How Ca2+ oscillations are generated and fine-tuned to yield versatile downstream responses remains to be elucidated. In hepatocytes, G protein-coupled receptor-linked Ca2+ oscillations report signal strength via frequency, whereas Ca2+ spike amplitude and wave velocity remain constant. IP3 uncaging also triggers oscillatory Ca2+ release, but, in contrast to hormones, Ca2+ spike amplitude, width, and wave velocity were dependent on [IP3] and were not perturbed by phospholipase C (PLC) inhibition. These data indicate that oscillations elicited by IP3 uncaging are driven by the biphasic regulation of the IP3 receptor by Ca2+, and, unlike hormone-dependent responses, do not require PLC. Removal of extracellular Ca2+ did not perturb Ca2+ oscillations elicited by IP3 uncaging, indicating that reloading of endoplasmic reticulum stores via plasma membrane Ca2+ influx does not entrain the signal. Activation and inhibition of PKC attenuated hormone-induced Ca2+ oscillations but had no effect on Ca2+ increases induced by uncaging IP3. Importantly, PKC activation and inhibition differentially affected Ca2+ spike frequencies and kinetics. PKC activation amplifies negative feedback loops at the level of G protein-coupled receptor PLC activity and/or IP3 metabolism to attenuate IP3 levels and suppress the generation of Ca2+ oscillations. Inhibition of PKC relieves negative feedback regulation of IP3 accumulation and, thereby, shifts Ca2+ oscillations toward sustained responses or dramatically prolonged spikes. PKC down-regulation attenuates phenylephrine-induced Ca2+ wave velocity, whereas responses to IP3 uncaging are enhanced. The ability to assess Ca2+ responses in the absence of PLC activity indicates that IP3 receptor modulation by PKC regulates Ca2+ release and wave velocity.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology