Influence of dietary omega-3 fatty acids on transmembrane signalling in rat submandibular salivary gland.

We have previously shown the incorporation of dietary omega-3 and omega-6 fatty acids from menhaden oil and corn oil, respectively, into membrane phospholipids of submandibular salivary gland (SMSG) of rat [Alam S. Q. and Alam B. S. (1988) Arch. Oral Biol. 33, 295-299]. We now demonstrate the influence of such incorporation on the regulation of G proteins and adenylate cyclase activity. Cholera toxin ribosylated three protein peptides (Mr 42,000, 44,000 and 46,000) to different extents in the two groups. We found 4.9- and 2.6-fold higher and 0.4-fold lower ribosylation of Mr 42,000, 44,000 and 46,000 peptides, respectively, in SMSG membranes of rats fed a diet containing 10% menhaden oil (group II) compared to those fed 10% corn oil (group I). Functional distinctions between different forms of these peptides are not known. Cholera toxin also exhibited radiolabelling of three peptides in the SMSG membranes from normal or fasting rats. In these membranes inhibitory G proteins were not detected by pertussis toxin dependent ADP ribosylation or by a low concentration of guanylyl 5-imidodiphosphate (10(-8) M), which selectively activates inhibitory G proteins which inhibit forskolin stimulated activity of adenylate cyclase. In group II membranes both basal and fluoride stimulated activities of adenylate cyclase were found to be significantly higher than the corresponding values in group I (P less than 0.02). In cholera toxin dependent ribosylated membranes of group I, basal and fluoride stimulated activities of adenylate cyclase were significantly higher than those obtained in the absence of cholera toxin (P less than 0.02). Surprisingly, corresponding values were found to be lower in ribosylated membranes of group II. This could be due either to conformational changes in heavily ribosylated G proteins, which alters coupling with the catalytic subunit of adenylate cyclase, or due to dissociation of excessive inhibitory beta gamma complex from alpha beta gamma complex upon the activation of G proteins.