Comparison of the effects of potassium and pH on the calibre of cerebral veins and arteries.

The vasomotor response of individual pial veins and arteries on the convexity of the cerebral cortex to perivascular microinjection of mock cerebrospinal fluid (CSF) containing various concentrations of potassium (K+) and of various pH (achieved by altering the bicarbonate, HCO-3 concentration) have been examined in cats anaesthetised with alpha-chloralose. Microapplication of CSF containing 0 mM HCO-3 (pH 4.80) effected significant increases in calibre of pial veins and arteries of 9.3 +/- 2.4% and 38.2 +/- 4% respectively (mean calibre change +/- SE), whereas CSF containing 22 mM HCO-3 (pH 7.45) which constricted pial arteries significantly (-18.5 +/- 2.9%) minimally altered venous calibre (-4.3 +/- 2.2%). Microapplication of CSF containing 0 mM potassium resulted in a significant reduction in pial arterial calibre (-11.4 +/- 2.8%) but failed to alter pial venous calibre (-0.3 +/- 0.6%). Perivascular microapplication of CSF containing moderately elevated potassium concentrations (10 mM) which effected marked, significant increases in pial arterial calibre (49.3 +/- 3.9%) did not significantly alter the calibre of the pial veins (mean response -1.6 +/- 2.4%). The perivascular administration of CSF containing a high concentration of potassium (40 mM) resulted in the significant constriction of both pial veins (-13.5 +/- 0.9%) and pial arteries (-47.2 +/-6.3%). The magnitude of the response was significantly smaller in the pial veins. The relative insensitivity to K+ and pH of the pial veins as compared to pial arteries suggests that alteration in the chemical composition of of the perivascular fluid are of lesser importance in the control of cerebrovascular capacitance than for the regulation of cerebrovascular resistance.