Ca$^2+$ transients measured in failing human ventricular myocytes
exhibit reduced amplitude, slowed relaxation, and blunted frequency
dependence. In the companion article (O'Rourke B, Kass DA, Tomaselli
GF, K��b S, Tunin R, Marb�n E. Mechanisms of altered excitation-contraction
coupling in canine tachycardia-induced heart, I: experimental studies.
Circ. Res.. 1999;84:562-570), O'Rourke et al show that Ca$^2+$
transients recorded in myocytes isolated from canine hearts subjected
to the tachycardia pacing protocol exhibit similar responses. Analyses
of protein levels in these failing hearts reveal that both SR Ca$^2+$
ATPase and phospholamban are decreased on average by 28\% and that
Na$^+$/Ca$^2+$ exchanger (NCX) protein is increased on average
by 104\%. In this article, we present a model of the canine midmyocardial
ventricular action potential and Ca$^2+$ transient. The model
is used to estimate the degree of functional upregulation and downregulation
of NCX and SR Ca$^2+$ ATPase in heart failure using data obtained
from 2 different experimental protocols. Model estimates of average
SR Ca$^2+$ ATPase functional downregulation obtained using these
experimental protocols are 49\% and 62\%. Model estimates of average
NCX functional upregulation range are 38\% and 75\%. Simulation of
voltage-clamp Ca$^2+$ transients indicates that such changes
are sufficient to account for the reduced amplitude, altered shape,
and slowed relaxation of Ca$^2+$ transients in the failing canine
heart. Model analyses also suggest that altered expression of Ca$^2+$
handling proteins plays a significant role in prolongation of action
potential duration in failing canine myocytes.