PMID: 7542710May 1, 1995Paper

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

The Journal of Physiology
R O ArellanoR Miledi

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 ...Continue Reading

Citations

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