Measuring Liver Mitochondrial Oxygen Consumption and Proton Leak Kinetics to Estimate Mitochondrial Respiration in Holstein Dairy Cattle

Summary

Here, we share methods for measuring mitochondrial oxygen consumption, a defining concept of nutritional energetics, and proton leak, the primary cause of inefficiency in mitochondrial generation of ATP. These results can account for 30% of the energy lost in nutrient utilization to help evaluate mitochondrial function.

Abstract

Oxygen consumption, proton motive force (PMF) and proton leak are measurements of mitochondrial respiration, or how well mitochondria are able to convert NADH and FADH into ATP. Since mitochondria are also the primary site for oxygen use and nutrient oxidation to carbon dioxide and water, how efficiently they use oxygen and produce ATP directly relates to the efficiency of nutrient metabolism, nutrient requirements of the animal, and health of the animal. The purpose of this method is to examine mitochondrial respiration, which can be used to examine the effects of different drugs, diets and environmental effects on mitochondrial metabolism. Results include oxygen consumption measured as proton dependent respiration (State 3) and proton leak dependent respiration (State 4). The ratio of State 3 / State 4 respiration is defined as respiratory control ratio (RCR) and can represent mitochondrial energetic efficiency. Mitochondrial proton leak is a process that allows dissipation of mitochondrial membrane potential (MMP) by uncoupling oxidative phosphorylation from ADP decreasing the efficiency of ATP synthesis. Oxygen and TRMP+ sensitive electrodes with mitochondrial substrates and electron transport chain inhibitors are used to measure State 3 and State 4 respiration, mitochondrial membrane PMF (or the potential to produce ATP) and proton leak. Limitations to this method are that liver tissue must be as fresh as possible and all biopsies and assays must be performed in less than 10 h. This limits the number of samples that can be collected and processed by a single person in a day to approximately 5. However, only 1 g of liver tissue is needed, so in large animals, such as dairy cattle, the amount of sample needed is small relative to liver size and there is little recovery time needed.

Introduction

Mitochondria are very sensitive to stress and their cellular environment can contribute to a wide variety of metabolic diseases. Oxygen consumption and proton leak in mitochondria are indicators of mitochondria health. The methods described in this paper estimate mitochondrial energy efficiency using RCR based on oxygen consumption with and without proton leak. These results can account for 30% of the energy lost in nutrient utilization¹. Changes in oxygen consumption and proton leak can identify mitochondrial dysfunction which contributes to metabolic disease and results in decreased energy efficiency. These methods can also be used to examine....

Protocol

All methods, protocol and studies described here were approved by the Institutional Animal Care and Use Committee (IACUC) of University of California, Davis.

1. Obtaining a Liver Biopsy from a Holstein Dairy Cow

NOTE: A liver biopsy should be performed by a licensed veterinarian. Liver biopsies can be performed on the dairy site where the cows are located. Lactating dairy cows can continue to be milked normally and milk does not need to be withdrawn from the food supply before or after the procedure. It is recommended that at least 4 people are needed to perform the liver biopsy on a dairy cow: a veterinarian to pe....

Results

Positive results showing RCR and proton leak kinetics are shown in Table 1 and Figure 15, respectively. In this study⁷, RCR and protein leak kinetics were measured in Holstein dairy cows at 70 days in milk after cows had been fed 1 of 5 different levels of Cu, Zn and Mn for 28 days. State 4, maximum proton leak-dependent respiration, had a tendency to be affected by mineral intake of Cu, Mn and Zn (p < 0.1.......

Discussion

The most critical point in the protocol is obtaining a representative liver tissue sample and beginning the isolation of mitochondria as soon as possible after biopsy. Variation in respiration measurements is low (Table 1) due to a short transport time from cow to laboratory. To reduce transport time, a small laboratory was set up in the office of the dairy, and liver samples were driven to the office laboratory as each was collected so that mitochondria were isolated within 10 min of biopsy. Setup and t.......

Materials

Name	Company	Catalog Number	Comments
Liver Biopsy
Equipment
Schackelford-Courtney bovine liver biopsy instrument	Sontec Instruments Englewood CO	1103-904
Suture	Fisher Scientific	19-037-516
Suture needles	NA	NA	Included with Suture
Scalpels	Sigma - Aldrich	S2896 / S2646	# for handle and blades
Surgery towels	Fisher Scientific	50-129-6667
Falcon tubes 50 mL	Fisher Scientific	14-432-22
Tweezers	Sigma - Aldrich	Z168750
50 mL syringes	Fisher Scientific	22-314387
Injection needles (22, 2 1/2)	VWR	MJ8881-200342
Cow halter	Tractor Supply Co.	101966599
Cotton swabbing	Fisher Scientific	14-959-102
cotton gauze squares (4x4)	Fisher Scientific	22-246069
Medical scissors	Sigma - Aldrich	Z265969
Chemicals
Coccidiosis Vaccine 0.75 bottle/cow			Provided by Veterinarian
Clostridia Vaccine			Provided by Veterinarian
Liver biopsy antibiotics excenel 2 cc/100 lbs for 3 days			Provided by Veterinarian
Providone Scrub	Aspen Veteterinary Resources	21260221
Ethanol 70%	Sigma - Aldrich	793213
Xylazine hydrochloride 100 mg/mL IV at 0.010-0.015 mg/kg bodyweight			Provided by Veterinarian
2% lidocaine HCl (10-15 mL)			Provided by Veterinarian
1 mg/kg IV injection of flunixin meglumine			Provided by Veterinarian
Isolation of Mitochondria (liver)
Equipment
Wheaton vial 30 mL with a Teflon pestle of 0.16 mm clearance	Fisher Scientific	02-911-527
Homogenizer Motor	Cole Parmer	EW-04369-10
Homogenizer Probe	Cole Parmer	EW-04468-22
Auto Pipette (10 mL)	Cole Parmer	SK-21600-74
Beaker (500 mL) with ice	Fisher Scientific	FB100600
Refrigerated microfuge	Fisher Scientific	75-002-441EW3
Microfuge tubes (1.5 mL)	Fisher Scientific	AM12400
Chemicals
Bicinchoninic acid (BCA) protein assay kit (microplates for plate reader)	abcam	ab102536
Sucrose	Sigma - Aldrich	S7903-1KG
Tris-HCl	Sigma - Aldrich	T1503-1KG
EDTA	Sigma - Aldrich	EDS-1KG
BSA (fatty acid free)	Sigma - Aldrich	A7030-50G
Mannitol	Sigma - Aldrich	M4125-1KG
Deionized water	Sigma - Aldrich	38796
Hepes	Sigma - Aldrich	H3375-500G
Use to create mitochondria isolation media: 220 mM mannitol, 70 mM sucrose, 20 mM HEPES, 20 mM Tris-HCl, 1 mM EDTA, and 0.1% (w/v) fatty acid free BSA,  pH 7.4 at 4 °C, will last 2 days in refrigerator
Mitochondrial Oxygen Comsuption
Equipment
Oxygraph Setup + Clark type oxygen electrode	Hansatech (PP Systems)	OXY1
Thermoregulated Water Pump	ADInstruments	MLE2001
Clark type Oxygen electrode	NA	NA
Autopipette (1 mL)	Cole Parmer	SK-21600-70	Included with Oxy1
Small magnetic stir bar	Fisher Scientific	14-513-95
Micropipette (10 μL)	Cole Parmer	SK-21600-60
pH meter	VWR
Chemicals
KCl	Sigma - Aldrich	P9333-1KG
Hepes	Sigma - Aldrich	H3375-500G
KH2PO4	Sigma - Aldrich	P5655-1KG
MgCl2	Sigma - Aldrich	M1028-100ML
EGTA	Sigma - Aldrich	E3889-100G
Use to make mitochondrial oxygen consumption media: 120 mM KCL, 5 mM KH2PO4, 5 mM MgCl2, 5 mM Hepes and 1 mM EGTA,  pH 7.4 at 30 °C with 0.3% defatted BSA
Rotenone (4 mM solution)	Sigma - Aldrich	R8875-5G
Succinate (1 M solution)	Sigma - Aldrich	S3674-250G
ADP (100 mM solution)	Sigma - Aldrich	A5285-1G
Oligomycin (solution of 8 μg/mL in ethanol)	Sigma - Aldrich	75351
FCCP	Sigma - Aldrich	C2920
Mitochondrial Membrane Potential and Proton Motive Force
Equipment
TPMP electrode	World Precision Instruments.	DRIREF-2
Chemicals-solutions do not need to be fresh but they do need to be kept in a freezer between runs
Malonate (0.1 mM solution)	Sigma - Aldrich	M1296
Oligomycin (8 μg/mL in ethanol), keep in freezer	Sigma - Aldrich	75351
Nigericin (80 ng/mL in ethanol), keep in freezer	Sigma - Aldrich	N7143
FCCP	Sigma - Aldrich	C3920
TPMP	Sigma - Aldrich	T200
TPMP solution: 10 mM TPMP, 120 mM KCL, 5 mM Hepes and 1 mM EGTA,  pH 7.4 at 30 °C with 0.3% defatted BSA