RNA was isolated from brains of 16-d-old rats and poly(A) samples were injected into stage V and VI oocytes. After allowing 2-5 d for expression, most oocytes were exposed to medium in which the K had been replaced by Cs for 24 hr prior to recording. Ba currents were usually measured in Cl-free Ba-methanesulfonate saline. IBa in noninjected oocytes was often undetectable, but ranged up to 50 nA (22 +/- 4 nA, n = 21). In contrast, injected oocytes showed a peak IBa of 339 +/- 42 nA (n = 33). The threshold for activation of IBa was -40 mV, with peak currents at +10 to +20 mV. After a peak, currents decayed to a nearly steady level along a single-exponential time course (tau = 650 +/- 50 msec at +20 mV). The maintained current was 67 +/- 6% (n = 9) of the early peak amplitude. A prepulse duration of 5 sec was needed to examine the inactivation of barium currents in injected oocytes. The inward IBa could be observed in BaCl2 solutions at potentials positive to ECl and also in Na-free salines, indicating that neither Cl- nor Na+ was carrying the inward current. Although IBa displayed voltage-independent blockade by Cd (50% inhibition at 6 microM), the peptide Ca channel antagonist, omega-CgTX (1 microM), and the organic Ca channel-blocking agents (verapamil, compound W-7, and nifedipine) were uniformly ineffective. No effects were observed with the dihydropyridine antagonist nifedipine (even at 10 microM, or when cells were held at -40 mV) or agonist Bay K-8644. However, IBa was enhanced via activation of protein kinase C with 4-beta-phorbol dibutyrate (PBT2). In contrast, use of forskolin to activate protein kinase A did not alter IBa.(ABSTRACT TRUNCATED AT 250 WORDS)