The effects of hormones such as catecholamine, vasopressin and angiotensin II on hepatic metabolism are mediated by alterations in the concentration of cytosolic free Ca2+ ([Ca2+]i). These [Ca2+]i changes are brought about, at least in part, by the action of the second messenger inositol 1,4,5-triphosphate (IP3). Biochemical studies in populations of hepatocytes have demonstrated a relatively simple relationship between IP3 and [Ca2+]i changes. However, studies carried out at the single cell level have revealed that the [Ca2+]i responses to these hormones are organized in the form of [Ca2+]i hormone concentration. These frequency-modulated [Ca2+]i oscillations may serve a number of signaling functions that cannot readily be achieved in other ways. There is a further level of organization of these [Ca2+]i signals at the subcellular level, since [Ca2+]i oscillations initiate from a discrete locus within each cell and then spread through the cell as waves of Ca2+ that propagate with constant velocity and amplitude. We suggest that Ca2+ waves play an important role in distributing [Ca2+]i signals, signal transduction from the plasma membrane may be essential for full hormonal responsiveness in large cytoplasm by binding to a limited sinusoidal membrane domain. Patterns of [Ca2+]i oscillation can be modified by a number of other agents, including insulin and glucagon, and as such the [Ca2+]i oscillation systems may be important loci for the interactions between various classes of hormones. The present proposal is directed towards elucidating the mechanisms that are responsible for generating [Ca2+]i oscillations and waves within hepatocytes. In addition, the subcellular distribution of major components of the Ca2+ signaling system will be examined to determine how these components give rise to specific polarized initiation site for Ca2+ waves, and how the intercellular organization contributes to the propagation of the Ca2+ waves. Experiments will be carried out using permeabilized cells to examine the structure and functional regulation of the intracellular Ca2+ storage pools, and intact cells to elucidate how these properties are integrated in the generation of oscillatory Ca2+ waves. A major components of these studies will rely on imaging approaches to the measurement of dynamic Ca2+ movements within both intact and permeabilized cells, including some novel techniques that permit spatially-resolved studies of Ca2+ stores and IP3 action at the subcellular level.
| Sela-Brown, A; Russell, J; Koszewski, N J et al. (1998) Calreticulin inhibits vitamin D's action on the PTH gene in vitro and may prevent vitamin D's effect in vivo in hypocalcemic rats. Mol Endocrinol 12:1193-200 |
| Patel, S; Santra, M; McQuillan, D J et al. (1998) Decorin activates the epidermal growth factor receptor and elevates cytosolic Ca2+ in A431 carcinoma cells. J Biol Chem 273:3121-4 |
