What information contains a rest contraction curve? a theoretical study with experimental results from the rabbit papillary muscle.

On the basis of a specialized two-Ca store model the degree of coupling q between the two stores was introduced. q is defined by the time constants of the model (leakage and coupling time constant) and varies between O (uncoupled stores) and 1 (maximal coupled stores). Provided that the contraction height in first approximation is proportional to the myoplasmic Ca concentration from the rest contraction curve the degree of coupling can be calculated using the coordinates of the maximum developed tension. From 13 rabbit papillary muscles the values of q were determined at different periods of rest thus demonstrating that q cannot be considered as constant in the first period of the pause. Accordingly, the time constants change rapidly at the beginning of the rest reaching relatively constant values after the half of the maximum time of the rest contraction curve. With respect to the hypothesis of intracellular recycling Ca controlling the contraction in cardiac muscle the great values of q (about 0.9 after a longer period of rest) suggest a very little loss of Ca from intracellular stores by leakages. From our calculations it is expected that in the maximum of the rest contraction curve the Ca concentration in both stores might differ insignificantly.