The invention provides methods for identifying modulators of cardiac troponin I activity as well as methods for using N-terminally truncated forms of cTnI to monitor the function of the circulatory system in general, and the heart in particular, of an organism such as man. Additionally, the invention provides methods of treating circulatory diseases, disorders or conditions, such as cardiac diseases and conditions, by administering a therapeutically effective amount of a modulator of the N-terminal structure of cTnI, such as a molecule capable of binding to the N-terminus of full-length cTnI, a molecule that modulates cTnI phosphorylation and/or N-terminal proteolytic cleavage, or by administering N-terminally truncated cTnI or a polynucleotide encoding that protein or polypeptide.