Effect of CO2 on intracellular pH and contraction of retinal capillary pericytes.

PURPOSE To test the potential participation of pericytes in the regulation of capillary blood flow according to metabolic needs. METHODS The authors measured the change in extracellular pH (pHo), intracellular pH (pHi), and contractile tone of cultured bovine retinal pericytes when the ambient carbon dioxide (CO2) level was changed. RESULTS Raising the partial pressure of CO2 (PCO2) from 5% to 10% or 20% acidified the medium, decreased the pHi, and relaxed the pericytes. Lowering the PCO2 from 5% to 0% or 2% alkalized the medium, raised the pHi, and contracted the pericytes. The reactions of pericytes in both cases were dose dependent. When the pHo was kept at approximately 7.4 as the PCO2 was changed by adjusting the ratio of NaHCO3 and NaCl in the medium (or by using HEPES buffer when PCO2 was zero), both elevating and lowering the PCO2 caused intracellular acidification but did not change the contractile tone of the retinal pericytes significantly. CONCLUSIONS The effect of CO2 on the pHi and the contractility of pericytes is mediated primarily by its influence on the pHo. Elevation of CO2 causes pericytes to relax, whereas dispersion of CO2 causes pericytes to contract under the condition that allows pHo to be affected by PCO2. Therefore, pericytes may contribute to the regulation of local blood flow by their response to the change in local metabolic conditions.