Calcium Flux in the Mammalian Ventricular Myocardium

The exchange of Ca⁴⁵ was studied in dog myocardium by means of a newly developed perfusion technique whereby an excised papillary muscle was perfused through its own artery. This makes possible the sequential and simultaneous correlation of ionic flux with ventricular myocardial function with each muscle serving as its own control. Calcium exchange has the following characteristics: (a) The major component of calcium flux is independent of the frequency of contraction. It demonstrates a rapidly equilibrating phase (half-time, 4 to 6 minutes) and a more slowly equilibrating phase with a progressively decreasing rate constant. The flux characteristics of the more rapidly equilibrating compartment are determined by a factor or factors, in addition to simple diffusion, which increase the time required for this compartment to achieve a steady-state with respect to the vascular compartment. (b) A lesser component of exchange is stimulus-rate dependent and is characterized by an alteration in the rate of calcium turnover such that the altered influx: efflux ratio requires 20 to 25 minutes to achieve equilibrium. After this time, despite the higher stimulus rate, there is no evidence of change in total tissue calcium. (c) The initial rate of the transient response is approximately proportional to the change in stimulus rate.