Regulation of fibrillar collagen types I and III and basement membrane type IV collagen gene expression in pressure overloaded rat myocardium. Left ventricular hypertrophy is based on cardiac myocyte growth. The hypertrophic process can be considered heterogeneous based on whether it also includes a remodeling and accumulation of fibrillar types I and III collagens that are responsible for impaired myocardial stiffness. In the heart, the messenger RNA (mRNA) for fibrillar collagen types I and III has been detected only in cardiac fibroblasts, whereas mRNA for basement membrane collagen type IV is present in both fibroblasts and myocytes. We studied the early and long-term expression of these collagenous proteins in rat myocardium after abdominal aortic banding with renal ischemia. Complementary DNA probes for rat pro-alpha 2 (I), mouse type III and mouse type IV collagens, and chicken beta-actin were used. Northern and dot blot analysis on total RNA extracted from left ventricular tissue indicated a sixfold increase in steady-state levels of mRNA for collagen type I on day 3 of abdominal aortic banding, which had declined to control levels by day 7 where it remained rather constant at 4 and 8 weeks. Type III collagen showed a similar pattern of gene expression after banding. mRNA levels for type IV collagen, on the other hand, were elevated on day 1 after banding, returning to control at day 7 and remaining constant. Actin mRNA levels also increased on day 1 of banding, followed by a rapid return to control levels. Monospecific antibody to types I and III collagens and immunofluorescent light microscopy on frozen sections of the myocardium revealed that at 1 week after banding, the distribution and density of these collagens were similar to those of control animals. 