Calcium exchange in rabbit myocardium during and after hypoxia: effect of temperature and substrate

Calcium influx and efflux were measured during hypoxia and on reoxygenation in the isolated arterially perfused septum of the rabbit heart. The uptake of 47Ca2+ was continuously followed with a NaI crystal and counter. The extracellular space (ECS) was measured in a similar manner with 51Cr-EDTA. Calcium efflux was recorded by collection of effluent drops after labelling with 45Ca2+. Hypoxia caused a rapid decline of developed tension followed by a rise in resting tension. The ECS increased initially but decreased as resting tension rose. 51Cr-EDTA did not have free access to the intracellular fluid. Calcium efflux did not change and calcium influx was either unchanged or reduced. On reoxygenation calcium influx increased immediately but efflux was unaltered and 51Cr-EDTA did not enter the cell. The effects of hypoxia were altered by manipulation of temperature and substrate. The recovery of mechanical function was related to the size of the calcium influx on reoxygenation. The experiments show that a rise of resting tension during hypoxia can occur in the absence of a net gain of calcium, calcium accumulation is closely associated with the extent of tissue damage, and that calcium influx on reoxygenation is probably due to a specific abnormality and not gross disruption of the cell membrane.