Fatty acid desaturase 1 knockout mice are lean with improved glycemic control and decreased development of atheromatous plaque.

Delta-5 desaturase (D5D) and delta-6 desaturase (D6D), encoded by fatty acid desaturase 1 (FADS1) and FADS2 genes, respectively, are enzymes in the synthetic pathways for ω3, ω6, and ω9 polyunsaturated fatty acids (PUFAs). Although PUFAs appear to be involved in mammalian metabolic pathways, the physiologic effect of isolated D5D deficiency on these pathways is unclear. After generating >4,650 knockouts (KOs) of independent mouse genes and analyzing them in our high-throughput phenotypic screen, we found that Fads1 KO mice were among the leanest of 3,651 chow-fed KO lines analyzed for body composition and were among the most glucose tolerant of 2,489 high-fat-diet-fed KO lines analyzed by oral glucose tolerance test. In confirmatory studies, chow- or high-fat-diet-fed Fads1 KO mice were leaner than wild-type (WT) littermates; when data from multiple cohorts of adult mice were combined, body fat was 38% and 31% lower in Fads1 male and female KO mice, respectively. Fads1 KO mice also had lower glucose and insulin excursions during oral glucose tolerance tests along with lower fasting glucose, insulin, triglyceride, and total cholesterol levels. In additional studies using a vascular injury model, Fads1 KO mice had significantly decreased femoral artery intima/media ratios consistent with a decreased inflammatory response in their arterial wall. Based on this result, we bred Fads1 KO and WT mice onto an ApoE KO background and fed them a Western diet for 14 weeks; in this atherogenic environment, aortic trees of Fads1 KO mice had 40% less atheromatous plaque compared to WT littermates. Importantly, PUFA levels measured in brain and liver phospholipid fractions of Fads1 KO mice were consistent with decreased D5D activity and normal D6D activity. The beneficial metabolic phenotype demonstrated in Fads1 KO mice suggests that selective D5D inhibitors may be useful in the treatment of human obesity, diabetes, and atherosclerotic cardiovascular disease.

High-fat feeding-induced hyperinsulinemia increases cardiac glucose uptake and mitochondrial function despite peripheral insulin resistance.

In obesity, reduced cardiac glucose uptake and mitochondrial abnormalities are putative causes of cardiac dysfunction. However, high-fat diet (HFD) does not consistently induce cardiac insulin resistance and mitochondrial damage, and recent studies suggest HFD may be cardioprotective. To determine cardiac responses to HFD, we investigated cardiac function, glucose uptake, and mitochondrial respiration in young (3-month-old) and middle-aged (MA) (12-month-old) male Ldlr(-/-) mice fed chow or 3 months HFD to induce obesity, systemic insulin resistance, and hyperinsulinemia. In MA Ldlr(-/-) mice, HFD induced accelerated atherosclerosis and nonalcoholic steatohepatitis, common complications of human obesity. Surprisingly, HFD-fed mice demonstrated increased cardiac glucose uptake, which was most prominent in MA mice, in the absence of cardiac contractile dysfunction or hypertrophy. Moreover, hearts of HFD-fed mice had enhanced mitochondrial oxidation of palmitoyl carnitine, glutamate, and succinate and greater basal insulin signaling compared with those of chow-fed mice, suggesting cardiac insulin sensitivity was maintained, despite systemic insulin resistance. Streptozotocin-induced ablation of insulin production markedly reduced cardiac glucose uptake and mitochondrial dysfunction in HFD-fed, but not in chow-fed, mice. Insulin injection reversed these effects, suggesting that insulin may protect cardiac mitochondria during HFD. These results have implications for cardiac metabolism and preservation of mitochondrial function in obesity.

Impact of physiological variables and genetic background on myocardial frequency-resistivity relations in the intact beating murine heart.

Conductance measurements for generation of an instantaneous left ventricular (LV) volume signal in the mouse are limited, because the volume signal is a combination of blood and LV muscle, and only the blood signal is desired. We have developed a conductance system that operates at two simultaneous frequencies to identify and remove the myocardial contribution to the instantaneous volume signal. This system is based on the observation that myocardial resistivity varies with frequency, whereas blood resistivity does not. For calculation of LV blood volume with the dual-frequency conductance system in mice, in vivo murine myocardial resistivity was measured and combined with an analytic approach. The goals of the present study were to identify and minimize the sources of error in the measurement of myocardial resistivity to enhance the accuracy of the dual-frequency conductance system. We extended these findings to a gene-altered mouse model to determine the impact of measured myocardial resistivity on the calculation of LV pressure-volume relations. We examined the impact of temperature, timing of the measurement during the cardiac cycle, breeding strain, anisotropy, and intrameasurement and interanimal variability on the measurement of intact murine myocardial resistivity. Applying this knowledge to diabetic and nondiabetic 11- and 20- to 24-wk-old mice, we demonstrated differences in myocardial resistivity at low frequencies, enhancement of LV systolic function at 11 wk and LV dilation at 20-24 wk, and histological and electron-microscopic studies demonstrating greater glycogen deposition in the diabetic mice. This study demonstrated the accurate technique of measuring myocardial resistivity and its impact on the determination of LV pressure-volume relations in gene-altered mice.

Design of instrumentation and data-acquisition system for complex admittance measurement.

Instantaneous left ventricular volume measurements have been made for many years using a tetrapolar conductance catheter. The main objective is to determine the efficiency of the beating heart, using a tetrapolar catheter inserted in the left ventricle of transgenic mice. The effect of the parallel myocardium contribution must be removed from the total measurement. A dual-frequency technique involving 1 kHz and 100 kHz was chosen because it has been established that the imaginary part (the capacitive reactance) of the complex admittance of the cardiac muscle is much smaller in the lower frequency than at the higher frequency. The design involves generation of an accurate frequency source for both the frequencies careful selection of operational amplifiers for the current conversion stage so that the current is not too large to kill the mouse and that it is capable of performing at high frequencies. The band pass filter stage involved careful design with minimal overlap of the pass bands of both the channels. The overall circuit was designed so that there is minimal shift in the phase due to the circuit elements alone. Work also involved design of GPIB--based data acquisition system using LabVIEW and a digital oscilloscope for effective data acquisition even at high frequencies, which are normally limited by the sampling frequency. This data acquisition system is currently being used in laboratory studies in vivo.

Enhanced left ventricular systolic function early in type 2 diabetic mice: clinical implications.

It is unclear whether the increase in availability of substrates for energy production in diabetes can lead to enhanced systolic function early in the disease, before the onset of structural changes to the myocardium. To examine this issue, BKS.Cg-m +/+ Lepr db (db/db) mice with type 2 diabetes and wild type controls had left ventricular pressure-volume relationships determined in situ. We demonstrated that the db/db mice, when compared to their wild type controls, generated greater left ventricular pressure and an enhancement of left ventricular systolic function based on enhanced power/EDV, positive dP/dt, preload recruitable stroke work, dP/dt--EDV relationship, and curvilinear end-systolic elastance. This enhancement in systolic function occurred despite the db/db mice having greater body weight, but similar preload (end-diastolic volume) and afterload (effective arterial elastance). We postulate that the previously described enhancement in renal glomerular filtration rate seen early in type 2 diabetes may be in part due to enhanced left ventricular systolic function early in this disease.

Evidence of time-varying myocardial contribution by in vivo magnitude and phase measurement in mice.

Cardiac volume can be estimated by a conductance catheter system. Both blood and myocardium are conductive, but only the blood conductance is desired. Therefore, the parallel myocardium contribution should be removed from the total measured conductance. Several methods have been developed to estimate the contribution from myocardium, and they only determine a single steady state value for the parallel contribution. Besides, myocardium was treated as purely resistive or mainly capacitive when estimating the myocardial contribution. We question these assumptions and propose that the myocardium is both resistive and capacitive, and its contribution changes during a single cardiac cycle. In vivo magnitude and phase experiments were performed in mice to confirm this hypothesis.

Enhancement of contractility with sustained afterload in the intact murine heart: blunting of length-dependent activation.

BACKGROUND: It has been hypothesized that because of its rapid heart rate, the intact murine heart functions near maximal contractility in the basal state. If this hypothesis is correct, then the fast and slow components of myocardial length-dependent activation should be blunted compared with larger mammals. METHODS AND RESULTS: Mice (n=24) were anesthetized, and via an open chest, LV pressure-volume relationships were determined by a dual-frequency conductance catheter system. Baseline pressure-volume relationships were determined during transient occlusion of the inferior vena cava, and repeat measurements were made after 1 (n=10) and 7 (n=21) minutes of sustained aortic occlusion. Control experiments were performed in a subset of mice (n=3). For baseline to 1 minute, an increase in afterload (maximal pressure 95+/-9 to 126+/-7 mm Hg; P<0.001) and effective arterial elastance (5.9+/-3.1 to 9.2+/-3.9 mm Hg/microl; P<0.001) resulted in an increase in end-diastolic volume (31+/-8 to 35+/-9 microL; P<0.001). The result was maintenance of stroke volume (17+/-6 to 15+/-6; P=NS) owing to an increase in contractility (leftward shift in V100 [the volume of end-systolic elastance at 100 mm Hg], 24+/-9 to 16+/-5 microL; P<0.001). No additional augmentation of systolic function was found at 7 minutes. CONCLUSIONS: This study demonstrates that the fast phase of length-dependent activation is intact but not the slow phase, consistent with murine myocardium functioning near maximal contractility in the basal state.