Re-evaluation of the P/Q Ca2+ channel components of Ba2+ currents in bovine chromaffin cells superfused with solutions containing low and high Ba2+ concentrations

 This study was undertaken to reassess the set of voltage-dependent Ca²⁺ channel subtypes expressed by bovine adrenal chromaffin cells maintained in primary cultures. Previous views on the pharmacology of such channels had to be revised in the light of the novel data which arose from the use in this study of low and high micromolar concentrations of ω-agatoxin IVA, and low (2 mM) and high (10 mM) concentrations of the charge carrier Ba²⁺. Whole-cell Ba²⁺ currents (I_Ba) through Ca²⁺ channels were elicited in voltage-clamped chromaffin cells, with a holding potential of –80 mV and depolarising pulses to 0 mV. Mean peak I _Ba was 425 pA in 2 mM Ba²⁺ (59 cells) and 787 pA in 10 mM Ba²⁺ (42 cells). In 2 mM Ba²⁺, ω-conotoxin MVIIC (3 μM) inhibited I _Ba by 79%; in 10 mM Ba²⁺, the blockade developed much more slowly and reached only 44%. A low concentration of ω-agatoxin IVA (20 nM) inhibited I _Ba by 9%; 2 μM inhibited I _Ba by 60%. This blockade was similar in low and high Ba²⁺ concentrations. After giving furnidipine (3 μM) and ω-conotoxin GVIA (1 μM), 2 μM ω-agatoxin IVA inhibited the remaining current (about 40–45%); this blockade was independent of the Ba²⁺ concentration. The current could be fully blocked by the cocktail furnidipine/ω-conotoxin GVIA/high ω-agatoxin IVA, both in low and high Ba²⁺ concentrations. The large Q-type channel component of I _Ba is blocked by micromolar concentrations of ω-agatoxin IVA and ω-conotoxin MVIIC. While solutions with a high Ba²⁺ concentration strongly delayed the development of blockade by ω-conotoxin MVIIC, the blockade by high concentrations of ω-agatoxin IVA was equally effective in solutions with a low or a high Ba²⁺ concentration. Hence, the use of appropriate Ba²⁺ and toxin concentrations in this study reveals that P-type Ca²⁺ channels are poorly expressed in bovine chromaffin cells; in contrast, a robust component of the current depends on Q-type Ca²⁺ channels. An R-type residual current is not present in these cells. Received: 22 April 1996 / Received after revision: 11 June 1990 / Accepted: 11 June 1996