Essential opposite roles of ERK and akt signaling in cardiac steroid-induced increase in heart contractility

Interaction of cardiac steroids (CS) with the Na⁺, K⁺-ATPase elicits, in addition to inhibition of the enzyme's activity, the activation of intracellular signaling such as extracellular signalregulated (ERK) and protein kinase B (Akt). We hypothesized that the activities of these pathways are involved in CS-induced increase in heart contractility. This hypothesis was tested using in vivo and ex vivo wild type (WT) and sarcoplasmic reticulum Ca(2+) atpase1a-deficient zebrafish (accordion, acc mutant) experimental model. Heart contractility was measured in vivo and in primary cardiomyocytes in WT zebrafish larvae and acc mutant. Ca²⁺ transients were determined ex vivo in adult zebrafish hearts. CS dose dependently augmented the force of contraction of larvae heart muscle and cardiomyocytes and increased Ca²⁺ transients in WT but not in acc mutant. CS in vivo increased the phosphorylation rate of ERK and Akt in the adult zebrafish heart of the two strains. Pretreatment of WT zebrafish larvae or cardiomyocytes with specific MAPK inhibitors completely abolished the CS-induced increase in contractility. On the contrary, pretreatment with Akt inhibitor significantly enhanced the CS-induced increase in heart contractility both in vivo and ex vivo without affecting CS-induced Ca²⁺ transients. Furthermore, pretreatment of the acc mutant larvae or cardiomyocytes with Akt inhibitor restored the CS-induced increase in heart contractility also without affecting Ca²⁺ transients. These results support the notion that the activity of MAPK pathway is obligatory for CS-induced increases in heart muscle contractility. Akt activity, on the other hand, plays a negative role, via Ca²⁺ independent mechanisms, in CS action. These findings point to novel potential pharmacological intervention to increase CS efficacy.