Change in Ca²⁺ Sensitivity of Cerebrovascular Smooth Muscle in Experimental Chronic Cerebral Vasospasm

Abstract

The mechanism of chronic cerebral vasospasm was investigated by examining the relation between cytosolic Ca²⁺ level ([Ca²⁺]_i) and muscle contraction in vascular smooth muscle after experimental vasospasm produced by the two-hemorrhage method in the canine basilar artery. [Ca²⁺]_i and tension were recorded simultaneously with a fluorimeter using the arterial strips with a Ca²⁺ indicator, fura-2. High K⁺ concentration (72.4 mM) and U-46619 (thromboxane A₂ analogue, 10^-8 M) were used as stimulants. The increase in contractile tension per [Ca²⁺]_i, per unit cross-sectional area was calculated, and compared for the vasospasm and the control groups. The increase in [Ca²⁺]_i in the vasospasm group was smaller than that in the control group, so the increase in tension per [Ca²⁺]_i per unit sectional area was larger in the vasospasm group. This value was much larger for the contraction induced by U-46619 than that induced by high K⁺ concentration in both groups. These results suggest that there is an increase in the basal Ca²⁺ sensitivity of the contractile elements in vasospastic vessels. In addition, the U46619-induced increase in Ca²⁺ sensitivity seemed to be augmented in the vasospasm group.