The Role of Calcium in Insulin Action - V. Importance of Cyclic Guanosine 3':5' Monophosphate and Calcium Ions in Insulin Stimulation of Lipoprotein Lipase Activity and Protein Synthesis in Adipose Tissue

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Abstract
The effects of insulin on cyclic 3':5' guanosine monophosphate (c-GMP), 3H leucine incorporation into total fat cell protein and lipoprotein lipase activity have been studied in rat adipose tissue in vitro. The influence of changes in calcium concentrations on these processes has also been examined. With isolated fat cells incubated in the presence of calcium (1.25 mM), insulin at concentrations between 10-10,000 µU/ ml produced an immediate rise in fat cell c-GMP and stimulation of protein synthesis. Insulin between 100-10,000 µU/ ml also increased the c-GMP content and enhanced the activity of the enzyme lipoprotein lipase in intact fat pads incubated with the hormone in vitro. Both with isolated fat cells and with fat pads the effect of insulin on c-GMP were transient, reaching maximal concentration after 2 minutes and returning to basal values in another 2-3 minutes. On the other hand the stimulation of lipoprotein lipase activity and of protein synthesis induced by the hormone continued for at least another 90 minutes. Omission of exogenous calcium from the incubation medium did not significantly influence the magnitude of the insulin stimulated c-GMP level, protein synthesis and lipoprotein lipase activity, while depletion of intracellular calcium by preincubation with EGTA (ethyleneglycol bis tetra-acetic acid 1 mM) markedly inhibited these effects. On the other hand, the calcium ionophore A23187 (0.2 nmol/ml) and lanthanide (1 µmol/ml) added to the incubation medium did simulate the insulin effects, caused an increase in adipose tissue c-GMP and enhanced the protein synthesis and lipoprotein lipase activity. In conclusion we suggest that c-GMP might mediate insulin action on protein synthesis and activation of lipoprotein lipase enzyme although calcium ions are necessary for the nucleotide to induce these effects. Calcium ions might also play an important role in transmission of signals from insulin receptor to enzymes regulating c-GMP concentration in adipose tissue.

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