In hepatocytes hormone-induced frequency-modulated intracellular Ca2+ oscillations (osc) control multiple signalling events. IP3 and Ca2+ oscillate synchronously but the question remains, are Ca2+ osc controlled by Ca2+ feedback on the IP3 receptor (IP3R) or is Ca2+ dependent +ve and/or −ve feedback on IP3 levels also required? Ca2+ imaging techniques were used to study Ca2+ osc in 24h cultured primary hepatocytes. Activation and inhibition of PKC decreased Ca2+ osc frequency and PLC activity induced by hormone. Photolysis of caged-IP3 resulted in transient Ca2+ release with Ca2+ osc observed in 40% of cells. These IP3 generated osc in the absence of hormone presumably arise via repetitive Ca2+induced Ca2+ release from the IP3R. However, FRET probes sensitive to changes in IP3 are currently under study to assess whether Ca2+ released by caged-IP3 is able to feedback on PLC and regenerate IP3. IP3R channel opening can be modulated by phosphorylation. Downregulation of PKC did not affect caged-IP3-induced Ca2+ osc occurrence or frequency, whilst acute activation of PKC increased Ca2+ osc frequency. These data suggest under basal conditions the IP3R is not regulated by PKC phosphorylation but hormone-induced PKC activity may affect IP3R opening. This study provides evidence that multiple PKC isoenzymes exert counteracting effects to shape hormone-induced Ca2+ osc. (Thomas P. Infusino Endowment to APT)