Abstract
A decrease in myocardial intracellular calcium concentration ([Ca2+]i) precedes relaxation, and a monoexponential function is typically used for fitting the decay of the Ca2+ transient. However, a logistic function has been shown to be a better fit for the relaxation force curve, compared to the conventional monoexponential function. In the present study, we compared the logistic and monoexponential functions for fitting the [Ca2+]i declines, which were measured using the aequorin method, and isometric relaxation force curves at 4 different onsets: the minimum time-derivative of [Ca2+]i (d[Ca2+]i/dt min) and force (dF/dtmin), and the 10%, 20% and 30% lower [Ca2+]i levels and forces over the data-sampling period in 7 isolated rabbit right ventricular and 15 isolated mouse left ventricular papillary muscles. Logistic functions were significantly superior for fitting the [Ca2+] i declines and relaxation force curves, compared to monoexponential functions. Changes in the normalized logistic [Ca2+] i decline and relaxation force time constants at the delayed onsets relative to their 100% values at d[Ca2+] i/dtmin and dF/dtmin were significantly smaller than the changes in the normalized monoexponential time constants. The ratio of the logistic relaxation force time constant relative to the logistic [Ca2+]i decline time constant was significantly smaller in mouse than in rabbit. We conclude that the logistic function more reliably characterizes the [Ca2+]i decline and relaxation force curve at any onset, irrespective of animal species. Simultaneous analyses using the logistic model for decay of the Ca2+ transient and myocardial lusitropism might be a useful strategy for analysis of species-specific myocardial calcium handling.
