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Glucose oxidation and oxygen consumption of isolated guinea pig and muskrat hearts
Camp. Biochem. Physiol. Vol. 86A, No. 2, pp. 381-382, Printed in Great Britain
GLUCOSE OXIDATION OF ISOLATED GUINEA
0300-9629187 $3.00 + 0.00 I(’ 1987 Pergamon Journals Ltd
1987
AND OXYGEN CONSUMPTION PIG AND MUSKRAT HEARTS*
THOMAS A. MCKEAN Department
of Biological
Sciences, University of Idaho, Telephone: (208) 885-8919 (Receiced
Moscow,
ID 83843, USA.
14 MUJ? 1986)
Abstract-l. Glucose in Krebs-Henseleit buffer was presented to isolated Langendorff perfused muskrat and guinea pig hearts that were paced at 240 beats/min. 2. Glucose uptake (amount removed from the perfusion fluid) was 3 times greater in the muskrat hearts than in the guinea pig heart. 3. Glucose oxidation (amount converted to CO,) and oxygen consumption did not differ in the hearts of the two species. 4. When glucose is the only exogenous substrate, isolated muskrat hearts extract more glucose than guinea pig hearts but oxidize similar amounts of glucose and have a similar myocardial oxygen consumption.
INTRODUCTION
Muskrats (Ondutra zibethicus) are among the most common freshwater diving mammals. Muskrats are good divers for their size with a submersion time of about 4 min (Jones et al., 1982). They show a number of adaptations to prolonged submersion including a well developed diving response consisting of a bradycardia and a peripheral vasoconstriction (Jones et al., 1982). The muskrat has large oxygen stores that are in the lower range of those reported for marine mammals (Snyder and Binkley, 1985). In spite of adaptations for oxygen conservation in diving mammals, hypoxemia may occur as arterial oxygen tension mav fall to levels as low as 15 torr in forced diving situations (Elsner et al., 1970 and McKean, 1982). To test the divine mammal’s cardiac tolerance to hypoxia, McKean and Landon, 1982 and McKean, 1984 compared the responses of muskrat and guinea pig (Cavia porcellus) hearts to oxygen deprivation and found the muskrat hearts much better suited to recover from 30 min of hypoxia than were guinea pig hearts. One of the reasons for this tolerance to hypoxia is that the isolated muskrat heart responds to hypoxia by an extreme bradycardia, sometimes to the point of asystole. In contrast, the hypoxic bradycardia of guinea pigs is about 60% of their normoxie heart rate. Presumably the arrested or slowly beating heart is using less ATP for contraction. A second mechanism for the muskrat’s heart being hypoxia-resistant is a greatly accelerated uptake of glucose and release of lactate during hypoxia compared to hypoxic guinea pig hearts. This would allow the muskrat heart to produce more ATP compared to the guinea pig heart. Previous studies from this laboratory (McKean, 1984 and McKean et al., 1986) have shown that, even
*Supported by a grant ation of Alaska.
from
the American
Heart
Associ-
under normoxic conditions, glucose uptake by isolated muskrat hearts is about three times greater than glucose uptake by isolated guinea pig hearts. The purpose of this study was to determine how much of the glucose that was taken up was oxidized to CO> and the oxygen consumption that was associated with this oxidation. MATERIALS
AND METHODS
Experiments were performed on adult animals of either sex. The muskrats were live-trapped in Idaho and Washington during the spring months and were used within 2 days of capture. The guinea pigs were purchased from a licensed animal dealer. Animals were anesthetized with ether and their hearts removed and placed in ice-cold Krebs-Henseleit bicarbonate solution with 2.5 mM Cal+, glucose 5.56 mM and insulin 12.5 Units/l. The hearts were perfused retrograde via the aorta (Langendorff preparation) with the perfusate heated to 37 ‘C. filtered with an in-line 0.65 micron filter and bubbled with 95% O,-5% COZ. The pH of the gassed solution was 7.4. Coronary flow was controlled with a roller pump so that aortic diastolic pressure was maintained at 60cm HzO. Left ventricular pressure was monitored with a fluid filled balloon inserted via the left atrium. Hearts were paced at 240 beats/min by means of stainless steel electrodes inserted into the ventricular wall. Oxygen partial pressure was measured in the aortic perfusion line and in a catheter that was placed in the pulmonary outflow tract. Clark type O2 electrodes in a 37 ‘C, stirred chamber were used. The pressures were converted to O2 content and myocardial O2 consumption (STPD) was calculated according to arteriovenous Or difference multiplied by the coronary flow divided by heart mass. In some experiments a fuel cell O2 content device (Lex-02-con) was used to measure arterial and venous oxygen content. Arterial and venous glucose concentrations were measured in quintuplicate using Sigma Chemical Co. glucose kits. Glucose uptake was calculated as the arteriovenous glucose difference multiplied by the coronary flow divided by the heart mass. One hundred microcuries of uniformly labeled 14C glucose were added to each liter of perfusion fluid. ?Z02 appeared when the glucose was oxidized by the myocardium. The 381
382
THOMAS A. MCKEAN Table
I. Oxygen consumptmn
and glucose oxidation
Oxygen consumption pljg per min Guinea pig (IV = 6) Muskrat (I!’ = 5)
coronary effluent from the pulmonary artery was collected under mineral oil. A 2 ml sample was injected into a rubber stoppered vial in which filter paper soaked in CO, absorber was suspended. Five drops of I N HCI were added to the sample to convert the HI4 CO, to 14C02 and the “‘CO, was absorbed by the filter paper. After 1 hr the paper was transferred to a scintillation vial and counted and compared to the 14C activity in the arterial perfusate. Validation of the technique with added H14C0, demonstrated 99% recovery of 14C as 14C0 Values are pfesented as the mean k SEM. The t-test was used to establish statistical significance at the 0.05 level or less.
RESULTS
51.4 ? 3.2 55.9 i 3.0
by heart
Glucose pmd/g
oxidation per min
0.34 + 0.04 0.41 & 0.09
compared to guinea pig hearts (McKean, 1984 and McKean et al., 1986) and accounts for most of the incomplete metabolism of glucose. The results of this study indicate that, when given glucose as the only exogenous substrate, isolated muskrat hearts extract more exogenous glucose than guinea pig hearts. Both species oxidize a similar amount of glucose and have a similar oxygen consumption. Acknowledgement-1 thank Margaret Dibble for technical assistance and Rolf Ingermann for helpful comments,
AND DISCUSSION REFERENCES
A comparison of exogenous glucose oxidation and O2 consumption for the hearts of the two species is shown in Table 1. There are no statistical differences between the hearts of the two species for either oxygen consumption or exogenous glucose oxidation. The oxygen consumption for guinea pig hearts in this study was only slightly lower than guinea pig heart oxygen consumption reported by Dewitt et al., 1983 In a different group of muskrats (N = 5) and guinea pigs (N = 4) glucose uptake and oxygen consumption (fuel cell O2 analyzer) were measured under the conditions of 240 beats/min pacing. In these animals, 0, consumption was 55.0 + 4.0 pmoles/g per min for muskrat hearts and 49.6 f 1.5 for guinea pig hearts. Glucose uptake was 0.42 f 0.10 pmol/min per g for guinea pig and 1.52k 0.52 for the muskrat hearts. The difference in glucose uptake by the hearts of the two species was statistically significant but the difference in oxygen consumption was not. A comparison of the glucose uptake vs glucose oxidation indicates that the guinea pig heart oxidizes most of the glucose that is taken up whereas the muskrat heart oxidizes about a third. Lactate production was about 5-6 times greater in muskrat
Dewitt D. F., Wangler R. D., Thompson C. I. and Sparks H. V. Jr. (1983) Phasic release of adenosine during steady state metabolic stimulation in the isolated guinea pig heart. Circ. Res. 53, 636643. Elsner R., Shurley J. T., Hammond D.D. and Brooks R. E. (1970) Cerebral tolerance to hypoxemia in asphyxiated Weddell seals. Respir. Physiol. 9, 287-297. Jones D. R., West N. H., Bamford 0. S., Drummond P. C. and Lord R. A., (1982) The effect of stress of forcible submergence on the diving response of muskrats (Ondatra zibethica). Can J. Zool. 60, 187-193. McKean T. (1984) Response of isolated muskrat and guinea pig hearts to hypoxia. Physiol Zool. 57, 5777562. McKean T. and Landon R., (1982) Comparision of the response of muskrat, rabbit, and guinea pig heart muscle to hypoxia. Am. J. Physiol. 243, R245-R250. McKean T. (1982) Cardiovascular adjustments to laboratory driving in beavers and nutria. Am. J. Physiol. 242, R434R440. McKean T., Schmidt D., Tingey J. M., Wingerson D. and Seeb W. (1986) The use of glucose, lactate, pyruvate, and palmitic acid by muskrat and guinea pig hearts. Physiol. Zool. 59, 283-292. Snyder G. K. and Binkley E. L. (1985) Oxygen transport, tissue glycogen stores, and tissue pyruvate kinase activity in muskrats. Can. J. Zool. 63, 144C-1444.
GLUCOSE OXIDATION OF ISOLATED GUINEA
0300-9629187 $3.00 + 0.00 I(’ 1987 Pergamon Journals Ltd
1987
AND OXYGEN CONSUMPTION PIG AND MUSKRAT HEARTS*
THOMAS A. MCKEAN Department
of Biological
Sciences, University of Idaho, Telephone: (208) 885-8919 (Receiced
Moscow,
ID 83843, USA.
14 MUJ? 1986)
Abstract-l. Glucose in Krebs-Henseleit buffer was presented to isolated Langendorff perfused muskrat and guinea pig hearts that were paced at 240 beats/min. 2. Glucose uptake (amount removed from the perfusion fluid) was 3 times greater in the muskrat hearts than in the guinea pig heart. 3. Glucose oxidation (amount converted to CO,) and oxygen consumption did not differ in the hearts of the two species. 4. When glucose is the only exogenous substrate, isolated muskrat hearts extract more glucose than guinea pig hearts but oxidize similar amounts of glucose and have a similar myocardial oxygen consumption.
INTRODUCTION
Muskrats (Ondutra zibethicus) are among the most common freshwater diving mammals. Muskrats are good divers for their size with a submersion time of about 4 min (Jones et al., 1982). They show a number of adaptations to prolonged submersion including a well developed diving response consisting of a bradycardia and a peripheral vasoconstriction (Jones et al., 1982). The muskrat has large oxygen stores that are in the lower range of those reported for marine mammals (Snyder and Binkley, 1985). In spite of adaptations for oxygen conservation in diving mammals, hypoxemia may occur as arterial oxygen tension mav fall to levels as low as 15 torr in forced diving situations (Elsner et al., 1970 and McKean, 1982). To test the divine mammal’s cardiac tolerance to hypoxia, McKean and Landon, 1982 and McKean, 1984 compared the responses of muskrat and guinea pig (Cavia porcellus) hearts to oxygen deprivation and found the muskrat hearts much better suited to recover from 30 min of hypoxia than were guinea pig hearts. One of the reasons for this tolerance to hypoxia is that the isolated muskrat heart responds to hypoxia by an extreme bradycardia, sometimes to the point of asystole. In contrast, the hypoxic bradycardia of guinea pigs is about 60% of their normoxie heart rate. Presumably the arrested or slowly beating heart is using less ATP for contraction. A second mechanism for the muskrat’s heart being hypoxia-resistant is a greatly accelerated uptake of glucose and release of lactate during hypoxia compared to hypoxic guinea pig hearts. This would allow the muskrat heart to produce more ATP compared to the guinea pig heart. Previous studies from this laboratory (McKean, 1984 and McKean et al., 1986) have shown that, even
*Supported by a grant ation of Alaska.
from
the American
Heart
Associ-
under normoxic conditions, glucose uptake by isolated muskrat hearts is about three times greater than glucose uptake by isolated guinea pig hearts. The purpose of this study was to determine how much of the glucose that was taken up was oxidized to CO> and the oxygen consumption that was associated with this oxidation. MATERIALS
AND METHODS
Experiments were performed on adult animals of either sex. The muskrats were live-trapped in Idaho and Washington during the spring months and were used within 2 days of capture. The guinea pigs were purchased from a licensed animal dealer. Animals were anesthetized with ether and their hearts removed and placed in ice-cold Krebs-Henseleit bicarbonate solution with 2.5 mM Cal+, glucose 5.56 mM and insulin 12.5 Units/l. The hearts were perfused retrograde via the aorta (Langendorff preparation) with the perfusate heated to 37 ‘C. filtered with an in-line 0.65 micron filter and bubbled with 95% O,-5% COZ. The pH of the gassed solution was 7.4. Coronary flow was controlled with a roller pump so that aortic diastolic pressure was maintained at 60cm HzO. Left ventricular pressure was monitored with a fluid filled balloon inserted via the left atrium. Hearts were paced at 240 beats/min by means of stainless steel electrodes inserted into the ventricular wall. Oxygen partial pressure was measured in the aortic perfusion line and in a catheter that was placed in the pulmonary outflow tract. Clark type O2 electrodes in a 37 ‘C, stirred chamber were used. The pressures were converted to O2 content and myocardial O2 consumption (STPD) was calculated according to arteriovenous Or difference multiplied by the coronary flow divided by heart mass. In some experiments a fuel cell O2 content device (Lex-02-con) was used to measure arterial and venous oxygen content. Arterial and venous glucose concentrations were measured in quintuplicate using Sigma Chemical Co. glucose kits. Glucose uptake was calculated as the arteriovenous glucose difference multiplied by the coronary flow divided by the heart mass. One hundred microcuries of uniformly labeled 14C glucose were added to each liter of perfusion fluid. ?Z02 appeared when the glucose was oxidized by the myocardium. The 381
382
THOMAS A. MCKEAN Table
I. Oxygen consumptmn
and glucose oxidation
Oxygen consumption pljg per min Guinea pig (IV = 6) Muskrat (I!’ = 5)
coronary effluent from the pulmonary artery was collected under mineral oil. A 2 ml sample was injected into a rubber stoppered vial in which filter paper soaked in CO, absorber was suspended. Five drops of I N HCI were added to the sample to convert the HI4 CO, to 14C02 and the “‘CO, was absorbed by the filter paper. After 1 hr the paper was transferred to a scintillation vial and counted and compared to the 14C activity in the arterial perfusate. Validation of the technique with added H14C0, demonstrated 99% recovery of 14C as 14C0 Values are pfesented as the mean k SEM. The t-test was used to establish statistical significance at the 0.05 level or less.
RESULTS
51.4 ? 3.2 55.9 i 3.0
by heart
Glucose pmd/g
oxidation per min
0.34 + 0.04 0.41 & 0.09
compared to guinea pig hearts (McKean, 1984 and McKean et al., 1986) and accounts for most of the incomplete metabolism of glucose. The results of this study indicate that, when given glucose as the only exogenous substrate, isolated muskrat hearts extract more exogenous glucose than guinea pig hearts. Both species oxidize a similar amount of glucose and have a similar oxygen consumption. Acknowledgement-1 thank Margaret Dibble for technical assistance and Rolf Ingermann for helpful comments,
AND DISCUSSION REFERENCES
A comparison of exogenous glucose oxidation and O2 consumption for the hearts of the two species is shown in Table 1. There are no statistical differences between the hearts of the two species for either oxygen consumption or exogenous glucose oxidation. The oxygen consumption for guinea pig hearts in this study was only slightly lower than guinea pig heart oxygen consumption reported by Dewitt et al., 1983 In a different group of muskrats (N = 5) and guinea pigs (N = 4) glucose uptake and oxygen consumption (fuel cell O2 analyzer) were measured under the conditions of 240 beats/min pacing. In these animals, 0, consumption was 55.0 + 4.0 pmoles/g per min for muskrat hearts and 49.6 f 1.5 for guinea pig hearts. Glucose uptake was 0.42 f 0.10 pmol/min per g for guinea pig and 1.52k 0.52 for the muskrat hearts. The difference in glucose uptake by the hearts of the two species was statistically significant but the difference in oxygen consumption was not. A comparison of the glucose uptake vs glucose oxidation indicates that the guinea pig heart oxidizes most of the glucose that is taken up whereas the muskrat heart oxidizes about a third. Lactate production was about 5-6 times greater in muskrat
Dewitt D. F., Wangler R. D., Thompson C. I. and Sparks H. V. Jr. (1983) Phasic release of adenosine during steady state metabolic stimulation in the isolated guinea pig heart. Circ. Res. 53, 636643. Elsner R., Shurley J. T., Hammond D.D. and Brooks R. E. (1970) Cerebral tolerance to hypoxemia in asphyxiated Weddell seals. Respir. Physiol. 9, 287-297. Jones D. R., West N. H., Bamford 0. S., Drummond P. C. and Lord R. A., (1982) The effect of stress of forcible submergence on the diving response of muskrats (Ondatra zibethica). Can J. Zool. 60, 187-193. McKean T. (1984) Response of isolated muskrat and guinea pig hearts to hypoxia. Physiol Zool. 57, 5777562. McKean T. and Landon R., (1982) Comparision of the response of muskrat, rabbit, and guinea pig heart muscle to hypoxia. Am. J. Physiol. 243, R245-R250. McKean T. (1982) Cardiovascular adjustments to laboratory driving in beavers and nutria. Am. J. Physiol. 242, R434R440. McKean T., Schmidt D., Tingey J. M., Wingerson D. and Seeb W. (1986) The use of glucose, lactate, pyruvate, and palmitic acid by muskrat and guinea pig hearts. Physiol. Zool. 59, 283-292. Snyder G. K. and Binkley E. L. (1985) Oxygen transport, tissue glycogen stores, and tissue pyruvate kinase activity in muskrats. Can. J. Zool. 63, 144C-1444.