Membrane depolarization as a cause of tension development in mammalian ventricular muscle

A method was devised which made it possible to depolarize ventricular muscle, over a short interval of length, by applying an external voltage and to record, from the same segment, the developed tension. A fiber bundle, 0.5–1.0 mm in diameter, from the right ventricular cavity of the sheep or calf, was inserted into a close-fitting hole in a plastic block, contained in a Tyrode bath. The fibers were depolarized where they exited from the block. The muscle was held, by suction, at two surface points of very small area, 0.4 mm apart. Measurements with microelectrodes showed that the depolarization was roughly uniform over the length of muscle segment from which tension was recorded. By this means, the action potential (normally lasting about 1/2 sec.) could be prolonged to a duration of 2 seconds. This caused tension to be maintained at near-peak levels; relaxation occurred only when the fibers were allowed to repolarize. Evidence was thus adduced in support of the view that the contractile mechanism is continuously responsive to depolarization.