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

¹Department of Biomedical Engineering and ²Department of Pharmacology and Physiology, University of Rochester, Rochester, New York 14642

Submitted 8 January 2004 ; accepted in final form 9 February 2004

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 P_2x 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 P_2x 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 P_2x or P₁ receptors. These transients may represent a mechanism by which individual EC responses are integrated to result in coordinated arteriolar responses in situ.

oscillations; fluorescence imaging; microvascular communication

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