PURPOSE: Release of calcium (Ca(2+)) from the sarcoplasmic reticulum (SR) 
induced by Ca(2+) influx through voltage-dependent sarcolemmal L-type Ca(2+) 
channels (CICR) in cardiac muscle cells has been implicated as a potential 
target contributing to anesthetic-induced myocardial depression. In an earlier 
study, we found that (1) a half-logistic (h-L) function, which represents a 
half-curve of a sigmoid logistic function with a boundary at the inflection 
point, curve-fits the first half of the ascending phases of the isometric 
myocardial tension and isovolumic left ventricular (LV) pressure waveforms 
better than a mono-exponential (m-E) function and (2) the h-L time constants are 
useful as inotropic indices. We report here our investigation of the potential 
application of an h-L function to the analysis of the first half of the 
ascending phase of the Ca(2+) transient curve (faCaT) that precedes and 
initiates myocardial contraction and the increase in LV pressure.
METHODS: Ca(2+) transients (CaT) were measured using the Ca(2+)-sensitive 
photoprotein aequorin, which was microinjected into seven isolated rabbit right 
ventricular and 15 isolated mouse LV papillary muscles. The faCaT data from the 
beginning of twitch stimulation to the maximum of the first-order time 
derivative of Ca(2+) concentration (dCa/dt(max)) was curve-fitted by the 
least-squares method using h-L and m-E function equations.
RESULTS: The mean correlation coefficient (r) values of the h-L and m-E 
curve-fits for the faCaTs were 0.9740 and 0.9654 (P < 0.05) in the rabbit and 
0.9895 and 0.9812 (P < 0.0001) in the mouse.
CONCLUSION: The h-L curves tracked the amplitudes and time courses of the faCaTs 
in cardiac muscles more accurately than m-E functions. Based on this result, we 
suggest that the h-L time constant may be a more reliable index than the m-E 
time constant for evaluating the rate of CICR from the SR in myocardial Ca(2+) 
handling. The h-L approach may provide a more useful model for the study of CICR 
during the contraction process induced by anesthetic agents.