Protein kinase C heterogeneity in GH₄C₁ rat pituitary cells: Characterization of a Ca²⁺-independent phorbol ester receptor

Clonal GH₄C₁ rat pituitary cells are heterogeneous with respect to phorbol dibutyrate receptors (PDBu-R) and protein kinase C (PKC) content. GH cell PDBu-Rs can be separated into two categories based on Ca²⁺-modulation of receptor affinity. Approximately 70% of the cytosolic PDBu-Rs demonstrate Ca²⁺-sensitive receptor affinity and redistribute from the soluble to the particulate fraction in the presence of excess Ca²⁺. The other 30% of the receptors remain in the cytosol in the presence of excess Ca²⁺. Their receptor affinity is Ca²⁺-independent. Northern blot hybridization and immunoblot analysis showed that GH₄C₁ cells express Ca²⁺-independent ∈-PKC as well as Ca²⁺-dependent α-and β-PKCs. Cell lysis in Ca²⁺ caused the redistribution of >95% of α- and β-PKC to the particulate fraction, whereas ∼90% of the ∈-PKC remained in the cytosol. In contrast, brief treatment of GH cell cultures with PDBu or thyrotropin-releasing hormone caused redistribution of all three isozymes. Prolonged treatment with PDBu down-modulated all three isozymes but at different rates and to different extents. In contrast, prolonged thyrotropin-releasing hormone treatment selectively down-modulated ∈-PKC. These results demonstrate that GH cells have both Ca²⁺-sensitive and -insensitive PKCs and PDBu-Rs and that both populations are regulated by agonists that control prolactin synthesis and secretion by these cells.