Procollagen processing by procollagen C-proteinase (PCP) is an important step in collagen deposition. This reaction is stimulated by another glycoprotein, known as PCP enhancer. The objective of this study was to identify factors that regulate the expression of PCP enhancer in cardiac fibroblasts and examine possible correlation with collagen expression. Rat heart fibroblasts were cultured in the presence or absence of three known stimulators of collagen synthesis: ascorbic acid, TGF-beta, and aldosterone. The mRNA and protein levels of PCP enhancer and collagen type I were each assessed using Northern and Western blotting, respectively. Expression of PCP was assessed by RT-PCR and its activity in the culture media was determined using radioactive procollagen as the substrate. The levels of PCP enhancer mRNA increased 1.5- to 2-fold in response to ascorbate, TGF-beta, or aldosterone. This increase was paralleled by an up to fourfold increase in the level of the pro alpha1(I) collagen chain transcript and was accompanied by a marked increase in the levels of the respective proteins in the culture media. PCP activity in the culture media was also increased, apparently, without effect on its expression. These results indicate that expression of PCP enhancer in cultured rat heart fibroblasts is coordinated with that of collagen. The observed augmentation of PCP activity may be a consequence of the increase in the levels of PCP enhancer in the culture media.