Quantitative properties and receptor reserve of the IP3 and calcium branch of Gq-coupled receptor signaling

G_q-coupled plasma membrane receptors activate phospholipase C (PLC), which hydrolyzes membrane phosphatidylinositol 4,5-bisphosphate (PIP₂) into the second messengers inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). This leads to calcium release, protein kinase C (PKC) activation, and sometimes PIP₂ depletion. To understand mechanisms governing these diverging signals and to determine which of these signals is responsible for the inhibition of KCNQ2/3 (K_V7.2/7.3) potassium channels, we monitored levels of PIP₂, IP₃, and calcium in single living cells. DAG and PKC are monitored in our companion paper(Falkenburger et al. 2013. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.201210887). Theresults extend our previous kinetic model of G_q-coupled receptor signaling to IP₃ and calcium. We find that activation of low-abundance endogenous P2Y₂ receptors by a saturating concentration of uridine 5'-triphosphate (UTP; 100 μM) leads to calcium release but not to PIP₂ depletion. Activation of overexpressed M1 muscarinic receptors by 10 μM Oxo-M leads to a similar calcium release but also depletes PIP₂. KCNQ2/3 channels are inhibited by Oxo-M (by 85%), but not by UTP (<1%). These differences can be attributed purely todifferences in receptor abundance. Full amplitude calcium responses can be elicited even after PIP₂ was partially depleted by overexpressed inducible phosphatidylinositol 5-phosphatases, suggesting that very low amounts of IP3 suffice to elicit a full calcium release.Hence, weak PLC activation can elicit robust calcium signals without net PIP₂ depletion or KCNQ2/3 channel inhibition.