gms | German Medical Science

Objective: The presence of acid sensing ion channels (ASICs) has recently been identified in non-neuronal tissues including the vessel wall. We have therefore addressed the question of whether these channels are expressed in rat cerebral arteries and whether they play any role in vasomotor response to acidosis.

Methods: Studies on the functional importance of ASICs were performed by measurement of isometric force in ring segments of rat middle cerebral and basilar artery (MCA, BA). Following precontraction with U46619 (0.3µM) acidosis was induced by cumulative application of lactic acid or HCl in HEPES buffer (pH range, 7.05 - 6.56) and in Krebs buffer (pH range, 6.96 - 6.41). We studied the effect of amiloride (30µM) – a known blocker of ASIC and ASIC-like channels on acid-induced relaxation as well as on resting tension and U46619-induced tone. Expression of the ASIC subtypes was studied using real time semi-quantitative PCR (rt-PCR) methodology in pial arteries with brain tissue serving as reference.

Results: In HEPES-buffered solution, acidosis induced a pH-related relaxation of MCA and BA ring segments with the maximal effect observed with 4mM lactic acid (pH=6.56). In MCA ring segments amiloride increased relaxation (expressed in % of precontraction induced by application of 0.3 µM U46619) in moderate acidosis (but decreased relaxation in severe acidosis (pH<6.65; p<0.01 in each case)). Similar trends (though not statistically significant) were observed in BA ring segments. In Krebs buffer, acidosis-induced relaxation was largely suppressed in MCA and BA segments. Amiloride also lowered resting tension (MCA: 18.5±1.7% vs 9.7±1.7% in solvent control conditions; p<0.01 and BA: 16.4±1.7% vs 1.9±0.8% in solvent control conditions; p<0.01) and reduced the contractile response to U46619 in MCA segments (contraction in % reference obtained in 124mM K⁺ Krebs solution: 71.7±2.8% vs 80±2.7%; p<0.01) but not in BA segments. Using rt-PCR methodology we found ASIC-1 and ASIC-2 mRNA in the MCA vessel wall, although at a markedly lower level than in brain tissue. In addition, ASIC-4 mRNA was detected in brain tissue but not in the vessel wall.

Conclusion: The results suggest expression in rat cerebral arteries of ASIC1 and ASIC2 which appear to modulate acidosis-induced vasomotor reactivity. In moderate acidosis ASICs may confer a propensity to contraction but may increase relaxation in marked acidosis. In addition, ASICs appear to be involved in regulation of tone in the physiological pH range. Moreover, the differences in HEPES and Krebs buffer strongly point to an important role of intracellular pH changes in extracellular acidosis due to accumulation of a fixed acid.