Tolerance of isolated cardiac muscle to hypoxia: force-frequency interrelationships (38566).

The preformance of isolated left ventricular muscle preparations during 60 min of hypoxia (95 percent N-2, 5% CO-2) and 30 min of reoxygenation (95% O-2, 5% CO2) was examined at differing stimulation rates (1, 12 and 60 stimuli/min). Since a change in inotropic state accompanies a change in cardiac rate, studies were carried out in two species with different force-frequency relationships over the heart rate range examined the rat and the guinea pig. Preparations were studied at the apices of their length tension curves while contracting isometrically 12 times a min at 28 degrees. At increasing stimulation rates, under oxygenated conditions, isometric tension fell in rat ventricular muscle while the reverse is true in the guinea pig. Despite the differing force-frequency responses, both species demonstrated progressively poor performance during hypoxia at increasing stimulation rates. This was manifested by a more rapid and greater decline in mechanical activity, more severe contracture during hypoxia and and slower recovery upon reoxygenation.