Actions of NAADP and other agents in cardiac myocytes

Modulation of cardiac rate and contraction through calcium-dependent and independent means are of central import to the ability of an organism to adapt to its environment. Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent calcium-releasing second messenger across a broad range of tissues and organisms. In cardiac myocytes, NAADP is thought to stimulate calcium release from acidic stores which then bolsters filling and release during CICR. Questions remain: as to the potential need for amplification to generate the size of responses observed and the physiological role of the NAADP pathway. In contractile myocytes, photorelease of NAADP caused significant increase in calcium transient amplitude and velocity of transient upstroke and decay. Effects were absent during NAADP photorelease in the presence of Ned-19 or CaMKII inhibitors. Cellular calcium transient responses to β-adrenergic stimulation were significantly reduced in the presence of inhibitors of the NAADP pathway. These data support the hypothesis that NAADP-induced calcium release is relevant during adrenergic stimulation and requires amplification through CaMKII. Rate modulation at the sino-atrial node can occur through the hyperpolarisation-activated current I(f). Basal cardiac rate is a major determinant in cardiac mortality and compounds which specifically affect rate have clinical utility. A compound currently used to treat inflammatory conditions was found to have a significant rate-reducing effect in sino-atrial node preparations mediated by inhibition of I(f). Apelin, an endogenous peptide, has been reported to potently generate improved contractility without development of hypertrophy. Study of its effects in single cells have provided conflicting information, at least in part because of the difficulty in working with the compound. A method for the consistent observation of apelin-mediated contractile responses is presented, focusing on the timecourse of cell contraction. These observations suggest a role for apelin in both inotropy and lusitropy and will enable further research.This thesis is not currently available in ORA