Membrane currents elicited by divalent cations in Xenopus oocytes.

Abstract

1. Membrane currents were recorded from voltage-clamped Xenopus oocytes in response to bath application of various divalent cations. 2. In oocytes from 93 of 160 frogs tested, Co2+ ions evoked slow, oscillatory membrane currents. Sensitivity to Co2+ varied greatly between oocytes from different frogs, but was relatively consistent for oocytes taken from the same ovary. Oocytes with high sensitivity had response threshold of 5-10 .mu.M, and gave currents > 1 .mu.A to 1 mM-CoCl2. In contrast, oocytes form some frogs gave no oscillatory response even to 10 mM-CoCl2. With responsive oocytes, Cd2+, Ni2+, Zn2+ Mn2+ and Cr2+ ions (5 .mu.M to 1 mM) also elicited oscillations, whereas Sr2+, Ba2+ and Ca2+ (0.1-10 mM) showed very little activity, and Mg2+ ions, none. 3. Responses to divalent cation were well preserved in defolliculated oocytes, indicating they were generated in the oocyte membrane itself, and were not dependent on the presence of enveloping follicular cells. 4. The oscillatory currents reversed around -20 mV (the chloride equilibrium potential) and rectified strongly at potentials more negative than about -60 mV. The oscillations were mimicked by intraoocyte injection of inositol 1,4,5-triphosphate (IP3), were largely preserved after removal of external Ca2+, but were abolished following chelation of intracellular Ca2+ by EGTA. Intraoocyte injection of Co2+ ions failed to generate oscillatory currents. 5. Currents elicited by divalent cations resembled the oocyte''s oscillatory responses to acetylcholine and a serum protein. However, the response to divalent cations was not blocked by atropine and furthermore, the relative sensitivities to these agonists varied independently between oocytes from different frogs. 6. We conclude that extracellular Cd2+, Ni2+, Zn2+, Co2+ Mn2+ and Cr2+ interact with the oocyte surface to raise cytosolic levels of inositol phosphates. This causes mobilization of intracellular Ca2+, in turn activating Ca2+-gated Cl- channels in the oocyte membrane. 7. In addition to the large oscillatory currents, divalent cations generated small (5-50 nA), smooth, maintained currents associated with decreases in membrane conductance. The size and ionic basis of these currents varied between oocytes from different frogs. 8. Zinc ions also elicited smooth currents, associated with an increase in membrane conductance, and carried predominantly by K+. This response was specific to Zn2+ and occurred independently of oscillatory Cl- currents. The K+ current was abolished by defolliculation, was potentiated by the cyclic AMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, and showed facilitation with K+ currents generated by the adenylate cyclase activator forskolin. All this suggests that Zn2+ ions interact with a cyclic nucleotide-activated K+ gating mechanism which appears to be located in follicular cells.