Changes in rat atrial ANF granules induced by hindlimb suspension

It is well known that the heart releases a factor called ANF (atrial natriuretic factor) or ANP (atrial natriuretic peptide) capable of inducing rapid diuretic and natriuretic actions. This factor is stored in secretory granules mainly located in myocytes in both atria. The main secretory stimulus is the distention of the atrial cavity resulting, for example, from enhanced venous return. However, the cellular events which occur after the stimulation remain to be clarified. The aim of this investigation was to study the intra-cellular events preceding the ANF release, using the rat hindlimb suspension as model of stimulation. In this model, Wistar rats were placed in a 30 degrees anti-orthostatic position and a blood shift towards the heart was obtained. Different durations (1/4 h, 1/2 h, 3/4 h, 1 h, 2 h and 6 h) were studied. The ANF plasma level was investigated by Radio Immuno Assay and granule immunoreactivity was measured by counting gold particles on micrographs. The ANF plasma level was significantly increased (+60%) after 1 h of suspension. The response was transient and then decreased to basal values. Morphological criteria established at the beginning of this study, and measured throughout the experiment, were found transiently modified after suspension. The surface of the perinuclear area was transitory enlarged by 36% 30 min after suspension. Moreover, in the same time immunoreactivity of the secretory granules was enhanced without changes in granule size. These results suggest an increase in the ANF synthesis and storage in the granules during the stimulation. However, the cellular regulatory mechanism of the ANF synthesis which could explain the transitory aspect of these events, requires further investigation.