Protein intake affects levels of G-protein subunits Gαi2, Gαi3, and Gβ in rat glomerular membranes

Protein intake affects levels of G-protein subunits G αi2 , G αi3 , and G β in rat glomerular membranes. Using toxin-catalyzed ADP-ribosylation and specific immunoblots we examined whether the mass of G-protein subunits, G αs , G αi (includes G αi2 , G αi3 , and G α0 ) and G αβ , in glomerular membranes was altered by dietary protein intake. ADP-ribosylation catalyzed by cholera toxin (CT) or pertussis toxin (PT) detected significant amounts of G αs or G αi in glomerular membranes from rats fed a low (6% casein) or a high (40% casein) protein diet. There was no significant difference in G αs content between glomerular membranes from low or high protein-fed rats. However, the amounts of G αi were significantly lower in glomerular membranes from rats fed a high protein diet when compared to glomerular membranes from rats fed a low protein diet. Two isoforms of immunoreactive G αs , 45 and 52 kDa proteins, were detected in glomerular membranes. The predominant isoform of G αs was a 52 kDa protein. As with ADP-ribosylation, immunoblots snowed no significant difference in G αs content between glomerular membranes obtained from the two diet groups of rats. Also, immunoreactive G αi2 G αi3 and G β were present in glomerular membranes. The mass of G αi2 and G αi3 was significantly lower in glomerular membranes of rats fed a high protein diet than in those of rats fed a low-protein diet. The decreased mass of total G αi , that is G αi2 and G αi3 , was comparable to that seen with PT-catalyzed ADP-ribosylation. By contrast, G β content was significantly greater in glomerular membranes of rats fed a high protein diet than in those of rats fed a low protein diet. G α0 was not immunodetected in glomerular membranes. In addition, any G-protein subunit examined in the present study was not detectable in cytosolic extracts of glomerular membranes. A decrease in G αi2 and G αi3 and an increase in G β may contribute in part to the greater production of eicosanoids by glomeruli of rats fed a high protein diet.