Vascular resistance is a major controller of blood pressure and blood flow. Contractile state of small-sized arteries determines their diameter, and therefore vascular resistance to blood flow. Vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) are the two major cell types in the arterial wall that are known to influence vascular resistance. VSMCs can contract or relax, resulting in vasoconstriction or vasodilation, respectively. ECs line the inner walls of all arteries, and can modulate the contractile state of the surrounding VSMCs. Traditionally, increases in VSMC Ca 2+ have been associated with vasoconstriction, whereas increases in endothelial Ca 2+ are thought to be vasodilatory. However, some highly localized Ca 2+ signals in VSMCs can couple with vasodilator effector pathways and lower vascular resistance.
| HOW ARE CALCIUM SIGNALS MEASURED?Ca 2+ -sensitive fluorescent dyes are the most commonly used tools for studies of intracellular Ca 2+ . The fluorescence properties of Ca 2+ dyes change when they are bound to Ca 2+ , thus enabling detection of intracellular Ca 2+ levels. Fluorescent dyes are commonly used as membrane-permeable acetoxymethyl (AM) esters, which are cell-permeable. Intracellular esterases release the membrane-impermeable anion form of the dye that is retained within the cell. Most fluorescent dyes are derivatives of the Ca 2+ chelator BAPTA (bis-[o-amino-phenoxy]-ethane-N,N,N'N 0 -tetraacetic acid) (Tsien, 1980). Fluorescent dyes can be ratiometric or nonratiometric. Ratiometric dyes show a shift in excitation (fura-2) or emission (indole-1) wavelength that is dependent on the concentration of free Ca 2+ . Non-ratiometric dyes (fluo family) show an increase in fluorescence intensity on binding Ca 2+ and are useful for qualitative changes in Ca 2+ . Ratiometric dyes are generally used for estimating Ca 2+ concentration, but methods have been developed to calculate Ca 2+ concentration using single wavelength fluorescence signals with non-ratiometric dyes (Maravall, Mainen, Sabatini, & Svoboda, 2000). Ratiometric dyes have the advantage that the ratio normalizes the variations in fluorescence with uneven dye loading, distribution, dye leakage, or photobleaching. However, the dyes require excitation in the ultraviolet range, and cannot record individual Ca 2+ signals that drive many physiological functions. Non-ratiometric dyes enable the recordings of localized Ca 2+ signals. However, an accurate analysis of kinetic and spatial properties of fast Ca 2+ signals may require the use of high-speed Ca 2+ imaging. Indeed, high-speed Ca 2+ imaging has enabled detailed studies of kinetic and spatial properties of several Ca 2+ signals, including rise time, duration, spatial spread, amplitudes of elementary signals, and functional coupling between neighboring signals (