A monovalent cationic conductance that is blocked by extracellular divalent cations in Xenopus oocytes.

Abstract

1. Native Xenopus oocytes were voltage clamped and exposed to Ringer solutions containing low concentrations of divalent cations. Oocytes, held at ‐60 mV, developed a reversible non‐inactivating smooth inward current (Ic) associated with an increase in membrane conductance. 2. Ic was selectively carried by cations (Na+, K+), indicating that the current was not the result of a non‐specific membrane breakdown, but was due instead to removal of a blocking effect of divalent cations on a specific population of endogenous ionic channels located in the oocyte membrane. 3. The blocking effects of Ca2+ and Mg2+ were voltage dependent, implying action at a binding site within the pore of the cationic channel. For example, the half‐maximal inhibition (IC50) of Ic by Ca2+ was 61 microM in oocytes held at ‐60 mV and 212 microM in oocytes held at 0 mV. 4. The Ic channels could be unblocked by depolarization of the membrane even in the presence of physiological concentrations of Ca2+ or Mg2+. The unblocking of the channels was observed as a slowly developing outward current. 5. The novel cationic current was substantially reduced following in vitro maturation of oocytes by treatment with progesterone (10 microM, 4‐5 h). 6. The physiological role of Ic channels remains to be elucidated. Nonetheless, their characteristics explain the ionic basis of the sensitivity of oocytes to reductions in extracellular divalent cations and raise the possibility that the channels play a role in calcium homeostasis.

Probing the pore region of recombinant N-methyl-D-aspartate channels using external and internal magnesium block.

“…Ca2+ was eliminated from the extracellular solutions to suppress the contamination of the measured currents by Ca2+-activated endogenous currents (26), which is particularly important for NMDA current measurements at potentials above 0 mV. Mg2+-free solutions (supplemented with 0.1 mM EDTA) and low Mg2+ solutions were only applied briefly because, upon complete removal of divalent cations, a slow but large endogenous cationic current develops in the oocyte (27). To avoid the development of this current, each low Mg2+ measurement was followed by a period of at least 3 min in a solution containing 1 mM Mg2+.…”

Evidence for Activation of Endogenous Transporters in Xenopus laevis Oocytes Expressing the Plasmodium falciparum Chloroquine Resistance Transporter, PfCRT

“…As reported for amino acid transporters, coupling of transport activities may serve to recycle ions or to buffer ionic or V m changes induced by the transporter of interest (61)(62)(63)(64). Alternatively, PfCRT may interfere with second messengers, such as Ca 2ϩ , which is known to regulate the activity of numerous membrane transporters, including the NHE (46,65), G cat (66), and H ϩ -ATPase (67). A function for PfCRT as an activator or modulator of other transporters would be consistent with most models proposed to explain chloroquine resistance in P. falciparum.…”

Channels involved in transient currents unmasked by removal of extracellular calcium in cardiac cells

“…For example, in cardiac and other cells, L-type and T-type Ca 2ϩ channels become highly permeable to monovalent cations when Ca o is decreased or removed (13,19). Finally, Ca o removal can also induce the opening of new channels, apparently not conductive in the presence of divalent cations (2,17,22,28,29,31). Thus the time-dependent, I to -like currents induced upon Ca o removal could be due either to a modified behavior of known channels or to the unmasking of novel ones.…”