H+ ions acutely affect cardiac excitation and contraction, but their long-term effect on gene expression is unknown. This is despite in vitro evidence for the pH-sensitivity of transcription factor (TF) binding to DNA. Major remodelling of cardiac pH regulation can occur in disease states, but little is known about the mechanisms that regulate nucleoplasm pH (pHnuc), and how these could be exploited to control gene expression. Previously (1), we have shown that pHnuc dynamics can be imaged using the DNA-binding dye Hoechst 33342. Nuclear pores allow the passage of medium-sized molecules (e.g. calcein), but H+ ions must first bind to mobile buffers in order to gain access to the nucleoplasm. Fixed H+ buffering residing in the nucleus of permeabilised cells was estimated to be very weak on the basis of the large amplitude of pHnuc transients evoked by photolytic H+-uncaging or exposure to weak acids/bases. Consequently, the majority of nuclear pH buffering is sourced from the cytoplasm in the form of mobile buffers, such as diffusible histidyl dipeptides (HDPs) present in abundance in adult cardiomyocytes, but detected at lower levels in neonatal or adult failing hearts. Low nuclear pore H+ permeability and weak nucleoplasm H+ buffering can allow pHnuc to change relative to cytoplasmic pH (pHcyt) under modest acid-base fluxes. Indeed, under certain stress conditions typically associated with hypertrophic remodelling, pHnuc can change substantially, with little change in pHcyt. For example, nuclear Ca2+ release evoked by inositol-1,4,5-trisphosphate signalling stably acidifies the nucleus. This novel pH-Ca2+ interaction is weakest in cells with low HDP content, but can be restored by long-term incubation with HDP precursors. We hypothesise that nuclear pH-Ca2+ coupling arises as a result of the diffusive exchange of Ca2+ for H+ ions through nuclear pores aboard HDPs. This mode of transport takes please because HDPs bind Ca2+ and H+ ions in a competitive manner (2). By controlling nuclear Ca2+/H+ coupling, HDP molecules are novel modulators of gene expression in cardiac development and disease.