The phosphorylation of rat cardiac microsomal proteins was investigated with 
special attention to the effects of okadaic acid (an inhibitor of protein 
phosphatases), inhibitor 2 of protein phosphatase 1 and inhibitor of cyclic 
AMP-dependent protein kinase (protein kinase A). The results showed that okadaic 
acid (5 microM) modestly but reproducibly augmented the protein kinase 
A-catalyzed phospholamban (PLN) phosphorylation, although exerted little effect 
on the calcium/calmodulin kinase-catalyzed PLN phosphorylation. Microsomes 
contained three other substrates (M(r) 23, 19 and 17 kDa) that were 
phosphorylated by protein kinase A but not by calcium/calmodulin kinase. The 
protein kinase A-catalyzed phosphorylation of these three substrates was 
markedly (2-3 fold) increased by 5 microM okadaic acid. Calmodulin was found to 
antagonize the action of okadaic acid on such phosphorylation. Protein kinase A 
inhibitor was found to decrease the protein kinase A-catalyzed phosphorylation 
of microsomal polypeptides. Unexpectedly, inhibitor 2 was also found to markedly 
decrease protein kinase A-catalyzed phosphorylation of phospholamban as well 
these other microsomal substrates. These results are consistent with the views 
that protein phosphatase 1 is capable of dephosphorylating membrane-associated 
phospholamban when it is phosphorylated by protein kinase A, but not by 
calcium/calmodulin kinase, and that under certain conditions, 
calcium/calmodulin-stimulated protein phosphatase (protein phosphatase 2B) is 
also able to dephosphorylate PLN phosphorylated by protein kinase A. 
Additionally, the observations show that protein phosphatase 1 is extremely 
active against the three protein kinase A substrates (M(r) 23, 19 and 17 kDa) 
that were present in the isolated microsomes and whose state of phosphorylation 
was particularly affected in the presence of dimethylsulfoxide. Protein 
phosphatase 2B is also capable of dephosphorylating these three substrates.