The effect of aldosterone on phospholipid and phosphoinositide metabolism in rat submandibular gland

Abstract

It is well known that aldosterone stimulates Na + transport in epithelia, including rat submandibular salivary gland. Aldosterone affects cells by causing nuclear-directed synthesis of aldosterone-induced protein. That protein then exerts its effect by: (1) phospholipid remodelling of the cell membrane (increasing permeability to Na +); (2) stimulating the Na + pump; or (3) increasing ATP production by mitochondria. The purpose of this study was to determine if aldosterone stimulates membrane phospholipid or phosphoinositide turnover in the rat salivary gland and duct. Paired salivary glands were removed from 12 rats and main salivary-gland excretory ducts were removed from 16 rats. One-half of each group was treated with aldosterone (10 −6 M) while being incubated in Ringer's containing [ 32P]-orthophosphate for 2 h at 37 °C. The other half (controls) received no aldosterone during incubation. Phospholipids were extracted, separated and detected by thin-layer chromatography, autoradiography, and quantified by liquid scintillation counting. Results were expressed in dis/min (μM PO 4) −1 (h) −1. The turnover of membrane phospholipid fractions or percentage fractions was not changed significantly by aldosterone ( p > 0.10). Paired submandibular salivary glands were also removed from 12 more rats and main submandibular excretory ducts from 18 rats. Tissue was incubated in 2 ml of Ringer's solution containing [ 3H]-myo-inositol for 2 hat 37 °C. Tissue was then placed in Ringer's containing LiCl and the experimental groups were challenged with aldosterone for 20 min at 37 °C. The tissues were then homogenized, and the inositol phosphate fractions quantified by column chromatography and liquid scintillation counting. Results were expressed as dis/min (μM PO 4) −1 (h) −1. Aldosterone did not stimulate formation of glycerophosphoinositol, inositol phosphate, inositol biphosphate, or inositol trisphosphate (IP 3) fractions in duct tissue. However, aldosterone significantly increased IP 3 in the glands of the experimental group (30.3 ± 3.5) compared to the control group (23.5 ± 2.8) ( p < 0.025). It is concluded that in the rat salivary gland, aldosterone does not exert an action on phospholipid metabolism, but does stimulate formation of IP 3. This suggests that aldosterone may exert its effect on Na + transport by stimulating the phosphoinositide pathway in salivary epithelia.