Calcium signaling in brown fat cells

Rat brown fat cells respond to the neurotransmitter norepinephrine in vivo through a number of signaling pathways including changes in intracellular calcium. This dissertation describes a previously unknown effect of beta-adrenergic stimulation on release of calcium from endoplasmic reticulum (ER) stores. Namely, that beta-adrenergic stimulation potentiates such release initiated by other receptor types such as alpha-adrenoceptors or P2 purinergic receptors. This finding suggests that norepinephrine elicits brown fat calcium signals in vivo through activation of both alpha 1- and beta-adrenoceptors. Simultaneous to ER calcium release, norepinephrine activates a calcium influx mechanism that contributes to intracellular calcium increases. Some have proposed that this influx is secondary to depletion of ER calcium stores---the "capacitative" calcium entry model. Pursuing this model, I have identified a store-operated calcium current (I SOC) in brown adipocytes activated by the fungal toxin cyclopiazonic acid. This current shares characteristics with the store-operated current ICRAC that has been described in other cells. The shared properties include inward rectification, voltage-dependent inactivation, blockage by lanthanides, and an apparent cation permeability sequence of Ca2+ > Na+ ≥ Cs+ based on membrane current magnitudes. Experiments characterizing ISOC also revealed two other cation currents that have not been previously described in brown fat. One current is regulated by intracellular MgATP and has characteristics nearly identical to a cation current known variously as IMIC or MagNuM and believed to be mediated by the TRPM7 gene product. The other current is a background cation conductance that is inhibited by increases in extracellular calcium, and enhanced by removal of extracellular divalents.; Two other accompanying studies are previously published reports associated with my work as a trainee on the NSF Training Grant "Nonlinear Dynamics in Biology" through the UC Davis Institute of Theoretical Dynamics. The first is an experimental project describing a chloride current in endothelial cells activated by mechanical shear stress (flow). The second project is a methodological review of a particular class of spatially explicit mathematical models known as "non-local" models. This usefulness of this theoretical approach is illustrated with problems from both ecology and cell biology.