LAZAROIDS AND DEFEROXAMINE ATTENUATE THE INTRACELLULAR EFFECTS OF OXYHEMOGLOBIN IN VASCULAR SMOOTH-MUSCLE

Objectives: While it is probable that the cerebrovascular spasm which occurs after subarachnoid haemorrhage results from the action of haemo globin, the mechanism of that process remains unclear. These studies w ere thus designed to test the hypothesis that the action of oxyhaemogl obin results from the iron-catalyzed formation of free radicals and su bsequent lipid peroxidation resulting in intracellular changes in the second messengers for contraction. Methods: Levels of intracellular ca lcium and of inositol (1,4,5)-trisphosphate were measured in cultured vascular smooth muscle cells derived from primate cerebral arteries. C ontractility of rings of canine cerebral vessels were examined in vitr o using standard pharmacological techniques. Vessels in spasm were obt ained from the ''two haemorrhage'' canine model and the presence of va sospasm was confirmed angiographically. In each case, the effects of o xyhaemoglobin and sometimes of free radicals generated from iron salts were examined in the presence and in the absence of free-radical scav enging agents or the iron chelating agent, deferoxamine. Results: Oxyh aemoglobin produces a slowly-developing sustained contraction of arter ial rings which is accompanied by a sustained elevation of intracellul ar calcium. It also produces a transient but significant elevation of inositol (1,4,5)-trisphosphate, but this is not correlated with the de velopment of sustained constriction. Deferoxamine and the lazaroid com pounds U-74389G and U-83836E were effective in preventing the effects of oxyhaemoglobin and free radicals in the models tested, although in vessels in spasm, all effects were smaller. Conclusions: The present s tudy provides results which are consistent with the hypothesis that th e actions of haemoglobin on vascular smooth muscle are mediated by the formation of free radicals which subsequently affect intracellular ca lcium concentrations. This also implies that agents which impair free radical production or other processes leading to iron-catalyzed lipid peroxidation, are of potential value in cerebrovascular spasm.