The capillary membrane and calcium kinetics in perfused hearts: independence of force decay on heart rate.

Earlier studies have indicated there is no correlation between heart rate (HR) and force decay during Ca depletion. However a compelling hypothesis states that because the sarcolemma is in an intrinsically different state during the action potential than between action potentials, the release of Ca, and hence the decay of force, during Ca depletion should depend on the proportion of time spent in the depolarised state and hence should depend on HR. In each of ten Langendorff-perfused rabbit hearts we measured the exponential rate constant for the decline of activity developed force at two different heart rates. Heart rates were in the range of 48 to 142 beats . min-1. Application of the t test to the pairs of rate constants showed the difference within pairs to be not different from zero (P greater than 0.10). In order to explain this contradiction of the hypothesis, we considered that the capillary membrane limits Ca washout and hence force decay. To examine this, advantage was taken of the variation in rate constants between hearts (0.023 to 0.114 s-1). Linear regression of force decay rate constants with resistance to flow (perfusion pressure/flow rate), upon which capillary exchange area might be expected to depend, yielded a correlation coefficient of -0.494, on application of the t test, significant at P = 0.05. This correlation, we believe, supports the concept that the capillary membrane limits Ca washout and therefore that Ca-kinetic events related to force development (eg, HR dependence) are "hidden" behind the capillary membrane.