MYOSIN LIGHT-CHAIN PHOSPHATASE AND KINASE ABNORMALITIES IN FETAL SHEEP PULMONARY-HYPERTENSION

Inasmuch as smooth muscle contractile protein abnormalities may accoun t for the maintenance of a high pulmonary vascular resistance, we eval uated the pulmonary arterial myosin light chain kinase (MLCK) and phos phatase (MLCP) in normal and pulmonary hypertensive (PH) fetal sheep. In addition, aorta and vena cava MLCP and MLCK activities were also me asured. The MLCK activity (nanomoles/min/mg) was determined by the inc orporation of [P-32]PO4-3 to the 20-kD smooth muscle myosin light chai ns and the MLCP activity by assaying for the dephosphorylation of the 20-kD myosin light chain (MLCP-light chain) and heavy meromyosin (MLCP -HMM). The MLCP content was determined by Western blot analysis. PH wa s characterized by a significant increase in the right-to-left ventric ular wall weight ratio from 0.99 +/- 0.04 in the control to 1.52 +/- 0 .12 in the experimental group (p < 0.01). The pulmonary MLCP-light cha in and MLCP-HMM activities in the experimental group were 2.0 +/- 0.2 and 1.3 +/- 0.2 and significantly lower than in the control group valu es (3.8 +/- 0.5 and 2.5 +/- 0.3; p < 0.01). The MLCK activity was 9.6 +/- 1.2 in the control and 7.8 +/- 0.7 in the experimental fetal pulmo nary artery (p = NS). The activities of both enzymes in the aorta and vena cava samples were not altered by PH. The MLCP content in experime ntal animals (0.50 +/- 0.09 OD x mm(2)) was significantly lower than t hat for the control pulmonary tissue (1.72 +/- 0.42; p < 0.01), sugges ting that PH down-regulates pulmonary vascular MLCP expression. In con clusion, the maintenance of a high pulmonary vascular resistance in PH may be secondary to abnormalities in tissue content and/or activity o f MLCP.