Influx of Calcium into Rabbit Myocardium in Relation to Its Ionic Environment

Abstract

Abstract. In the first 150 seconds after contracture of rabbit myocardium has been induced by shifting from perfusion with zero [Ca2+1 and low [K+] to solutions with normal levels of those cations, there is a large influx of Ca2+ 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 Ki for Na+ in perfusion fluids is also associated with an increased influx, but of lesser magnitude. The latter typ( s of contracture are reversible while the former is not. It seems probable that the irreversible contracture is induced by the large Ca2 + influx.Previous studies' have shown that the net influx of Ca2+ into heart muscle after a period of exposure to zero [Ca2+] [Ca2+] in the perfusate effluent and the extravascular, extracellular fluid. 45Ca was employed as a tracer for calcium movement by switching the perfusate source to a reservoir containing the isotope at the time of beginning the Ca2+ uptake period. After this period the perfusion was instantaneously shifted to solutions of identical composition except for the Ca2+ isotope. Nonradioactive perfusion was continued for exactly 180 sec in order to eliminate the majority of the high activity perfusate. The perfusate effluent was collected at short intervals and it was assumed that the extracellular space was in equilibrium with the perfusate during the final collection period. The 180 sec washout was employed because Ca2+ entrance into muscle cells is slow and large errors could be introduced into 45Ca values for the intracellular compartments by small errors in sucrose-space measurements. According to previous studies on dog papillary muscle by Langer4 and others, the half time for the 603