Standards in kidney transplantation have been significantly improved during the past years [7,42-44]. They were accompanied by a large number of experimental studies using animal kidney perfusion models [1-5,16,45,46] However, exact reference values for different perfusion conditions have not been described so far and the present studies aimed to address this issue by defining reference values of renal functional parameters in both laboratory and slaughterhouse animal kidneys under different perfusion conditions.
When analyzing the blood parameters of the perfusion groups, a slight increase for free plasma hemoglobin was found in all groups. This increase can be explained by a moderate cell damage by the blood pumps which is commonly found in perfusion systems [4,41]. Also, there was a slight decrease in total blood protein in all perfusion groups that might be explained by protein adsorption at the perfusion system tubes  and a certain urinary protein excretion. Likewise, the slight decrease in blood hemoglobin can be explained by a loss of erythrocytes due to blood sampling as previously found in different perfusion settings [19,41].
Kidney function was studied at first at the level of glomerular filtration and four parameters: creatinine U/P quotient (U/Pcrea), urine-flow (VU), creatinine-clearance (Clcrea) and water-reabsorption (RF H2O) were analyzed by help of a special grapho-analytical method (figure 1).
This separating analysis is crucial since the Clcrea is commonly used as the approximation of the glomerular filtration rate and thus can be taken as one of the principal indicators of renal function quality with a physiological mean value in the control group of 76.1 ml/min*100 g (table 6), represented in figure 1 as the dotted green line and as cross symbols for the single measurements. In comparison to this physiological in vivo control, the measurements for group A kidneys, presented in figure 1 and table 6, resulted in a mean value of 59.2 at 60 min of perfusion duration what is fairly comparable to the control level of the creatinine clearance.
Comparable levels of Clcrea, as depicted in figure 1, means that the different values arrange along straight declining lines in the nomogram. Using this approach, two hypergroups or clusters of kidneys were found (figure 1): The first cluster containing groups CON and A arrange in a falling linear band (dotted lines) between 60–80 ml/min*100 g. The second cluster consists of groups (B, C, E) showing a broader Clcrea value scattering than CON and A with a range of mean values between 27.6 ml/min*100 g (B) and 16.3 ml/min*100 g (E). A minimum Clcrea of 1.6 ml/min*100 g was found in group D.
Focussing only on the parameter creatinine-clearance, group A seems to contain the best performing experimental kidneys so far. This could be supposed since group A consists of OP kidneys with no preservation at all.
Comparing the closely related Clcrea levels of groups CON and A, however, one has to consider the different underlying physiological conditions (see figure 1): In the CON group the Clcrea values are determined under a normal diuresis of about 1 ml/min*100 g with an U/Pcrea of about 100. In group A, contrarily a strong poliuric state is present under isolated perfusion conditions with a 15-fold increased diuresis. Concomitant with this finding an extremely low U/Pcrea value of 5.2 indicates a significantly reduced water reabsorption RF H2O of 76.7 % (against 98.9 % in the control kidneys). This can be explained in part by the absence of ADH control in the isolated kidney [5,48].
After kidney function analysis on the basis of these 4 parameters it seems to be doubtful to define group A as best performing in the sense of comparability to normal organ function in living animals. Rather one could presume that isolated organs partly follow their own rules, thus exhibiting a functional behaviour what could be defined as "free-running". This state is due to the kidney's secession from any higher organ control (humoral and nervous system).
With regard to the oppositional postglomerular water flow situations, observed between group A and the control group, it seems to be necessary, to consider further renal parameters and also functional and metabolic mechanisms to qualify the outcome of the isolated kidney.
In this respect, considering the nephron's handling of substrates, the tubular reabsorption of sodium is a further prominent mechanism and the fractional reabsorption of sodium RFNa is the representative parameter for this function.
An application of RFNa as an useful indicator to qualify renal function under isolated organ perfusion has been demonstrated in a previously published study  for isolated pig kidneys with differing experimental protocols.
In the experiments of the actual study however, the RFNa value means of groups A, B, C, and E exhibit almost equal levels in a range from 82.3 % (A) to 88.7 % (E) without any significant differences. Only group D separates significantly with a low value of 38.4 %.
Sodium reabsorption is an energy consuming process and the physiological coupling between sodium reabsorption and oxygen consumption appeared to be a further promising tool to analyze renal sodium handling in the isolated kidney. This process is described in the literature as of linear proportionality for the mammalian kidney [31-34,49]. That relation is illustrated in figure 2 and table 7 for the experimental groups of kidneys examined in this study : The line which connects the cross symbols (denoted DE) in figure 2 in an almost ideal regression between the oxygen consumption on the y-axis and the sodium reabsorption on the x-axis, represents the physiological in-vivo situation. That part of the diagram (group DE) was adapted from in vivo studies  resulting in the following regression equation:
O2-consumption = 0.121 + 0.0332 * TNa
The first term on the right side of the equation (0.121) represents the basal oxygen consumption of the kidney without any sodium transport at all. The second term in its reciprocal expression equals in the following value: 30.1 mmol Na/mmol O2, representing the number of Na-ions per oxygen-molecule actively and O2 -consuming being transported back into the blood. The equation and the values are very similarly reported in other studies [32,33,51].
Out of the isolated kidney groups in figure 2 there were found statistically acceptable (R2 > 0.4) regression lines only for groups A and C (see table 7) with the following TNa/O2 cons qotients: group A 42.0 and group C: 80.6.
Taking the slope of the green line in figure 2 as the in vivo standard, steeper angles of regression lines, as could be constructed for groups D, E, would result in values of the TNa/O2 cons coupling qotient lower than the normal 30.1 mmol Na/mmol O2. This situation is sometimes discussed as "decoupled". Because there is more than normal oxygen per unit sodium consumed, the generation of heat shock proteins (HSPs) is proposed  as one reason of that imbalance. These HSPs play a major roll in renal ischemia and reperfusion injury [14,53], as occurring in perfusion studies, here for the experimental groups D (longest cold ischemia) and E (longest warm ischemia).
In contrast to this situation, as found for groups A, B, C, reduced slopes of regression lines (resulting in higher TNa/O2 cons quotients, with more than the physiologically normal 30.1 Na-ions per consumed O2-molecule, appearing in the renal venous blood), may represent tubular leakage processes [30,54].