Antibodies binding to sites on the alpha-subunit of the (Na++K+)-ATPase increase cardiac contraction of both ventricular myocytes and mouse heart. In particular, antibodies binding to the RSATEEEPPNDD (SEQ ID NO: 1) or DVEDSYGQQWTYEQR (SEQ ID NO: 2) peptides (or isoforms/derivatives thereof) of the alpha-subunit of the (Na++K+)-ATPase, have been found to be highly inotropic. Both the antibodies and the peptides are important for the treatment of human heart failure and other contractile disorders.