To our knowledge, this study presents the first comprehensive examination of the mechano-energetics of the diabetic heart. We have extended the practice (common in the literature) of reporting only a single value of myocardial efficiency of the diabetic heart by reporting its behaviour as a function of afterload. Efficiency is a complex function of afterload, with values ranging from zero to a peak (which occurs at somewhere in the mid-value of afterload). It is thus incomplete, and misleading, to report only a single value. We show that the efficiency-afterload relation allows reconciliation of the conflicting conclusions in the literature regarding the effect of diabetes on myocardial efficiency.
Achievement of the diabetic state
A single injection of STZ at age 6-7 weeks, clearly rendered the rats diabetic 7-8 weeks later. This is demonstrated by the 4.4-fold higher concentration of blood glucose, the 30% lower body mass and the 23% lower heart mass but 11% higher ratio of heart mass to body mass (Table 1). The STZ rats developed LV hypertrophy, as indicated by a greater average LV wall thickness relative to heart mass (Table 1), consistent with the finding of MacDonald et al. . Compared to their SHAM controls, the STZ rats had a greater average lung mass per body mass, consistent with the findings of Ofulue et al. . They also had lower average intrinsic heart rates, as measured in vivo and in vitro (at 32°C). Bradycardia has been a consistent finding in STZ-diabetic hearts [19, 20, 22–25, 32, 34–36]. We obviated any influence of bradycardia on the mechano-energetics variables by pacing the excised hearts at 4 Hz (i.e., above the intrinsic heart rates at 32°C).
Adequacy of oxygen supply to the isolated working-heart
The use of a crystalloid perfusate demands examination of the adequacy of oxygen supply. We exploited the inverse relationship between temperature and oxygen solubility–lowering the temperature of the perfusate to 32°C, which, in turn, allowed us to reduce the pacing frequency to 4 Hz. Thus we simultaneously increased oxygen supply and reduced oxygen demand, both of which manoeuvres enhance tissue oxygenation. As seen in Figure 2, the average coronary venous PO2 was 25 mmHg at the lowest workload examined (40 mmHg afterload). This value can be put in perspective with reference to Tune et al.  and the comprehensive review of Zong et al. , who reported that resting PO2 values from the right and left coronary veins of dogs are, respectively, 30 mmHg and 20 mmHg. It is unlikely that our observed values indicate hypoxic conditions, given the substantial coronary reserve revealed by the progressive increase of venous PO2 values with afterload (consistent with the in situ canine data of Yonekura et al. ). Note that the average highest value of venous PO2 (which occurred at the maximal afterload) was 90 mmHg for diabetic hearts and 140 mmHg for control hearts. PO2 values above 100 mmHg exceed those associated with normoxia in vivo. Also observed in Figure 2 is a hint of autoregulation in the mid-range of afterloads and the probable appearance of Gregg’s Phenomenon at higher rates of work demand. These considerations give us confidence that the supply of oxygen to the isolated hearts was adequate and allow us to focus on the effects of diabetes on cardiac mechano-energetics.
Blood pressure in vivo
Given the development of bradycardia and LV hypertrophy in the diabetic rats, it is somewhat surprising that no difference of either systolic or diastolic blood pressure was observed in vivo (Table 1). Systolic blood pressure, as well as mean arterial pressure and pulse pressure, have been shown to be higher and to increase with the duration of diabetes in Type 1 diabetic patients [40, 41]. Elevated blood pressure has been used as an index of arterial stiffening in diabetic patients [40, 41]. Arterial stiffness is known to be higher in diabetic human tissue post-mortem as well as in the arteries [43–45] and myocytes  of diabetic patients. These effects are commonly attributed to increased fibrosis , deposition of advanced glycation end-products [44, 45], increased oxidative stress  or low-grade inflammation . The absence of any difference of blood pressure indices between STZ and SHAM rat hearts probably reflects the difference between acute (present study) and chronic diabetes (human patients).
Aortic flow, coronary flow and maximal afterload in vitro
The effect of STZ-induced diabetes was primarily manifested on the rate of aortic flow (Figure 3B). The extent of decrease of the aortic flow with increasing afterload was greater for the STZ hearts, resulting in negligible flow at an average afterload of 105 mmHg (Figure 3). In contrast, the SHAM hearts did not achieve zero aortic flow until challenged by an average afterload of 150 mmHg. This demonstrates that the LV of the STZ heart fails to generate as high a pressure as the normal heart in vitro. This finding is consistent with the reduced peak developed LV pressure of aortic-occluded diabetic hearts  and the reduced peak LV systolic pressure reported by others [4, 9, 21–25].
However, there are also studies showing no effect of STZ on LV systolic pressure [1, 2, 19, 20]. Nevertheless, it is striking that the clear difference in maximum LV pressure development between STZ and SHAM hearts in vitro (Figure 3) stands in contrast to the absence of any differences among systolic, mean or diastolic pressures measured in vivo (Table 1). The literature on these discrepant results once again lacks consensus. Whereas it is not uncommon to read that STZ-diabetic cardiomyopathy in rats lowers mean arterial pressure significantly [46–49], most groups have reported no difference [19, 20, 23, 34, 35, 49–51], while some have even reported higher mean values [52, 53]. Clearly, any observed response of either mean arterial pressure or LV pressure can be accommodated. In fact, both Radovits et al.  and Cheng et al.  reported lower in vivo LV systolic pressure with no change of mean arterial pressure in the STZ-diabetic rats. Our results, showing reduced maximal afterload (Figure 3) and similar mean arterial pressure (Table 1), are in accord.
At the maximum afterload (when the aortic flow rate is near zero), the coronary flow rate is maximal. STZ-treatment has negligible effect on the maximum rate of coronary flow (8-10 mL min-1 g-1), in accord with the finding of Joffe et al.  who showed no effect of STZ on coronary perfusion pressure of the in vitro heart. This suggests that autoregulation of the heart, which is responsible for the relatively constant coronary flow within an afterload region between around 60 mmHg to 100 mmHg (Figure 3), is unaffected by STZ treatment. At the maximum afterload, since the coronary flow rate is maximal, the derived work output of the heart is small but non-zero (since aortic flow rate is near zero; Figure 4A). In consequence, the calculated total efficiency (Equation 1) is a small value (Figure 4E). This has prompted us to extrapolate the curves fitted to the work-afterload and efficiency-afterload data to predict the peak value of afterload. These extrapolated values approximate the systolic pressure of an isovolumic contraction. Since the predicted maximum afterload of the STZ hearts is lower (105 mmHg versus 150 mmHg; Figure 3), the predicted peak afterload (or isovolumic pressure) of the STZ hearts is consistently lower (114 mmHg versus 169 mmHg; Figure 5E and F). Reassuringly, the predicted peak afterload does not differ in value whether it is extrapolated from the work-afterload curve (Figure 5E) or from the efficiency-afterload curve (Figure 5F).
Peak values of work and of total efficiency in vitro
These two peak values occur at neither the maximum afterload nor the predicted peak afterload. Instead, they occur at their optimal afterloads, as shown in Figure 4 and quantified in Figure 5C and D. Given that STZ reduces the maximum afterload and the predicted peak afterload, the optimal afterload is necessarily reduced. Despite this effect, the peak value of total efficiency (14-15%) is unaffected by STZ (Figures 4F and 5B). These values of total efficiency fall within the range of the literature values reported for the normal rat heart .
Note that the average efficiency-afterload curve of the STZ hearts intersects that of the SHAM hearts at an afterload of approximately 70 mmHg (Figure 4F). At this afterload, diabetes has negligible effects (Table 2) on either aortic or coronary flow rates, work output, total oxygen consumption, total efficiency, or basal rate of oxygen consumption. Correction for the basal rate of oxygen consumption (Equation 2) reveals that the mechanical efficiency, at least at afterload 70 mmHg, is unaffected by STZ.
Optimisation of the heart for mechano-energetics
As is also evident in Figures 4 and 5, the optimal afterload for peak work is different from that for peak total efficiency. The former occurs at a greater value of afterload than the latter, and this is independent of diabetic status. This result, in the intact whole-heart, is consistent with the results obtained in isolated one-dimensional cardiac preparations, such as papillary muscles [10–16] and trabeculae carneae [17, 18].
We can calculate the value of work at which total efficiency is at its peak, and vice versa (Figure 5). But since the optimal afterload for peak work is greater than that for peak total efficiency, this has raised a question as to whether the STZ and SHAM hearts, in vivo, operate at some high afterload (90 mmHg, for example) or at the afterload that optimises either work or total efficiency. If they operate at their optimal afterloads for peak work, then they have the same total efficiency. If they operate at their optimal afterloads for peak total efficiency, then they again have the same total efficiency. But if they both operate at high afterload, then the STZ heart would have a relatively lower total efficiency (Table 3). The literature on the concept of ‘optimisation’ (the matching of either peak work or peak efficiency to arterial load) is inconclusive [55–62], but, as pointed out by Elzinga and Westerhof  and by de Tombe et al. , it scarcely matters. The afterloads at which these two maxima occur do not differ greatly, i.e., both the work and efficiency are greater than 90% of their respective optima. Our results, for both the STZ and SHAM hearts, are in accord. Thus, from the literature, we think that the heart operates within the range of optimal afterloads for peak work and for peak total efficiency. Our results, on isolated hearts, show that the STZ-diabetic heart operates optimally at afterloads 60-70 mmHg, lower than those for the healthy heart (80-100 mmHg). Despite operating at a lower afterload, the STZ-diabetic heart pumps as efficiently as the healthy heart (14.7%). We thus infer that the diabetic heart, under normal conditions, pumps as efficiently as the healthy heart, but if it is challenged with any intervention that requires it to pump at high afterloads, then its efficiency will be lower than that of the healthy heart.
Reporting a single value of efficiency
Because of the complex dependencies of efficiency on afterload, it is unwise to perform experiments at only a single afterload, and to report a single value of efficiency, unless one has a priori knowledge of the relation between efficiency and afterload. If we had performed only a single experiment at a high afterload (for example, 90 mmHg), to examine the effect of diabetes, then we would have reported that the diabetic heart has a lower value of efficiency. We consider it necessary that experiments be performed at a range of afterloads since, by doing so, the optimal afterload at which the heart operates can be ascertained. If only a single value of efficiency is reported, then it is essential to report the afterload at which that value was obtained. In fact, this was done by Penpargkul et al.  who reported that they had performed all measurements at a fixed afterload of approximately 60 mmHg. It was their serendipitous choice of this value that allowed them to find no effect of diabetes upon myocardial work output, oxygen consumption or total efficiency – a result in accord with our findings reported in Figure 4.