LOSS OF MYOCARDIAL CAPILLARY ENDOTHELIAL-CELL ALKALINE-PHOSPHATASE (ALP) ACTIVITY IN PRIMARY ENDOTHELIAL-CELL CULTURE

Primary cultures of rat myocardial capillary endothelial cells were es tablished and characterized. A range of typical endothelial cell-speci fic markers were retained in vitro. Cell kinetic studies in confluent endothelial-cell cultures in vitro revealed a roughly 50-fold increase in the proportion of cells in s-phase, indicating a very considerable shortening of cell turnover time, compared to in vivo conditions. Alk aline phosphatase enzyme activity and encoding mRNA are strongly expre ssed in myocardial capillary endothelial cells in vivo, but were not d etectable in vitro. This was true in cell cultures from two strains of rat, which revealed significantly different enzyme expression levels in vivo. In co-cultures of pericytes and endothelial cells, positive A LP enzyme reaction was detected in pericytes, which in vivo show only very weak enzyme reactivity. Treatment of cell cultures with less than or equal to 10 M retinoic acid had no effect in pure endothelial cell cultures, but did increase ALP expression of pericytes in co-cultures . The observation of a loss of endothelial ALP expression in vitro sup ports other in vitro as well as our own in vivo observations, indicati ng a negative correlation of ALP expression and proliferative activity of endothelial cells.