Figure 1. Schematic diagram of the ionic current, Ca²⁺ fluxes and exocytosis in human β-cell. Transmembrane currents are: I_CaP is the voltage-gated P-type Ca²⁺ current, that is also responsible for insulin release (see text), I_CaL is the voltage-gated L-type Ca²⁺ current, I_CaT is the voltage-gated T-type Ca²⁺ current; I_PCa, plasma membrane Ca²⁺ pump current; I_Na, voltage-gated Na⁺ current; I_Nab, Na⁺ background current I_KDr, delayed rectifier K⁺ current; I_KCaB, Ca²⁺ and high voltage-activated K⁺ (BK channel) current; I_KCa, Ca²⁺ activated K⁺ (SK channel) current; I_Kher is the human-ERG K⁺ channel current and I_KATP, ATP-sensitive K⁺ channels current. Calcium enters the β-cells primarily through voltage-activated Ca²⁺ channels by diffusion along an inwardly directed electrochemical gradient. At the plasma membrane two processes are involved in transporting Ca²⁺ out of the cell; a Ca²⁺ pump and removal of Ca²⁺ sequestrated in insulin granules by exocytosis (coefficient k_sq).