PULSATILE AND STEADY FLOW-INDUCED CALCIUM OSCILLATIONS IN SINGLE CULTURED ENDOTHELIAL-CELLS

The Influence of flow-imposed shear stress on the intracellular calciu m concentration ([Ca2+](i)) of cultured endothelial cells (ECs) remain s incompletely understood. In the present study, we measured [Ca2+](i) in single bovine aortic ECs, using fluorescence ratiometric image ana lysis. The effects of several flow patterns were analysed: steady shea r stress (5-70 dyn/cm(2)), 1-Hz pulsatile shear stress (nonreversing 4 0 +/- 30 dyn/cm(2), reversing 20 +/- 40 dyn/cm(2), or purely oscillato ry 0 +/- 20 dyn/cm(2)), or changing shear stress levels. Under all flo w conditions, single-cell analyses revealed flow-induced asynchronous [Ca2+](i) oscillations, which occurred randomly over the monolayer and which were not seen in the average [Ca2+](i) signal corresponding to the monolayer response. The number of single-cell [Ca2+](i) oscillatio ns and the corresponding oscillation frequency rose as the shear stres s associated with the steady flow increased: 0.06 +/- 0.02 min(-1) at 5 dyn/cm(2), 0.19 +/- 0.03 min(-1) at 20 dyn/cm(2), and 0.28 +/- 0.02 min(-1) at 70 dyn/cm(2) (means +/- SD). Also: the number of oscillatio ns was greater for any type of pulsatile flow (0.53 +/- 0.07 min(-1) a t 40 +/- 20 dyn/cm(2), 0.54 +/- 0.08 min(-1) at 20 +/- 40 dyn/cm(2), a nd 0.39 +/- 0.07 min(-1) at 0 +/- 20 dyn/cm(2)), as compared to any le vel of steady flow. The most dramatic finding was that purely oscillat ory flow induced numerous single-cell [Ca2+](i) oscillations, yet the average [Ca2+](i) response for the monolayer did not change. Furthermo re, an EC monolayer switched from low to high (or from high to low) st eady flow consistently showed an increase (or a decrease) in the numbe r of single-cell [Ca2+](i) oscillations. These experiments show that E Cs respond to different flow conditions by varying single-cell [Ca2+]( i) oscillatory activity, This may have important implications in the e ndothelium-dependent control of vascular physiology, such as the relea se of vasoactive substances.