EQUIBIAXIAL STRAIN AND STRAIN-RATE STIMULATE EARLY ACTIVATION OF G-PROTEINS IN CARDIAC FIBROBLASTS

Cardiac fibroblasts are responsible for the production of the extracel lular matrix of the heart, with alterations of fibroblast function imp licated in myocardial infarction and cardiac hypertrophy. Here the rol e of heterotrimeric GTP-binding proteins (G proteins) in the mechanotr ansduction of strain in rat cardiac fibroblasts was investigated. Cell s in an equibiaxial stretch device were incubated with the photoreacti ve GTP analog azidoanalido [alpha-P-32]GTP (AAGTP) and were subjected to various regimens of strain. Autoradiographic analysis showed a 42-k Da protein labeled for cells exposed to 12 cycles of 3% strain or 6 cy cles of 6% strain over 60 s (strain rate of 1.2%/s), whereas 6 cycles of 3% strain (0.6%/s) elicited no measurable response. To further inve stigate the role of strain rate, a single 6% cycle over 10 or 60 s (1. 2% and 0.2%/s, respectively) was applied, with the more rapid cycle st imulating AAGTP binding, whereas the lower strain rate showed no respo nse. In cells subjected to a single 6% cycle/10 s, immunoprecipitation identified the AAGTP-labeled 42-kDa band as the G protein subunits G alpha(q) and G alpha(i1). These results demonstrate that G protein act ivation represents one of the early mechanotransduction events in card iac fibroblasts subjected to mechanical strain, with the rate at which the strain is applied modulating this response.