Pertussis toxin blocks activin A-induced production of inositol phosphates in rat hepatocytes

Regulation of cellular and system function by activin

Activin is an important molecule that regulates hormonogenesis, cellular homeostasis (divide or die pathways), and differentiation programs (developmentally and in adult cells). The cellular mechanisms that integrate an activin signal into a physiological response include a binary receptor complex and tandem serine threonine kinases, intracellular signal mediators, and nuclear transcription factors. Activin antagonists (inhibins) and bioneutralizing binding proteins (follistatins) act as gating molecules to ensure accurate delivery of activin signals to cellular machinery. Correct execution of an activin cue intracellularly permits actions as fundamental as embryonic mesoderm development, neuronal survival, hematopoietic function, and reproductive cyclicity. Absent or incorrect activin signaling results in phenotypes as catastrophic as embryonic lethality, tumor formation, and infertility. The general ways in which a cell senses and responds to an activin signal will be reviewed in the first part of this paper. The role of this ligand in reproductive function will also be examined as a specific example of activin activity.

Effects of inhibin on activin A-Induced glucose metabolism in rat hepatocytes

This study was conducted to investigate the effects of inhibin on hepatic glucose metabolism. We have previously reported that activin A induced a dose-dependent glycogenolytic action on hepatocytes, and that 10(-9) M activin A induced a maximum glycogenolytic effect. Inhibin itself induced no increase or decrease in glucose output at any dose tested. At a concentration of 10(-10) M, inhibin was seen to inhibit 10(-9) M activin A-induced glucose output by 30% as compared to the control. In contrast to its inhibitory effect on the action of activin A, 10(-10) M and higher concentrations of inhibin did not inhibit angiotensin II-or vasopressin-induced glycogenolysis. We further investigated the mechanism of the inhibitory effect of inhibin on activin A-induced glycogenolysis, and found that 10(-10) M inhibin did inhibit the increase in cytoplasmic-free calcium concentration that was seen with 10(-9) M activin A.We also investigated the effects of inhibin on the activin A-induced production of inositol trisphosphates, and the results showed that 10(-10) M inhibin inhibited the activin A-induced production of inositol trisphosphates by 30% compared to the control. Furthermore, it was demonstrated that inhibin did not affect the binding of activin A to isolated hepatocytes. These data demonstrated that inhibin inhibited the activin A-induced glycogenolysis by inhibiting the increases of inositol trisphosphates and cytoplasmic free calcium concentrations.