Essential Opposite Roles of ERK and Akt Signaling in Cardiac Steroid‐Induced Increase in Heart Contractility

Abstract

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 ERK and 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 wt and SERCA1a 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 and 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.Support or Funding InformationThis work was supported by grants from the Ministry of Trade and Industry (NOFAR), Israel and the Szolts Foundation, The Hebrew University of Jerusalem, and the Walter and Greta Chair in Heart Studies (to D.L).