Cyclic AMP stimulates luteinizing-hormone (lutropin) exocytosis in permeabilized sheep anterior-pituitary cells. Synergism with protein kinase C and calcium.

Journal article

Sheep anterior-pituitary cells permeabilized with Staphylococcus aureus alpha-toxin were used to investigate the role of cyclic AMP (cAMP) in exocytosis of luteinizing hormone (lutropin, LH) under conditions where the intracellular free Ca2+ concentration ([Ca2+]free) is clamped by Ca2+ buffers. At resting [Ca2+]free (pCa 7), cAMP rapidly stimulated LH exocytosis (within 5 min) and continued to stimulate exocytosis for at least 30 min. When cAMP breakdown was inhibited by 3-isobutyl-1-methylxanthine (IBMX), the concentration giving half-maximal response (EC50) for cAMP-stimulated exocytosis was 10 microM. cAMP-stimulated exocytosis required millimolar concentrations of MgATP, as has been found with Ca2(+)- and phorbol-ester-stimulated LH exocytosis. cAMP caused a modest enhancement of Ca2(+)-stimulated LH exocytosis by decreasing in the EC50 for Ca2+ from pCa 5.6 to pCa 5.9, but had little effect on the maximal LH response to Ca2+. Activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate (PMA) dramatically enhanced cAMP-stimulated LH exocytosis by both increasing the maximal effect 5-7-fold and decreasing the EC50 for cAMP to 3 microM. This synergism between cAMP and PMA was further augmented by increasing the [Ca2+]free. Gonadotropin-releasing hormone (gonadoliberin, GnRH) stimulated cAMP production in intact pituitary cells. Since GnRH stimulation is reported to activate PKC and increase the intracellular [Ca2+]free, our results suggest that a synergistic interaction of the cAMP, PKC and Ca2+ second-messenger systems is of importance in the mechanism of GnRH-stimulated LH exocytosis.

Authors: Macrae, M; Davidson, J; Millar, R; Van Der Merwe, P

• Journal: Biochemical journal
• Volume: 271
• Issue: 3
• Pages: 635-639
• Publication date: 1990-11-01
• EISSN: 1470-8728
• ISSN: 0264-6021
• Language: English
• Keywords: Sheep; Exocytosis; Protein Kinase C; Pituitary Gland, Anterior; Cyclic AMP; Calcium; Tetradecanoylphorbol Acetate; Cell Membrane Permeability; Adenosine Triphosphate; Intracellular Fluid; Drug Synergism; 1-Methyl-3-isobutylxanthine; Stimulation, Chemical; Animals; Gonadotropin-Releasing Hormone; Luteinizing Hormone