Mechanoelectric transduction in the isolated cardiac cell: the effects of swelling

Cell swelling determines an increase in cell volume and has been shown to induce the activation of stretch activated channels. No direct evidence has been reported concerning the effects of swelling on the action potential of single cardiac myocytes with respect to cell volume changes. Aim of our study was to investigate the effects of cell swelling on action potential shape and duration and on the main ionic currents underlying the action potential of guinea pig ventricular myocytes. Adult guinea pig ventricular myocytes were studied by means of the patch-clamp technique in Whole Cell Configuration (WCR), in current clamp mode and in voltage clamp mode to record membrane potentials and ionic currents respectively. Images of the cells were captured simultaneously with electrical recordings by means of a SIT camera coupled to the microscope to correlate the effects induced by swelling on the electrophysiological parameters with the changes in cell volume. Cell were swolllen by decreasing the ionic strength of the external solution with respect to the intracellular and extracellular control solutions (from 290mOsm/L in control to 172 mOsm/L). The average volume increase during swelling was 45.5±7.2% (n=10, p<0.001), while cells length wasn't significantly changed. Swelling induced modifications of the action potential: Action Potential Duration was significantly shortened, APD90 had a mean value of 488.2±45.1 ms in control conditions and 256.7±24.9 ms during swelling (n=17, p<0.001); resting potential was significantly depolarized going from a mean value of -75.6±1.7 mV in control conditions to -71.1±1.9 mV during the hyposmotic challenge (n=17, p<0.001). Both effects were partially reversible upon washout. Action potential shortening was significant at all phases of repolarization but the more consistent shortening of APD occurred during the plateau phase (phase 2 repolarization) as APD20-50 shortened by 44.6% while APD50-90 shortened by 21%. The stretch of cells induced by swelling determines modifications at the membrane level that bring to the activation and/or modulate some ionic channels. Experiments performed to study the ionic currents involved in action potential changes show that swelling induces a consistent increase in the amplitude of the delayed rectifier potassium current (IK), this effect may partially account for the AP shortening observed during phase 2 and phase 3 repolarization The activation of a chloride current. outwardly rectifying at Vm>ECl and inward at Vm<ECl observed in our cells, may contribute at positive potentials to AP shortening occurring during the plateau phase and at negative potentials to the depolarization of Vrest. Our data also show that IKl is increased in amplitude by cell swelling and may contribute to the effect observed on Vrest. We conclude that cell swelling, by means of the mechano-electrical feedback, can modify the refractoriness of cardiac tissue (inducing action potential shortening and resting potential depolarization), and may predispose the heart to arrhythmias