Effects of angiotensin II (ANG II) on osmosensory transduction in rat supraoptic nucleus neurons

Action potential firing rate from magnocellular neurosecretory cells (MNCs), and the resulting vasopressin (VP, antidiuretic hormone) secretion from the neurohypophysis, correlate linearly with extracellular fluid (ECF) osmolality. Previous studies suggest that angiotensin II (Ang II) released centrally may enhance the sensitivity (gain) of this osmoregulatory reflex by increasing the slope of the relation between the firing rate of MNCs and ECF osmolality. The intrinsic osmosensitivity of MNCs is mediated by a cell volume-dependent regulation of stretch inactivated cation (SIC) channels. Therefore Ang II might increase gain by modulating the effects of ECF osmolality on cell volume, or the coupling between volume changes and SIC channel activation. Using digital imaging and morphometry, we found that rat MNCs change their volume osmometrically without volume regulation, and that Ang II does not change the relation between cell volume and ECF osmolality. However whole cell patch clamp analysis revealed that Ang II increases the apparent mechanosensitivity of SIC channels as quantified by the mechanosensitivity index (MI = conductance change/volume change) in isolated MNCs. The effect of Ang II on MI was inhibited by chelerythrine and by chelation of intracellular calcium ions with BAPTA. In contrast, phorbol 12-myristate 13-acetate (PMA) mimicked the effects of Ang II, indicating that protein kinase C (PKC) is involved in mediating the effects of the peptide on MI. The effects of Ang II and PMA on MI were accompanied by an increase in the density of actin filaments (F-actin) in the submembrane cortical cytoskeleton, as revealed by confocal microscopic analysis of fluorescent phalloidin staining. The effect of Ang II on F-actin density could also be blocked by chelerythrine. Moreover, disruption of F-actin with cytochalasin D prevented the increase in MI caused by Ang II, whereas stabilization of F-actin with jasplakinolide enhanced MI and occluded the effect of Ang II. We conclude that Ang II increases the osmosensitivity of MNCs by enhancing the mechanosensitivity of SIC channels through a PKC-dependent accumulation of F-actin in the submembrane cytoskeleton