Modification of cytosolic calcium signaling by subplasmalemmal microdomains.

To investigate the hypothesis that Na(+) concentration in subplasmalemmal microdomains regulates Ca(2+) concentrations in cellular microdomains (Ca), the cytosol (Ca), and sarcoplasmic reticulum (SR; Ca), we modeled transport events in those compartments. Inputs to the model were obtained from published measurements in descending vasa recta pericytes and other smooth muscle cells. The model accounts for major classes of ion channels, Na(+)/Ca(2+) exchange (NCX), and the distributions of Na(+)-K(+)-ATPase alpha(1)- and alpha(2)-isoforms in the plasma membrane. Ca(2+) release from SR stores is assumed to occur via ryanodine (RyR) and inositol trisphosphate (IP(3)R) receptors. The model shows that the requisite existence of a significant Na(+) concentration difference between the cytosol (Na) and microdomains (Na) necessitates restriction of intercompartmental diffusion. Accepting the latter, the model predicts resting ion concentrations that are compatible with experimental measurements and temporal changes in Ca similar to those observed on NCX inhibition. An important role for NCX in the regulation of Ca(2+) signaling is verified. In the resting state, NCX operates in "forward mode," with Na(+) entry and Ca(2+) extrusion from the cell. Inhibition of NCX respectively raises and reduces Ca and Na by 40 and 30%. NCX translates variations in Na(+)-K(+)-ATPase activity into changes in Ca, Ca, and Ca. Taken together, the model simulations verify the feasibility of the central hypothesis that modulation of Na can influence both the loading of Ca(2+) into SR stores and Ca(2+) variation.