Activin A and follistatin are normally present in relatively low amounts in the circulation. Heparin administration elicits a rapid and robust release of these proteins, although this phenomenon is poorly defined. In the present studies, the response to heparin administration was evaluated in the plasma of adult ewes in terms of whether it was dose-dependent, could be neutralized, was responsive to multiple stimulation, and the nature of the activin A and follistatin released. Activin A and follistatin were rapidly released by heparin in a dose-dependent manner (25, 100 or 250 IU/kg), with differences in the response as adjudged by peak concentration, timing of the peak and area under the curve. The heparin response could be blocked by pretreatment with protamine; conversely protamine injection alone (2 mg/kg) elicited release of follistatin but not activin A. Repeat administration of heparin at three-hourly intervals resulted in activin and follistatin responses to each injection, but each subsequent stimulation increased and extended the responses, consistent with saturation of the heparin clearance mechanism. Size exclusion chromatography of plasma samples confirmed that the majority of activin and follistatin released by heparin was a complex, whereas follistatin released by protamine was unbound. These data are consistent with a large pool of activin A and follistatin resident on extracellular matrices, with the rapid response implicating the vascular endothelium as the prime site of release following administration of these commonly used anticoagulant therapies.

A series of experiments were conducted in adult ewes to delineate the release profile of activin A and its relationship to other cytokines following an i.v. injection of the bacterial cell wall component, lipopolysaccharide (LPS). Following this challenge, plasma activin A increased rapidly and appeared to be released in a biphasic manner, slightly preceding the release of tumour necrosis factor-alpha (TNFalpha) and before elevation of interleukin (IL)-6 and follistatin levels. The concentration of activin A was correlated with body temperature during the response to LPS. A second experiment compared cytokine concentrations in matched blood and cerebrospinal fluid (CSF) samples. This revealed that activin A was not released centrally in the CSF following a peripheral LPS injection, nor was TNFalpha or the activin binding protein, follistatin, but IL-6 showed a robust elevation. In a third experiment, the stimulus for activin A release was examined by blocking prostaglandin synthesis. Flurbiprofen, a prostaglandin synthesis inhibitor, effectively attenuated the fever response to LPS and partly inhibited cortisol release, but the cytokine profiles were unaffected. Finally, the bioactivity of TNFalpha and/or IL-1 was blocked using soluble receptor antagonists. These treatments did not affect the initial release of activin A, but blockade of TNFalpha depressed the second activin peak. These studies define more rigorously the release of activin A into the circulation following acute inflammatory challenge. The response is rapid and probably biphasic, independent of prostaglandin- mediated pathways and does not depend upon stimulation by TNFalpha or IL-1. The data suggest that activin A release is an early event in the inflammatory cascade following the interaction of LPS with its cellular receptor.