Coordination of cellular activity through Ca²⁺ enables β cells to secrete precise quantities of insulin. To explore how the Ca²⁺ response is orchestrated in space and time, we implement optogenetic systems to probe the role of individual β cells in the glucose response. By targeted β cell activation/inactivation in zebrafish, we reveal a hierarchy of cells, each with a different level of influence over islet-wide Ca²⁺ dynamics. First-responder β cells lie at the top of the hierarchy, essential for initiating the first-phase Ca²⁺ response. Silencing first responders impairs the Ca²⁺ response to glucose. Conversely, selective activation of first responders demonstrates their increased capability to raise pan-islet Ca²⁺ levels compared to followers. By photolabeling and transcriptionally profiling β cells that differ in their thresholds to a glucose-stimulated Ca²⁺ response, we highlight vitamin B6 production as a signature pathway of first responders. We further define an evolutionarily conserved requirement for vitamin B6 in enabling the Ca²⁺ response to glucose in mammalian systems.