Effect of Acidosis on Isolated Porcine Retinal Vessels

To study the effect of normocapnic (NA) and hypercapnic acidosis (HA) on the tone, the intracellular calcium level ([Ca(2 +)](i)), and the membrane potential of smooth muscle cells in porcine retinal arterioles.Twenty-four porcine retinal arterioles were mounted in a wire myograph for isometric recording of the wall tension. The vessels were precontracted with 0.3 microM U46619 and were exposed to NA (pH = 7.0) and HA (pH = 7.0). Intracellular calcium was measured using the fluorophore Fura-2AM (n = 12). In six vessels, 0.1 mM NG-nitroarginine methyl ester (L-NAME) was added to block NO synthesis. The membrane potential of smooth muscles cells was measured in situ with sharp glass electrodes (n = 12).NA and HA induced both a decrease in wall tension from 1.04 +/- 0.06 N/m to 0.65 +/- 0.1 N/m (p0.01) (NA) and 0.56 +/- 0.1 N/m (p0.01) (HA) and a decrease in [Ca(2 +)](i) as evidenced from the change in the Fura-2 fluorescence emission ratio from 0.66 +/- 0.03 to 0.57 +/- 0.05 (p = 0.005) (NA) and 0.56 +/- 0.05 (p = 0.002) (HA). These results were unaffected by inhibition of NO-synthesis. NA and HA also both induced hyperpolarization of the smooth muscle membrane from -18 +/- 0.7 mV during precontraction to -26 +/- 1.9 mV (p = 0.002) (NA) and -24 +/- 2.6 mV (p = 0.02) (HA).Acidosis-induced relaxation of the tone in preconstricted isolated porcine retinal arterioles is associated with a decrease in intracellular calcium and a hyperpolarization of the smooth muscle cells. The acidosis-induced relaxation is independent of CO(2) and is not mediated through NO.