Regulation of Cl(-) channel conductance by Ins(3,4,5,6)P(4) provides receptor-dependent control over salt and fluid secretion, cell volume homeostasis, and electrical excitability of neurones and smooth muscle. Ignorance of how Ins(3,4,5,6)P(4) is synthesized has long hindered our understanding of this signaling pathway. We now show Ins(3,4,5,6)P(4) synthesis by Ins(1,3,4,5,6)P(5) 1-phosphatase activity by an enzyme previously characterized as an Ins(3,4,5,6)P(4) 1-kinase. Rationalization of these phenomena with a ligand binding model unveils Ins(1,3,4)P(3) as not simply an alternative kinase substrate, but also an activator of Ins(1,3,4,5,6)P(5) 1-phosphatase. Stable overexpression of the enzyme in epithelial monolayers verifies its physiological role in elevating Ins(3,4,5,6)P(4) levels and inhibiting secretion. It is exceptional for a single enzyme to catalyze two opposing signaling reactions (1-kinase/1-phosphatase) under physiological conditions. Reciprocal coordination of these opposing reactions offers an alternative to general doctrine that intracellular signals are regulated by integrating multiple, distinct phosphatases and kinases.

In cystic fibrosis (CF) airways, abnormal epithelial ion transport likely initiates mucus stasis, resulting in persistent airway infections and chronic inflammation. Mucus clearance is regulated, in part, by activation of apical membrane receptors coupled to intracellular calcium (Ca storage capacity of apically confined endoplasmic reticulum (ER) Ca 2؉ stores. To address whether the ER upregulation resulted from ER retention of misfolded ⌬F508 CFTR or was an acquired response to chronic luminal airway infection/inflammation, three approaches were used. First, ER density was studied in normal and CF sweat duct human epithelia expressing high levels of ⌬F508 CFTR, and it was found to be the same in normal and CF epithelia. Second, apical ER density was morphometrically analyzed in airway epithelia from normal subjects, ⌬F508 homozygous CF patients, and a disease control, primary ciliary dyskinesia; it was found to be greater in both CF and primary ciliary dyskinesia. Third, apical ER density and P2Y 2 -R activation-mobilized Ca

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