Stretch-Activated Channels in Heart Cells

Summary: Stretch-activated channels have been proposed as the transduction mechanism between load and protein synthesis in cardiac hypertrophy. Under this hypothesis, cardiac deformation is linked to an increased sodium (Na) influx, which, in turn, increases protein synthesis. We have tested whether stretch actually increases Na influx by applying patch-clamp techniques to cultured chick embryo cardiac myocytes and to freshly isolated adult guinea pig cardiomyocytes. Our experiments, in excised and cell-attached patches, revealed the existence of ionic channels that opened, or increased their frequency of opening, upon the application of negative pressures to the lumen of the patch-clamp pipettes. These stretch-sensitive channels allowed the passage of the major monovalent physiological cations, Na and potassium (K), and, to a much lesser extent, the major divalent cations calcium (Ca) and magnesium (Mg). Under normal conditions, the channels had a high open channel noise that prevented the customary, straightforward statistical analysis of single channel data. However, when one of the major monovalent cations was iso-osmotically replaced by sucrose, the open channel noise decreased significantly and permitted a good delineation of the open and closed channel states and, therefore, application of standard patch-clamp, statistical analysis techniques. Under these “sucrose,” “monoionic” conditions, the reversal potential was, as one should expect, close to the equilibrium potential for the major monovalent cation present. When high extracellular K solution was used to minimize the cell resting potential, the reversal potential for these stretch-activated currents was estimated to be around -40 mV. Therefore, under normal conditions, stretch should induce an inward, depolarizing current, carried mostly by Na ions. The voltage dependence of this stretch-activated current would insure that, during stretch, membrane depolarization does not go beyond the reversal potential of the stretch-activated channel. Finally, these channels were not permeant to anions and were completely blocked by 100 μM gadolinium. Our experimental results support the concept that increased load leads to an augmented sodium influx as proposed.