The Dual Anaplerotic Model (DAM): Integral Roles of Pyruvate Carboxylase and the GABA Shunt in Beta Cell Insulin Secretion

We present a simplified phenomenological computational framework that integrates the GABA shunt into established metabolic mechanisms underlying pancreatic beta cell insulin secretion. The GABA shunt introduces carbon into the tricarboxylic acid (TCA) cycle via succinate, thereby functioning as an anaplerotic pathway. This anaplerotic input is coupled to oscillatory cataplerotic fluxes, primarily involving α-ketoglutarate, whose effective extrusion requires coordinated counter-fluxes of malate and aspartate. Within the model, these cataplerotic exchanges are facilitated by UCP2-mediated transport processes and necessitate complementary anaplerotic replenishment through pyruvate carboxylase (PC). Based on this functional interdependence, we introduce the Dual Anaplerotic Model (DAM), which conceptually links two anaplerotic routes-the GABA shunt-mediated pathway and the glucose-dependent PC pathway-into a unified metabolic response module. DAM describes a coordinated, breathing-like redistribution of carbon between mitochondrial and cytosolic metabolite pools, while efficient oxidative metabolism of glucose-derived carbon entering the TCA cycle via pyruvate dehydrogenase is maintained. The model is driven by experimentally observed ATP/ADP and Ca²⁺ dynamics and is not intended to generate autonomous oscillations. Instead, it enables qualitative, phase-dependent visualization of how dual anaplerotic fluxes constrain and shape oscillatory metabolic states in beta cells. DAM provides an integrative conceptual scaffold for interpreting experimental observations and for motivating future quantitative modeling and experimental studies addressing metabolic regulation in physiological and pathophysiological contexts.