The role of insulin, glucagon, and cAMP in the regulation of hepatocyte guanylate cyclase activity.

Rat liver regeneration is regulated by a humoral signal that includes insulin and a sustained elevation in glucagon. The intracellular response is characterized by a rise in cAMP as well as altered cGMP metabolism, i.e. increased particulate guanylate cyclase activity. To evaluate the role of hormones in the regulation of guanylate cyclase during liver regeneration, the enzyme activity of primary cultures of rat hepatocytes was examined. Hepatocytes were maintained for 22 h in medium containing various combinations of insulin, glucagon, and cAMP. The cells were then harvested and homogenized and the guanylate cyclase activity was assessed in vitro. Hepatocytes maintained in 100 nM insulin exhibited a 42% (p < 0.001) increase in guanylate cyclase activity when compared to cells cultured in medium alone. Incubation with glucagon (100 nM) produced a 52% (p < 0.01) rise. In the presence of insulin (100 nM), culturing with as little as 5 nM glucagon resulted in increased activity, and 100 nM glucagon produced a 161% (p < 0.001) rise above cultures maintained in insulin alone. Thus, the combination of the two hormones produced an effect that was significantly (p < 0.01) greater than additive. Dibutyryl cAMP and 8-bromoadenosine 3':5'-monophosphoric acid were at least as effective as glucagon; the enzyme activity of cells maintained in 5 microM N6,02'-dibutyryl adenosine 3':5'-monophosphoric acid and 100 nM insulin was 243% (p < 0.001) above those in insulin alone. The findings suggest that the activity of hepatocyte guanylate cyclase is regulated by a synergistic action of insulin and glucagon and that positive interactions between the two cyclic nucleotide second messenger systems exist.