Quantitative properties and receptor reserve of the IP(3) and calcium branch of G(q)-coupled receptor signaling

G(q)-coupled plasma membrane receptors activate phospholipase C (PLC), which hydrolyzes membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) into the second messengers inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol (DAG). This leads to calcium release, protein kinase C (PKC) activation, and sometimes PIP(2) depletion. To understand mechanisms governing these diverging signals and to determine which of these signals is responsible for the inhibition of KCNQ2/3 (K(V)7.2/7.3) potassium channels, we monitored levels of PIP(2), IP(3), 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). The results extend our previous kinetic model of G(q)-coupled receptor signaling to IP(3) and calcium. We find that activation of low-abundance endogenous P2Y(2) receptors by a saturating concentration of uridine 5′-triphosphate (UTP; 100 µM) leads to calcium release but not to PIP(2) depletion. Activation of overexpressed M(1) muscarinic receptors by 10 µM Oxo-M leads to a similar calcium release but also depletes PIP(2). KCNQ2/3 channels are inhibited by Oxo-M (by 85%), but not by UTP (<1%). These differences can be attributed purely to differences in receptor abundance. Full amplitude calcium responses can be elicited even after PIP(2) was partially depleted by overexpressed inducible phosphatidylinositol 5-phosphatases, suggesting that very low amounts of IP(3) suffice to elicit a full calcium release. Hence, weak PLC activation can elicit robust calcium signals without net PIP(2) depletion or KCNQ2/3 channel inhibition.