Localized transient increases in endothelial cell Ca2+in arterioles in situ: implications for coordination of vascular function

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Abstract
Intracellular Ca2+ transients were identified in endothelial cells (ECs) in intact blood-perfused arterioles. ECs in cremaster muscle arterioles (diameter ∼45 μm) in anesthetized mice were loaded with the Ca2+ indicator fluo 4-AM by intraluminal perfusion, after which blood flow was reestablished. Confocal microscopy was used to visualize Ca2+ as a function of fluo-4 intensity in real time. Separate sets of experiments were performed under the following conditions: control, ischemia, during inhibition of P2x or P1 purinoreceptors, and with the application of exogenous adenosine. In controls, spontaneous EC Ca2+ transients displayed a wide range of activity frequency (1–32 events/min) and about one-third of these transient events were synchronized between adjacent ECs. The increase in Ca2+ remained localized and did not spread to encompass the entire cell body. Ca2+ transient activity decreased significantly with ischemia (from 9.9 ± 0.6 to 3.1 ± 0.3 events/min, n = 135) but was unaffected by P2x or P1 receptor inhibition. Exogenous adenosine significantly increased the frequency of Ca2+ transients (to 12.8 ± 0.9 events/min) and increased synchronization so that 50% of all Ca2+ events were synchronized between ECs. This response to adenosine was not due to an increase in shear stress. These data indicate that localized Ca2+ transients are sensitive to flow conditions and, separately, to metabolically active pathways (exogenous adenosine), although the basal activity occurs independently of P2x or P1 receptors. These transients may represent a mechanism by which individual EC responses are integrated to result in coordinated arteriolar responses in situ.