The invention provides methods for treating, ameliorating or protecting (preventing) an individual or a patient having or at risk of having heart disease or heart failure, or decreased cardiac function, comprising: providing a cyclic adenosine monophosphate-incompetent (cAMP-incompetent) adenylyl cyclase type 6 (AC6) protein or polypeptide (also called “an AC6mut”), or an AC6mut-encoding nucleic acid or a gene operatively linked to a transcriptional regulatory sequence.