Effect of H+ and elevated PCO2 on membrane electrical properties of rat cerebral arteries

Some membrane electrical properties of muscle cells from the middle cerebral artery of the rat were recorded with intracellular microelectrodes. The resting membrane potential (Em) of this preparation was -63 mV. Reduction of extracellular pH to 7.0 in the face of a constant PCO2 of 40 mm Hg had no significant effect on Em. Similarly the slope of the steady-state voltage/current curves was not different at pH 7.0 compared to control at pH 7.4. In marked contrast, when PCO2 was elevated to around 60 to 70 mm Hg there was a rapid hyperpolarization and reduction in the slope of the voltage current curve suggesting an increased conductance for one or more ionic species. In addition elevation of PCO2 increased the slope of the Em vs. log[K]0 curve from 46 mV/decade to 59 mV/decade which is in good agreement with a Nernstian potential for a K+ selective membrane. These data suggest that while the smooth muscle cells of rat cerebral arteries are relatively insensitive to a small reduction in extracellular pH; reduction of intracellular pH by elevating PCO2 induces hyperpolarization by increasing K+ conductance (gk). However, it is not clear from these experiments if the PCO2 effects are mediated entirely by changes in pH or if there is a direct membrane action of CO2.