolarization.
Preston
R R
RR
Laboratory of Molecular Biology, University of Wisconsin-Madison 53706.
Saimi
Y
Y
Kung
C
C
D000818:Animals; D002118:Calcium / Q000502:physiology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D004553:Electric Conductivity; D005071:Evoked Potentials / Q000187:drug effects / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D008839:Microelectrodes; D016805:Paramecium tetraurelia / Q000187:drug effects / Q000502:physiology*
D015220:Calcium Channels; D002118:Calcium