Mechanism underlying mannitol-induced relaxation in isolated monkey cerebral arteries

The addition of mannitol (25, 50, and 100 mM) increased osmotic pressures of the bathing media from 293 to 317, 340 and 383 mosmol, respectively, and elicited a dose-related relaxation in monkey cerebral artery strips precontracted with K+ or prostaglandin F2 alpha. This relaxation was attenuated by ouabain, amiloride, catalase and oxyhemoglobin but was not influenced by superoxide dismutase and indomethacin. Combined treatment of the strips exposed to ouabain and amiloride with catalase produced an additional inhibition. H2O2 produced a contraction, which was abolished by catalase. Replacement of entire NaCl with choline chloride markedly suppressed the mannitol-induced relaxation. Relaxations caused by the addition of hypertonic NaHCO3 (25 and 50 mM) were also attenuated by ouabain and amiloride but were unaffected by catalase and oxyhemoglobin. It may be concluded that the mannitol-induced cerebroarterial relaxation is associated with an activation of the electrogenic Na+ pump and the Na(+)-H+ exchange and probably with scavenging of hydroxyl radicals responsible for the arterial contraction. On the other hand, hypertonic NaHCO3 relaxes monkey cerebral arteries, possibly due to an activation of the Na+ pump and the Na(+)-H+ exchange.