INTRACELLULAR MECHANISMS INVOLVED IN THE RESPONSES OF CEREBROVASCULARSMOOTH-MUSCLE CELLS TO HEMOGLOBIN

An investigation was undertaken of the mechanism by which oxyhemoglobi n and its analog methemoglobin might cause cerebrovascular spasm. The effect of these compounds on the levels of intracellular inositol trip hosphate and calcium in cultured primate cerebrovascular smooth-muscle cells and the contractile action of oxyhemoglobin on isolated rings o f primate cerebral arteries were also examined. Oxyhemoglobin, but not methemoglobin, produces a transient but highly significant increase i n the intracellular levels of inositol triphosphate. Intracellular cal cium levels in these cells are increased by thrombin, aluminum tetrafl uoride, and oxyhemoglobin, and the sustained elevation in intracellula r calcium is prevented by ethyleneglycol tetra-acetic acid and the pho spholipase C inhibitor neomycin. Removal of the oxyhemoglobin after as long as 48 hours' incubation with this compound allowed cells to rapi dly reduce their intracellular calcium levels to near normal. Oxyhemog lobin produced contractions of isolated rings of both normal and spast ic cerebral arteries, although the response of spastic vessels was sig nificantly smaller. This effect was inhibited by neomycin. The additio n of neomycin relaxed arteries that were contracted with oxyhemoglobin , 5-hydroxytryptamine, or potassium chloride. lt is thus likely that a ctivation of phospholipase C is a critical step in the development of vasospasm, but the transient nature of the response to inositol tripho sphate suggests that the sustained contraction may arise from other ph ospholipase C-dependent mechanisms.