Ca²⁺ channels in clonal rat anterior pituitary cells (GH₃/B₆)

Abstract.

In clonal rat somatomammotroph cells (GH₃/B₆) Ca²⁺ influx through voltage-dependent Ca²⁺ channels is important for regulating the Ca²⁺ concentration that mediates hormone secretion. To study the Ca²⁺ channel subtypes in GH₃/B₆ cells, Ca²⁺ channel currents were recorded with the whole-cell configuration of the patch-clamp technique using Ba²⁺ as the charge carrier. Forty-nine percent of the total Ba²⁺ current amplitude was mediated by a nifedipine-sensitive current (L-type). In addition, three other high-voltage-activated Ca²⁺ channel current components could be distinguished pharmacologically: 10 nM ω-agatoxin-IVA-sensitive current (22%; P-type), ω-conotoxin-MVIIC-sensitive current (18%; Q-type), and toxin-resistant current (24%). Since ω-conotoxin GVIA (2 µM) had no blocking effect, N-type Ca²⁺ channels are assumed not to be present in GH₃/B₆ cells. The T-type Ca²⁺ channel current was either absent or very small. Different pore-forming α₁ subunits of Ca²⁺ channels were found to be expressed in GH₃/B₆ cells, which could be the molecular correlates of the different Ba²⁺ current subtypes: α_1G of T-type, α_1C, α_1D and α_1S of L-type, and α_1A of P/Q-type current. In addition, transcripts for β1, β2 and β3 subunits were detected. Blockage of L-type channels with 10 µM nifedipine or P/Q-type channels with 10 nM ω-agatoxin MVIIC + 200 nM ω-conotoxin blocked action potential firing in GH₃/B₆ cells and decreased basal prolactin secretion.