A MONOVALENT CATIONIC CONDUCTANCE THAT IS BLOCKED BY EXTRACELLULAR DIVALENT-CATIONS IN XENOPUS OOCYTES

1. Native Xenopus oocytes were voltage clamped and exposed to Ringer s olutions containing low concentrations of divalent cations. Oocytes, h eld at -60 mV, developed a reversible non-inactivating smooth inward c urrent (I-c) associated with an increase in membrane conductance. 2. I ,was selectively carried by cations (Na+, K+), indicating that the cur rent 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 s pecific population of endogenous ionic channels located in the oocyte membrane. 3. The blocking effects of Ca2+ and Mg2+ were voltage depend ent, implying action at a binding site within the pore of the cationic channel. For example, the half-maximal inhibition (IC50) of I-c by Ca 2+ was 61 mu M in oocytes held at -60 mV and 212 mu M in oocytes held at 0 mV. 4. The I-c channels could be unblocked by depolarization of t he membrane even in the presence of physiological concentrations of Ca 2+ or Mg2+. The unblocking of the channels was observed as a slowly de veloping outward current. 5. The novel cationic current was substantia lly reduced following in vitro maturation of oocytes by treatment with progesterone (10 mu M, 4-5 h). 6. The physiological role of I-c chann els remains to be elucidated. Nonetheless, their characteristics expla in the ionic basis of the sensitivity of oocytes to reductions in extr acellular divalent cations and raise the possibility that the channels play a role in calcium homeostasis.