Calcium spots: elementary signals in response to mechanical stress in vascular endothelial cells

Nitric oxide signaling in the microcirculation

Several apparent paradoxes are evident when one compares mathematical predictions from models of nitric oxide (NO) diffusion and convection in vasculature structures with experimental measurements of NO (or related metabolites) in animal and human studies. Values for NO predicted from mathematical models are generally much lower than in vivo NO values reported in the literature for experiments, specifically with NO microelectrodes positioned at perivascular locations next to different sizes of blood vessels in the microcirculation and NO electrodes inserted into a wide range of tissues supplied by the microcirculation of each specific organ system under investigation. There continues to be uncertainty about the roles of NO scavenging by hemoglobin versus a storage function that may conserve NO, and other signaling targets for NO need to be considered. This review describes model predictions and relevant experimental data with respect to several signaling pathways in the microcirculation that involve NO.

From nitric oxide to endothelial cytosolic Ca2+: a negative feedback control

Nitric oxide production is stimulated by an increase of the concentration of cytosolic Ca(2+) in vascular endothelial cells. Recent evidence suggests that nitric oxide and cGMP might attenuate Ca(2+) influx and, at the same time, initiate a Ca(2+) removal mechanism, thereby decreasing the intracellular concentration of endothelial Ca(2+) in a negative feedback fashion. Such a negative feedback mechanism could serve to protect the endothelial cells from the detrimental effects of excessive nitric oxide and Ca(2+).

Optical Bioimaging: From Living Tissue to a Single Molecule: Calcium Imaging in Blood Vessel In Situ Employing Two-Photon Excitation Fluorescence Microscopy

Recent developments in optoelectronics permit real-time Ca(2+) imaging of thin planes within cells utilizing laser scanning confocal microscopy (LSCM). However, a major complication associated with this imaging system involves increased phototoxicity with improved spatiotemporal resolution. Two-photon excitation microscopy (TPEM) helps to minimize phototoxicity due to the restriction of this technique to the volume proximal to the geometric focus of the light. In this study, the capability of Ca(2+) imaging was investigated employing recently developed real-time TPEM, RTS2000MP (Bio-Rad, Tokyo) with a mode-locked Ti-sapphire laser. Z-axis resolution of RTS2000MP with high NA objectives defined as full-width at half maximum (FWHM) with a 0.5-microm fluorescent bead provided values nearly identical to those obtained with LSCM at a small pinhole (0.2 mm) (approximately 0.6 microm). When serial sectioning of 21 sequential images at 0.3-microm intervals in cultured endothelial cells loaded with calcein and tetramethyl-rhodamine methylester were performed with TPEM, the z-axis resolution was higher than that observed with LSCM; moreover, the photobleaching rate was significantly lower than that obtained with LSCM. Maximum fluorescence intensities were detected at 780 nm in excitation spectra of fluo-3 and fluo-4 Ca(2+)-sensitive probes with TPEM. Fluorescence images in mouse arterial endothelial cells loaded with fluo-4 could be clearly visualized by TPEM in situ. Application of acetylcholine caused oscillatory increase in [Ca(2+)](i) of endothelial cells; subsequently, relaxation along the major axis of smooth muscle cells was evident. Furthermore, consecutive long-lasting experiments could be repeated with identical response in the same microscopic field. In conclusion, fluorescence imaging employing TPEM is useful for Ca(2+) imaging in blood vessels in situ.