LAUSR

Significance The heart autoregulates its contractile strength to maintain cardiac output when pumping blood against the mechanical afterload from vascular resistance. Increased afterload in cardiovascular diseases is associated with cardiac arrhythmias. However, the underlying mechanisms remain elusive. Here, we studied the afterload effect on electrophysiology by embedding single cardiomyocytes in a three-dimensional viscoelastic hydrogel to apply afterload during cell contraction. We found that afterload is acutely transduced via nitric oxide synthase 1 (NOS1) signaling to modulate multiple ion channels to prolong action potential, increase Ca2+ transient, and enhance contractility. However, higher afterload causes a discordant alternans that is arrhythmogenic. Hence, our findings reveal an afterload-induced mechano-chemo-electro-transduction pathway that closes feedback loops in cardiac excitation–contraction coupling to enable autoregulation of contractility. The heart pumps blood against the mechanical afterload from arterial resistance, and increased afterload may alter cardiac electrophysiology and contribute to life-threatening arrhythmias. However, the cellular and molecular mechanisms underlying mechanoelectric coupling in cardiomyocytes remain unclear. We developed an innovative patch-clamp-in-gel technology to embed cardiomyocytes in a three-dimensional (3D) viscoelastic hydrogel that imposes an afterload during regular myocyte contraction. Here, we investigated how afterload affects action potentials, ionic currents, intracellular Ca2+ transients, and cell contraction of adult rabbit ventricular cardiomyocytes. We found that afterload prolonged action potential duration (APD), increased transient outward K+ current, decreased inward rectifier K+ current, and increased L-type Ca2+ current. Increased Ca2+ entry caused enhanced Ca2+ transients and contractility. Moreover, elevated afterload led to discordant alternans in APD and Ca2+ transient. Ca2+ alternans persisted under action potential clamp, indicating that the alternans was Ca2+ dependent. Furthermore, all these afterload effects were significantly attenuated by inhibiting nitric oxide synthase 1 (NOS1). Taken together, our data reveal a mechano-chemo-electrotransduction (MCET) mechanism that acutely transduces afterload through NOS1–nitric oxide signaling to modulate the action potential, Ca2+ transient, and contractility. The MCET pathway provides a feedback loop in excitation–Ca2+ signaling–contraction coupling, enabling autoregulation of contractility in cardiomyocytes in response to afterload. This MCET mechanism is integral to the individual cardiomyocyte (and thus the heart) to intrinsically enhance its contractility in response to the load against which it has to do work. While this MCET is largely compensatory for physiological load changes, it may also increase susceptibility to arrhythmias under excessive pathological loading.