ACTIVITY OF SMOOTH-MUSCLE PHOSPHATASE-1 AND PHOSPHATASE-2A IN RABBIT BASILAR ARTERY IN VASOSPASM

Background and Purpose Subarachnoid hemorrhage frequently leads to a l ong-term cerebral artery narrowing called vasospasm. Recently, the inv olvement of myosin light chain kinase has been found in experimental v asospasm in our laboratory. We therefore measured the activity of seri ne/threonine protein phosphatases 1 and 2A in the rabbit basilar arter y in vasospasm and in vasocontraction to study their role, particularl y in regard to vasospasm compared with vasocontraction. Methods Vasosp asm was produced in the rabbit basilar artery by a two-hemorrhage meth od. Vasocontraction was induced by local application of KCl or seroton in to the rabbit basilar artery after a transclival exposure. The cont rol animals were treated with saline instead of fresh blood. Serine/th reonine protein phosphatase activity in the basilar artery was assayed with the use of [P-32]phosphorylase-a as a substrate; protein phospha tase 1 activity was evaluated as protein phosphatase activity in the p resence of 1 nmol/L okadaic acid, whereas protein phosphatase 2A activ ity was assessed as protein phosphatase activity inhibited by 1 nmol/L okadaic acid. Results Values of mean activity of protein phosphatase 1 in myofibrillar extract were 3.58+/-0.26 nmol/min per milligram in t he control group, 3.22+/-0.12 nmol/min per milligram in the spastic gr oup on day 2, and 3.01+/-0.16 nmol/min per milligram in the spastic gr oup 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. Valu es of mean activity of protein phosphatase 2A in cytosolic extract wer e 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+/-0.1 7 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 phosphata se 2A in cytosolic extract in the KCl and serotonin groups. Conclusion s Protein phosphatase 1 in myofibrillar extract is reported to catalyz e the dephospholylation of myosin light chain and calponin, whereas pr otein phosphatase 2A in cytosolic extract catalyzes the dephosphorylat ion of calponin and caldesmon. In addition, the phosphorylation of cal ponin and caldesmon results in the loss of their ability to inhibit sm ooth muscle contraction. Therefore, the significant decrease in activi ty of protein phosphatases 1 and ZA in vasospasm may result in uninter rupted vascular smooth muscle contraction by the preservation of phosp horylation of not only myosin light chain but also calponin and caldes mon.