Membrane currents in immature oocytes of the Rana perezi frog

Immature oocytes of the Rana perezi frog were studied electrophysiologically to see if some of the unusual ionic channels found in Xenopus oocytes were also expressed in these cells. Growing oocytes showed a fairly linear current/voltage relationship (from -200 to +60 mV), whereas fully grown cells had several voltage-dependent conductances. Depolarizing pulses elicited a potassium current blocked by tetraethylammonium (TEA) and two kinetically different Ca2+-dependent Cl- currents (ICl(Ca)), both sensitive to niflumic acid. ICl(Ca), which have not been previously observed in Rana immature oocytes, were also found in response to acetylcholine or rabbit serum superfusion or intracellular injection of Ca2+. In addition, three different Cl- currents were activated in these cells by hyperpolarization: (1) a transient inward current dependent on a critical intracellular Ca2+ concentration; (2) an inward rectifier Cl- current, which was Ca2+ independent; and (3) a high threshold (over -140 mV), slow Cl- current, blocked by several divalent cations, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetamido-4-isothiocyanatostilbene-2, 2'-disulphonic acid (SITS). The presence of most of these infrequent currents in immature oocytes of several frogs and toads suggests that they are not merely the result of random genomic expression but a programmed decision, probably related to a definite functional role.