Mechanotransduction via No Signaling Auto-Regulates Cardiomyocyte Contractility

Abstract

Background: The heart must adjust its contractile force to compensate for the blood pressure changes in daily life (due to changes in posture, physical activity, emotional state, etc.) in order to maintain adequate cardiac output. Here we investigate the mechano-chemo-transduction (MCT) mechanisms that transduce mechanical load to regulate cardiac Ca2+ signaling and contractility. Methods: We used a Cell-in-Gel system to embed freshly isolated rabbit ventricular myocytes in a 3-D viscoelastic hydrogel. The cell in-gel was continuously perfused with Tyrode's solution and electrically paced at 0.5 Hz. Self-control experiments were conducted by (1) studying the myocytes contracting in-gel under mechanical load, (2) then dissolving the gel to release the cell into Tyrode's solution and (3) studying the cell contracting in solution load-free. Results: The cardiomyocytes contracting in-gel exhibited a larger cytosolic Ca2+ transient than load-free cells (Fura-2 ratio 2.71±0.07 vs 2.08±0.04), revealing the MCT effect on regulating intracellular Ca2+. The SR Ca2+ content was higher and the fractional release of Ca2+ from SR was also larger when the cell was in-gel than load-free (measured with Fluo-5N), which should contribute to increasing the cytosolic Ca2+ transient. Furthermore, the MCT effects on the SR Ca2+ and cytosolic Ca2+ were abolished by using 1 mM L-NAME to inhibit nitric oxide synthases (NOS), suggesting a critical role of NOS-NO signaling in mediating MCT. The cell in-gel contraction was reduced from 13.5±0.4% in untreated cell to 10.7±0.6% after L-NAME treatment, showing the MCT effect on enhancing contraction. Conclusion: Mechano-chemo-transduction in cardiomyocyte is mediated by NOS-NO signaling to regulate the SR Ca2+ release to increase the cytosolic Ca2+ transient, which in turn enhances the contractility in compensatory response to increased load. This MCT mechanism contributes to the Anrep effect and is fundamental to cardiac regulation in health and disease.