Calcium homeostasis is critical for cardiac myocyte function and must be tightly regulated. The guiding hypothesis of this study is that a carboxyl-terminal cleavage product of the cardiac L-type calcium channel (Ca_V1.2) autoregulates expression. First, we confirmed that the Ca_V1.2 C terminus (CCt) is cleaved in murine cardiac myocytes from mature and developing ventricle. Overexpression of full-length CCt caused a 34±8% decrease of Ca_V1.2 promoter activity, and truncated CCt caused an 80±3% decrease of Ca_V1.2 promoter (n=12). The full-length CCt distributes into cytosol and nucleus. A deletion mutant of CCt has a greater relative affinity for the nucleus than full-length CCt, and this is consistent with increased repression of Ca_V1.2 promoter activity by truncated CCt. Chromatin immunoprecipitation analysis revealed that CCt interacts with the Ca_V1.2 promoter in adult ventricular cardiac myocytes at promoter modules containing Nkx2.5/Mef2, C/EBp, and a cis regulatory module. The next hypothesis tested was that CCt contributes to transcriptional signaling associated with cellular hypertrophy. We explored whether fetal cardiac myocyte Ca_V1.2 was regulated by serum in vitro. We tested atrial natriuretic factor promoter activity as a positive control and measured the serum response of Ca_V1.2 promoter, protein, and L-type current (I_Ca,L) from fetal mouse ventricular myocytes. Serum increased atrial natriuretic factor promoter activity and cell size as expected. Serum withdrawal increased Ca_V1.2 promoter activity, mRNA, and I_Ca,L. Moreover, serum withdrawal decreased the relative nuclear localization of CCt. A combination of promoter deletion mutant analyses, and the response of promoter mutants to serum withdrawal support the conclusion that CCt, a proteolytic fragment of Ca_V1.2, autoregulates Ca_V1.2 expression in cardiac myocytes. These data support the novel mechanism that a mobile segment of Ca_V1.2 links Ca handling to nuclear signaling.