Sarcolemmal-bound calcium and contractility in the mammalian myocardium

It is proposed that the contractile state of the septum is proportional to the fraction of these sarcolemmal calcium receptors which are occupied and which control the kinetics of changes in contractility.

Abstract

Abstract Using the perfused rabbit interventricular septum we have studied the kinetics of changes in contractility when the calcium concentration of the perfusate is either raised or lowered. We find that the rate of decline of d T d t in low calcium is first-order ( t 1 2 ∼45 s ) and independent of initial perfusate calcium levels. The redevelopment of d T d t when the calcium level is raised is 3 to 5 times faster. Lanthanum inhibits contraction with a t 1 2 of about 6 s. Analysis of these kinetics in the light of morphological considerations leads us to conclude that there are specific sarcolemmal calcium receptors which are rapidly accessible ( t 1 2 ∼6 s ) to the perfusate and which control the kinetics of changes in contractility. Based on this model we propose that the contractile state of the septum is proportional to the fraction of these receptors which are occupied. Although one type of receptor appears to dominate, the presence of a second receptor is indicated and we are able to estimate the association binding constant of each.