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Enhanced proliferation of adventitial fibroblasts is a major contributor to the structural remodeling of the pulmonary artery (PA) that occurs during hypoxia-induced pulmonary hypertension. The mechanisms responsible for the exuberant growth of fibroblasts are unknown; however, protein kinase C (PKC) isozymes have previously been shown to be important in the enhanced growth properties of immature PA fibroblasts. We tested the hypotheses that PA adventitial fibroblasts from neonatal calves exposed chronically to hypoxia after birth would express augmented growth responses compared with fibroblasts from the control adventitia and that these properties would be associated with selective changes in expression of PKC isozymes. We studied the effects of serum, purified mitogens, and hypoxia on the growth of aggregate populations of fibroblasts isolated from the PA of neonatal control calves (Neo-C) and calves chronically exposed to hypoxia for 2 wk beginning on Day 1 of life (Neo-Hyp). Neo-Hyp fibroblasts demonstrated higher proliferative capabilities than did Neo-C cells in response to all the stimuli tested. Importantly, hypoxia was found to act synergistically with peptide mitogens (platelet-derived growth factor, basic fibroblast growth factor, insulin-like growth factor-I) to stimulate growth in Neo-Hyp but not in Neo-C cells. Using PKC-isozyme nonselective and selective inhibitors and immunoblot analysis, we found differences in utilization of PKC isozymes in Neo-Hyp and Neo-C fibroblasts and have identified PKC-βI and -ζ as key contributors to the augmented growth of Neo-Hyp fibroblasts. Although the activity of PKC-βI and -ζ isozymes was increased by hypoxia in serum-deprived Neo-C and Neo-Hyp fibroblasts, under normoxia, quiescent Neo-Hyp fibroblasts had higher PKC-ζ-specific activity than did Neo-C cells. These results suggest that neonatal PA adventitial fibroblasts acquire new growth properties in the setting of hypoxia- induced pulmonary hypertension and that the augmented proliferative characteristics of the Neo-Hyp fibroblasts might be associated with changes in specifc PKC isozyme expression and activation patterns.