Temperature-dependent effects of phosphodiesterase inhibitors for cardiovascular support in hypothermic patients - Effects on cellular elimination of cAMP and cGMP

Hypothermia-induced cardiac dysfunction (HCD), commonly seen during rewarming, is one of the feared complications of accidental hypothermia. Animal studies with phosphodiesterase (PDE) inhibitors and sodium nitroprusside have demonstrated positive effects on reversing HCD. These medications act by indirectly increasing intracellular levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) which lead to inodilation and vasodilation, respectively. Cyclic nucleotides are also regulated by active expulsion out of the cell by ATP-binding cassette (ABC) transporters, ABCC4 and ABCC5. We investigated therefore pharmacodynamic properties of three PDE3 inhibitors (milrinone, amrinone, levosimendan) and two PDE5 inhibitors (sildenafil, vardenafil) in hypothermic settings to explore their potential use in pharmacological treatment of HCD . We also explored the isolated effects of differentiated hypothermia on intracellular cAMP levels and cellular elimination of cyclic nucleotides. In vitro studies were performed to determine intracellular cAMP levels in hypothermia and the activity of recombinant PDE3 and PDE5, as well as ABCC4 and ABCC5 function, in temperatures down to 20°C. The degree of cellular access of each drug was assessed. Dose-response curves were created, which provided the foundation for calculating IC50 and Ki values. Our research showed that moderate hypothermia, but not severe hypothermia, increased intracellular cAMP. All drugs were able to cross the cell membrane at low temperatures. Cyclic nucleotide elimination by PDEs and ABC transporters exhibited temperature dependent reductions in activity. Both PDE3 and PDE5 inhibitors were able to inhibit cyclic nucleotide elimination through enzyme and transporter suppression below 30°C. As these target proteins exhibit temperature-dependent reduction in activity, there is a need for temperature-adjusted dosage reduction in future potential treatment algorithms