Orexin-B Augments Voltage-Gated L-Type Ca ²⁺ Current via Protein Kinase C-Mediated Signalling Pathway in Ovine Somatotropes

Abstract

Orexins, orexigenic neuropeptides, are secreted from lateral hypothalamus and orexin receptors are expressed in the pituitary. Since growth hormone (GH) secreted from pituitary is integrally linked to energy homeostasis and metabolism, we studied the effect of orexin-B on voltage-gated Ca²⁺ currents and the related signalling mechanisms in primary cultured ovine somatotropes using whole-cell patch-clamp techniques. With a bath solution containing TEA-Cl (40 m M) and Tetrodotoxin (TTX) (1 µ M), three subtypes of Ca²⁺ currents, namely the long-lasting (L), transient (T), and N currents, were isolated using different holding potentials (–80 and –30 mV) in combination with specific Ca²⁺ channel blockers (nifedipine and ω-conotoxin). About 75% of the total current amplitude was contributed by the L current, whereas the N and T currents accounted for the rest. Orexin-B (1–100 n M) dose-dependently and reversibly increased only the L current up to approximately 125% of the control value within 4–5 min. Neither a specific protein kinase A (PKA) blocker (H89, 1 µ M) nor an inhibitory peptide (PKI, 10 µ M) had any effect on the increase in L current by orexin-B. The orexin-B-induced increase in the L current was abolished by concurrent treatment with calphostin C (Cal-C, 100 n M), protein kinase C (PKC) inhibitory peptide (PKC_19–36, 1 µ M), or by pretreatment with phorbol-12,13-dibutyrate (PDBu) (0.5 µ M) for 16 h (a downregulator of PKC). Orexin-B also increased in vitro GH secretion in a dose-dependent manner. We conclude that orexin-B increases the L-type Ca²⁺ current and GH secretion through orexin receptors and PKC-mediated signalling pathways in ovine somatotropes.