Abstract:
Calcium (Ca²⁺) oscillations in hepatocytes control many critical cellular functions, includ-ing glucose metabolism and bile secretion. The mechanisms underlying repetitive Ca²⁺ oscillations and how these mechanisms regulate these oscillations is not fully understood. Recent experimental evidence has shown that both Ca²⁺ regulation of the inositol 1,4,5-trisphosphate (IP3) receptor and agonist-activated Ca²⁺ regulation of IP3 metabolism gen-erates Ca²⁺ oscillations and co-exist in hepatocytes. Furthermore, agonist-activated Ca²⁺ oscillations in hepatocytes are shown to have a wide dynamic range. In particular, recent experimental evidence shows that agonist stimulation of the P2Y family of receptors leads to qualitatively diverse Ca²⁺ oscillations. We investigate the effects of the feedback mecha-nisms on the Ca²⁺ response in two parts.
First, we construct a mathematical model of the Ca²⁺ signalling network in hepatocytes. The model accounts for the biphasic regulation of Ca²⁺ on the IP3 receptor (IP3R) and the positive feedback from Ca²⁺ on IP3 metabolism, via activation of phospholipase C (PLC) by agonist and Ca²⁺. Model simulations show that Ca²⁺ oscillations exist for both constant [IP3] and [IP3] changing dynamically. We show, both experimentally and in the model, that as agonist concentration increases, Ca²⁺ oscillations transition between simple narrow-spike oscillations and complex broad-spike oscillations. The model predicts that narrow-spike os-cillations persist when Ca²⁺ transport across the plasma membrane is blocked. This predic-tion has been experimentally validated. In contrast, broad-spike oscillations are terminated when plasma membrane transport is blocked.
Next, we present a new model of Ca²⁺ oscillations in hepatocytes based on the experi-ments to investigate the mechanisms controlling P2Y-activated Ca²⁺ oscillations. The model builds upon the previous model to include protein kinase C (PKC) regulation of multiple cel-lular substrates. Utilising the model, we suggest the activity and intensity of PLC and PKC necessary to explain the qualitatively diverse Ca²⁺ oscillations in response to P2Y receptor activation.
We conclude that multiple feedback mechanisms participate in regulating Ca²⁺ oscilla-tions in hepatocytes.