We have observed recently that experimental
renal failure in the rat is accompanied by increases in circulating concentrations of the
cardiotonic steroid,
marinobufagenin (MBG), and substantial cardiac
fibrosis. We performed the following studies to examine whether MBG might directly stimulate cardiac fibroblast
collagen production. In vivo studies were performed using the 5/6th
nephrectomy model of experimental
renal failure (PNx), MBG infusion (MBG), PNx after immunization against MBG, and concomitant PNx and
adrenalectomy. Physiological measurements with a Millar
catheter and immunohistochemistry were performed. In vitro studies were then pursued with cultured isolated cardiac fibroblasts. We observed that PNx and MBG increased MBG levels, blood pressure, heart size, impaired diastolic function, and caused cardiac
fibrosis. PNx after immunization against MBG and concomitant PNx and
adrenalectomy had similar blood pressure as PNx but less
cardiac hypertrophy, diastolic dysfunction, and cardiac
fibrosis. MBG induced increases in procollagen-1 expression by cultured cardiac fibroblasts at 1 nM concentration. These increases in
procollagen expression were accompanied by increases in
collagen translation and increases in procollagen-1
mRNA without any demonstrable increase in procollagen-1 protein stability. The stimulation of fibroblasts with MBG could be prevented by administration of inhibitors of
tyrosine phosphorylation, Src activation,
epidermal growth factor receptor transactivation, and N-acetyl
cysteine. Based on these findings, we propose that MBG directly induces increases in
collagen expression by fibroblasts, and we suggest that this may be important in the cardiac
fibrosis seen with experimental
renal failure.