Effects of Leptin on Intracellular Calcium Concentrations in Isolated Porcine Somatotropes

Abstract

Leptin, the product of the obese gene, is a protein that is secreted primarily from adipocytes. Leptin can influence the function of the pituitary gland through its action on the hypothalamus, but it can also directly act at the level of the pituitary gland. The ability of leptin to induce an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in somatotropes was examined in dispersed porcine pituitary cells using a calcium imaging system. Somatotropes were functionally identified by the application of human growth hormone releasing hormone. Leptin increased [Ca²⁺]_i in porcine somatotropes in a dose-dependent manner. The application of 100 n M leptin for 3 min did not have a significant effect on [Ca²⁺]_i, while a 3-min application of 1 µ M leptin increased [Ca²⁺]_i in about 50% of the somatotropes (p < 0.01). The application of a second leptin challenge (1 µ M) evoked a response in only 18% of the observed somatotropes. The stimulatory effect of leptin was abolished in low calcium saline and blocked by nifedipine, an L-calcium channel blocker, suggesting an involvement of calcium channels. Pretreatment of the cultures with AG 490, a specific Janus kinase inhibitor, and with SB 203580, a mitogen-activated protein kinase (MAP kinase) inhibitor, abolished the increase in [Ca²⁺]_ievoked by leptin. In the presence of N^ω-nitro- L-arginine methyl ester ( L-NAME), a nitric oxide synthase (NOS) inhibitor, the magnitude of the increase in [Ca²⁺]_i evoked by 1 µ M leptin was not significantly changed. However, in the presence of L-NAME only 24% of the somatotropes responded to leptin, while in parallel control cultures 70% of the somatotropes responded to leptin. These results imply an involvement of Janus kinase/signal transducer and activator or transcription, MAP kinase and NOS-signaling pathways in the stimulatory effect of leptin on porcine somatotropes.