CHANGES IN RAT ATRIAL ANF GRANULES INDUCED BY HINDLIMB SUSPENSION

It is well known that the heart releases a factor called ANF (atrial n atriuretic factor) or ANP (atrial natriuretic peptide) capable of indu cing 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, f or example, from enhanced venous return. However, the cellular events which occur after the stimulation remain to be clarified. The aim of t his investigation was to study the intra-cellular events preceding the ANF release, using the rat hindlimb suspension as model of stimulatio n. In this model, Wistar rats were placed in a 30 degrees anti-orthost atic position and a blood shift towards the heart was obtained. Differ ent durations (1/4 h, 1/2 h, 3/4 h, 1 h, 2 h and 6 h) were studied. Th e ANF plasma level was investigated by Radio Immune Assay and granule immunoreactivity was measured by counting gold particles on micrograph s. The ANF plasma level was significantly increased (+60%) after 1 h o f suspension. The response was transient and then decreased to basal v alues. Morphological criteria established at the beginning of this stu dy, and measured throughout the experiment, were found transiently mod ified after suspension. The surface of the perinuclear area was transi tory enlarged by 36% 30 min after suspension. Moreover, in the same ti me immunoreactivity of the secretory granules was enhanced without cha nges in granule size. These results suggest an increase in the ANF syn thesis and storage in the granules during the stimulation. However, th e cellular regulatory mechanism of the ANF synthesis which could expla in the transitory aspect of these events, requires further investigati on.