Activity of smooth muscle phosphatases 1 and 2A in rabbit basilar artery in vasospasm

Subarachnoid hemorrhage frequently leads to a long-term cerebral artery narrowing called vasospasm. Recently, the involvement of myosin light chain kinase has been found in experimental vasospasm in our laboratory. We therefore measured the activity of serine/threonine protein phosphatases 1 and 2A in the rabbit basilar artery in vasospasm and in vasocontraction to study their role, particularly in regard to vasospasm compared with vasocontraction.

Vasospasm was produced in the rabbit basilar artery by a two-hemorrhage method. Vasocontraction was induced by local application of KCl or serotonin to the rabbit basilar artery after a transclival exposure. The control animals were treated with saline instead of fresh blood. Serine/threonine protein phosphatase activity in the basilar artery was assayed with the use of [32P]phosphorylase-a as a substrate; protein phosphatase 1 activity was evaluated as protein phosphatase activity in the presence of 1 nmol/L okadaic acid, whereas protein phosphatase 2A activity was assessed as protein phosphatase activity inhibited by 1 nmol/L okadaic acid.

Values of mean activity of protein phosphatase 1 in myofibrillar extract were 3.58 +/- 0.26 nmol/min per milligram in the control group, 3.22 +/- 0.12 nmol/min per milligram in the spastic group on day 2, and 3.01 +/- 0.16 nmol/min per milligram in the spastic group on day 4 (a significant decrease in protein phosphatase 1 activity in the spastic group on days 2 and 4). In contrast, these values did not show any significant changes in the KCl and serotonin groups. Values of mean activity of protein phosphatase 2A in cytosolic extract were 0.90 +/- 0.07 nmol/min per milligram in the control group, 0.75 +/- 0.10 nmol/min per milligram in the spastic group on day 2, and 0.62 +/- 0.17 nmol/min per milligram in the spastic group on day 4 (a significant reduction in protein phosphatase 2A in the spastic group on days 2 and 4). There was no evidence of significant changes of protein phosphatase 2A in cytosolic extract in the KCl and serotonin groups.

Protein phosphatase 1 in myofibrillar extract is reported to catalyze the dephosphorylation of myosin light chain and calponin, whereas protein phosphatase 2A in cytosolic extract catalyzes the dephosphorylation of calponin and caldesmon. In addition, the phosphorylation of calponin and caldesmon results in the loss of their ability to inhibit smooth muscle contraction. Therefore, the significant decrease in activity of protein phosphatases 1 and 2A in vasospasm may result in uninterrupted vascular smooth muscle contraction by the preservation of phosphorylation of not only myosin light chain but also calponin and caldesmon.