Dynamics of calmodulin and cyclic AMP phosphodiesterase in plasma membranes of rat livers and ascites hepatomas

Abstract

1. Plasma membranes from ascites hepatoma cells (AH-7974, AH-130) contained much smaller amounts of calmodulin (about half) and cyclic AMP phosphodiesterase (about one-third) compared to plasma membranes of rat liver. 2. Some of calmodulin molecules in liver plasma membranes were released by repeated washing. The ‘washed’ liver plasma membranes showed the presence of specific binding sites for externally added calmodulin molecules (bovine brain) ( N = 140 pmol/mg protein, K d = 7.9 · 10 −8 M). The calmodulin content of AH-7974 plasma membranes was not reduced by repeated washing. The binding of calmodulin to the ‘washed’ AH-7974 plasma membranes was only of nonspecific nature with negative cooperativity. 3. Plasma membranes (liver and AH-7974) appeared to contain both calmodulin-dependent and calmodulin-independent phosphodiesterase, but the stimulation by externally added Ca 2+ plus calmodulin was rather small. Externally added calmodulin-dependent phosphodiesterase (bovine brain) was bound more to ‘washed’ liver plasma membranes than to ‘washed’ AH-7974 plasma membranes. Newly bound phosphodiesterase appeared to be more sensitive to the stimulation by Ca 2+ plus calmodulin in ‘washed’ hepatoma plasma membranes than in ‘washed’ liver plasma membranes. 4. Preincubation of ‘washed’ plasma membranes (liver and hepatoma) with calmodulin did not affect the binding of phosphodiesterase, but the sensitivity of phosphodiesterase to the stimulation by Ca 2+ plus calmodulin in hepatoma plasma membranes was lost.