Mechanisms of AVP-induced glucagon release in clonal α-cells In-R1-G9: involvement of Ca2+-dependent and -independent pathways

摘要

The mechanisms underlying AVP-induced increase in [Ca2+]i and glucagon release in clonal α-cells In-R1-G9 were investigated.AVP increased [Ca2+]i and glucagon release in a concentration-dependent manner. After the administration of AVP, glucagon was released within 30 s, quickly reached the maximum within 2 min, and maintained a steady-state concentration for at least 15 min.In Ca2+-containing medium, AVP increased [Ca2+]i in a biphasic pattern; a peak followed by a sustained plateau. In Ca2+-free medium, the Ca2+ response to AVP became monophasic with lower amplitude and no plateau. Both the basal and AVP-induced glucagon releases were lower in the absence than in the presence of extracellular Ca2+. When [Ca2+]i was stringently deprived by BAPTA, a Ca2+ chelator, AVP still significantly increased glucagon release.Pretreatment with thapsigargin, a microsomal Ca2+ ATPase inhibitor, abolished both the Ca2+ peak and sustained plateau.AVP increased intracellular concentration of IP3.U-73122 (8 μM), a phospholipase C inhibitor, abolished AVP-induced increases in [Ca2+]i, but only reduced AVP-induced glucagon release by 39%.Pretreatment with nimodipine, an L-type Ca2+ channel blocker failed to alter AVP-induced glucagon release or increase in [Ca2+]i.The results suggest that AVP causes glucagon release through both Ca2+-dependent and -independent pathways. For the Ca2+-dependent pathway, the Gq protein activates phospholipase C, which catalyzes the formation of IP3. IP3 induces Ca2+ release from the endoplasmic reticulum, which, in turn, triggers Ca2+ influx. Both Ca2+ release and Ca2+ influx may contribute to AVP-induced glucagon release.