Influx of calcium into rabbit myocardium in relation to its ionic environment.

In the first 150 seconds after contracture of rabbit myocardium has been induced by shifting from perfusion with zero [Ca(2+)] and low [K(+)] to solutions with normal levels of those cations, there is a large influx of Ca(2+) as measured both by isotopic tracer flux and by total tissue [Ca]. Tracer studies indicate that the influx is 90 per cent complete in 90 seconds. Contracture due to substitution of either Li(+) or K(+) for Na(+) in perfusion fluids is also associated with an increased influx, but of lesser magnitude. The latter types of contracture are reversible while the former is not. It seems probable that the irreversible contracture is induced by the large Ca(2+) influx.