Mechanical transduction of nitric oxide synthesis in the beating heart

NO alters contractile and relaxant properties of the heart. However, it is not known whether changes in ventricular loading conditions affect cardiac NO synthesis. To understand this potential contractile-relaxant autoregulatory mechanism, production of cardiac NO in response to mechanical stimuli was measured in vivo using a porphyrinic sensor placed in the left ventricular myocardium. The beating rabbit heart exhibited cyclic changes in [NO], peaking at 2.7+/-0.1 micromol/L near the endocardium and 0.93+/-0.20 micromol/L in the midventricular myocardium (concentrations were 15+/-4% lower in the rat heart). In the present study, we demonstrate for the first time that increasing or decreasing ventricular preload in vivo is followed by parallel changes in [NO], which may represent a novel autoregulatory mechanism to adjust cardiac performance or perfusion on a beat-to-beat basis. To quantify the relationship between applied force and NO synthesis, intermittent compressive or distending forces applied to ex vivo nonbeating hearts were shown to cause bursts of NO synthesis, with peak [NO] linearly related to ventricular transmural pressure. Experiments in which denuding cardiac endothelial and endocardial cells abrogated the NO signal indicate that these cells transduce mechanical stimulation into NO production in the heart. Taken together, these studies may help explain load-dependent relaxation, cardiac memory for mechanical events of preceding beats, diseases associated with myocardial distension, autoregulation of myocardial perfusion, and protection from thrombosis in the turbulent flow environment within the beating heart.