Figure 8.

Hypothesized model of Ca²⁺-free medium-induced Ca²⁺ signaling correlated with the activation of lipolysis. Ca²⁺ -free medium activates Cx43 hemichannels and induces vesicular ATP release. ATP release activates of the P2Y1 purinoreceptors of nearby adipocytes and triggers a signaling cascade involving the phosphoinositide signaling system—phospholipase C (PLC)—which leads to the mobilization of Ca²⁺ ions through the IP₃ receptor (IP₃R) from the endoplasmic reticulum (ER). The increase in [Ca²⁺]_i in white adipocytes is periodic in the form of prolonged Ca²⁺ oscillations in the cytosol, which causes Ca²⁺-dependent ATP release and paracrine activation of the fat cells’ monolayer. This behavior of the Ca²⁺ signaling system correlates further with an increase in the expression of the key lipolysis genes Lipe, Sirt1 and Atgl1, and a decrease in the expression of the Igf2 gene. As a result, there is a decrease in the number of lipid droplets and the level of glycerol.