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 mM) and Tetrodotoxin (TTX) (1 μM), 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 nM) 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 μM) nor an inhibitory peptide (PKI, 10 μM) 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 nM), protein kinase C (PKC) inhibitory peptide (PKC19-36, 1 μM), or by pretreatment with phorbol-12,13-dibutyrate (PDBu) (0.5 μM) 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.
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