Table 4.

Model assumptions

No.	Assumption	Clarification
1	Compounds selectively influence either CO or TPR.	Although some compounds may have a combined mechanism of action, i.e. have an effect on both CO and TPR, it was assumed that only including the direct/primary effect was sufficient for identifying the system. Therefore, any changes observed in the other parameters were assumed to be a result of the feedback (indirect/secondary effect).
2	All compounds influence the production rates of CO or TPR rather than the dissipation rates.	This assumption is based on the MoA of the selected compounds (Table 1).
3	For compounds for which the maximum effect was not observed, complete inhibition (i.e. Emax = 1) was assumed at infinite concentrations to ensure identification of the EC50.	To evaluate the validity of this assumption, the influence of different values of the Emax (i.e. Emax = 0.8) on the estimates of the system parameters was tested. This was done for one of the compounds (amlodipine).
4	The PK do not differ between rat strains and can be scaled between rabbit and rat on the basis of an allometric function (West et al., 1999; Anderson and Holford, 2009).	Although published information on the PK of all selected compounds was available, the PK was often evaluated in different rat strains and, for prazosin, even in a different species (rabbit).