TRANSFORMING GROWTH FACTOR-BETA-1 RAPIDLY ACTIVATES PHOSPHORYLASE IN A CALCIUM-DEPENDENT MANNER IN RAT HEPATOCYTES

Transforming growth factor-beta1 (TGF-beta1) rapidly activated phosphorylase in isolated rat hepatocytes (half-maximal rate of activation with approximately 0.1 ng/ml). Removal of Ca2+ from the external medium just before TGF-,81 addition markedly attenuated phosphorylase activation. TGF-beta1 (1 ng/ml) produced a small increase in [Ca2+]i (approximately 10% increase after 30 s), which appears sufficient to account for phosphorylase activation. These observations indicate that activation of the TGF-beta1 signal transduction system in hepatocytes is linked with a small increase in [Ca2+]i, and external Ca2+ may contribute in part to this increase.