Can second messenger systems cross-talk with each other?

Yes, cross-talk can occur between second messenger systems and may involve the convergence and/or divergence of signals. For example, for cAMP and calcium, if it is a mutually inhibitory interaction, cross-talk between the two can increase the range of agonist concentrations in which oscillations occur. If it is mutually activating, cross-talk decreases the oscillation range.  The interactions between pathways may reduce, amplify, or prevent the response. In the heart, cGMP acts as a regulator of the activity of cAMP-hydrolyzing PDEs. It acts such that the intracellular concentration of cGMP can influence the intracellular concentration of cAMP.