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Coronary ligation reduces maximum sustained swimming speed in chinook salmon, Oncorhynchus tshawytscha
Camp. Biochem. Physiol. Printed in Great Britain
Vol.87A, No. I, pp.
35-37,
0300-9629187 $3.00 + 0.00 Q 1987 Pcrgamon Journals Ltd
1987
CORONARY LIGATION REDUCES MAXIMUM SUSTAINED SWIMMING SPEED IN CHINOOK SALMON, ONCORHYNCHUS
TSHA WYTSCHA
A. P. FARRELL and J. F. SmrrENsEh’*t Biological Sciences, Simon Fraser University, Bumaby, British Columbia, Canada, V5A lS6. Telephone: (604) 291475; *Zoology Department, University of British Columbia, Vancouver, B.C., Canada, V6T 2A9; iPresent address, Dept. of Zoophysiology, University of Aarhus, DK-8000 Aarhus, Denmark (Received 3 July 1986)
Abstract-l. The maximum aerobic swimming speed of Chinook salmon (Oncorhynchus rshuwyrsha) was measured before and after ligation of the coronary artery. 2. Coronary artery ligation prevented blood flow to the compact layer of the ventricular myocardium, which represents 30% of the ventricular mass, and produced a statistically significant 35.5% reduction in maximum swimming speed. 3. We conclude that the coronary circulation is important for maximum aerobic swimming and implicit in this conclusion is that maximum cardiac performance is probably necessary for maximum aerobic swimming performance.
INTRODUCTION
MATERIALS AND METHODS
A coronary circulation is not found in all fish, but when present it supplies arterial blood to the outer compact myocardium of the ventricle (Santer, 1985). The remainder of the ventricle, or the entire ventricle in fish lacking a coronary circulation, is comprised of spongy myocardium which derives oxygen from the venous blood being pumped through the ventricle. The relative proportion of compact myocardium is directly related to the activity level of the fish species. Compact myocardium accounts for 6@-70% of the ventricle mass in active fish such as the tuna, 30-40% in salmonids, and is absent in sedentary fish (Santer and Greer Walker, 1980). Thus, it appears that the coronary circulation provides an essential myocardial oxygen supply for the high level of cardiac performance that is required during sustained swimming, given that fish hearts generally operate aerobically and myocardial oxygen consumption is linearly related to myocardial power output (Turner and Driedzic, 1980; Driedzic et al., 1983; Farrell, 1984; Farrell et al., 1985). Daxboeck (1982) tested this hypothesis by measuring the maximum sustained swimming speed, Lice,, before and after coronary ablation in S. gairdneri. Coronary ablation had no effect on Um,. However, Daxboeck (1982) noted coronary regrowth around the ablation site after the 7 day post-surgery recovery, which may have provided an adequate oxygen supply to the compact myocardium during swimming. In view of the theoretical argument which suggests that the coronary circulation should be essential for maximum aerobic performance of the heart, the present investigation essentially repeated the swimming experiments of Daxboeck (1982). However, the coronary artery was occluded acutely in an attempt to circumvent the problems of coronary vessel regrowth.
Oncorhynchuy rshawytscha (0.9-2.2 kg, N = 9) were obtained from sea pens at the Pacific Biological Station, Nanaimo, British Columbia. They were transported to Bamfield Marine Station, Bamtield, British Columbia, where they were held indoors in large tibreglass tanks supplied with flowing, aerated sea-water. The fish were acclimated to these conditions for l-2 weeks, but were not fed during this period. The experiments were conducted at the ambient water temperature, 10.5-l 1°C.
35
Swim tunnel and measurement of exercise performance The experiments were performed in the original Brett swimming respirometer (Brett, 1964) which has a volume of 1201. Vertical stripes of tape on the swimming section provided visual cues for orientation of the swimming fish, and an electrified screen encouraged the fish to swim continuously. Water oxygen tension in the respirometer was kept above 90% saturation throughout the experiment by a continuous flow (1 S-20 l/min) of aerated sea-water through the respirometer. Swimming speed was calculated by correcting the water velocity for the solid blocking effect of the fish (Bell and Terhune, 1970). Swimming performance was determined by increasing the swimming velocity by 0.25 bl/sec every 20 min until fatigue occurred (bl = body length). Critical swimming speed, C.Jti,, was calculated as described by Brett (1964) using:
UC,,,= u, + (&, x ri,) where fI, = the highest velocity maintained for the 20-min period (cm,&ec); L’,,= velocity increment (cm/s); li = time fish swam at the “fatigue” velocity (min); I,, = 20-min period of swimming. This formula interpolates for those fish that did not fatigue exactly at the beginning or the end of the 20-min swimming period. Experiment protocols
Each fish was anaesthetized (1:5,000 solution of ethyl m-amino benzoate, Sigma Chemicals) and placed supine on an operating sling with the gills irrigated (1: 15,000 ethyl m-amino benzoate). A small incision (< 1 cm) was made in the side of the isthmus to reveal the most cephalic portion
36
A. P. FAKKELL and J. F. STEFFFNSEN
of the ventral aorta inside the pericardial cavity. Here the coronary artery joins the surface of the ventral aorta. A #3-O silk thread was looped around the coronary artery and the two ends were Icd out of the incision. The incision was closed with #3-O silk. The fish, which was usually reviving, was quickly transferred to the swim tunnel, where it recovered its righting reflex and swimming ability within l-2 min. The operation usually took 5 min and bleeding was usually negligible. The fish recovered overnight (> 8 hr) at a swimming speed of 0.8 I blisec. After fatigue was established, the fish was quickly removed from the respirometer, lightly anacsthetized (I : 15,000 ethyl m-amino benzoatc), and placed on the operating sling. The silk loop was tightened to occlude the coronary artery and the fish was returned to the swim tunnel. This procedure took about 6 min and the fish quickly recovered its righting reflex in the swim tunnel. The swimming speed was progressively increased from 0.5 blisec to 1.0 bl/sec during the recovery period. G:,+ was remeasured after a minimum recovery time of 4 hr. Long recovery periods were not used for three reasons. First, WC wished to avoid coronary regrowth around the ablation site. Second, electrical power failures in Bamfield made shortterm experiments more desirable. Third, Brett (1964) found that the 0, debt associated with swimming to fatigue is largely paid off 4 hr after swimming. One Bsh was swum twice with the coronary artery ligated, once after a 4-hr recovery and then again after a further overnight recovery. CJti, was reduced by a similar amount after both recovery periods (51 and 42%, respectively), but small vessels were already forming around the ligature site. fish were killed by a Following the LJm, measurements, sharp blow to the head. The patency and placement of the coronary ligature were checked by visual inspection. The ventricle was removed, blotted, weighed and placed in 10% buffered formaldehyde. The relative proportion of compact and spongy myocardium was determined by dissecting the
compact layer away and comparing the weights of the two tissues afte; drying to a consiant weight at-60°C. Statistical differences (P < 0.05) were determined the Student’s paired f-test.
using
RESULTS
ligation was The mean U,,,, before coronary 1.90bl!sec (SD = 0.253, N = 5). U,, was significantly reduced after acute coronary ligation (I .23 blisec, SD = 0.345) (Table I). This 35.5% reduction in the maximum sustained swimming speed provides support for the hypothesis that the coronary circulation is necessary for maximum aerobic swimming performance. The compact myocardium represented 30.4% (SD = 4.0, N = 5) of the ventricles mass. In an additional fish, only the left branch of the coronary artery was occluded because the ligature was inadvertently placed downstream of the coronary branch and thus, a portion of its 30.5% compact myocardium continued to receive coronary flow. In this fish, coronary ligation reduced Uci,,,,by 12%. DISCUSSION u Crl, was determined following anaesthesia, a brief operation and overnight recovery. Glova and McInerney (1977) who were unable to demonstrate a significant difference in swimming performance between young coho salmon (Oncorhynchus kisurch), allowed 1 and I2 hr recovery in the swimming chamber. As well, Randall et al. (in preparation) measured Uai, in unoperated. but smaller Chinook salmon at 10°C. Their Ucri, values of 59.6cm/sec
Table I. Body length (W, cm --45.5(5.lcn
of
coronary artery ligation on formance in Chinook Salmon
Rody WI kg I .3 I (0.542)
Control
per-
c’c,,,. bl;sL*: Alicr ligation
I .90 (0.253)
Values presented as mean (SD) for five fish. ‘Denotes significant difference in CC,,, from (P < 0.05).
* I .23 (0.345) c~n~ml
value
(SEM = 9.9) and 83.5 cm/see (SEM = 13.2) for fish weighing 76 g (SEM = 12) and 319 g (SEM = 39) compare well with our CJcn,value of 86.5 cmjsec for a 1.31-kg fish. Furthermore, the U,,, was similar in four additional fish (86.0cm/sec), one of which had only the left branch of the coronary ligated later and three of which died as a result of either a power failure or surgical mishaps. Consequently, we believe that the anaesthesia and surgery had a minimal effect on Ucricafter the overnight recovery. There was at least a 4-hr recovery between Ua, determinations before and after coronary artery ligation. The 4-hr recovery time was a compromise between adequate recovery from the exercise and avoidance of coronary vessel regrowth. It is unlikely that the initial exercise regime affected the UC,, dctcrmination after coronary ligation because the 0, debt would have been largely paid off (Brett, 1964). The 35.5% reduction in UCi,,,after coronary artery ligation occurred because coronary flow was eliminated and rendered the compact myocardium hypoxic. With about a third of the ventricle hypoxic, it is unlikely that the heart can generate its maximum power output (Turner and Driedzic, 1980; Farrell et ul., 1985) and maximum aerobic swimming pcrformance is precluded by the inability of the heart to perform maximally. Therefore, by implication, maximum cardiac performance is required for swimming at UC,,,.This is also supported by our observation that when only half of the coronary circulation was tied off in one fish, U,,, was better maintained (only a 12% reduction). Our conclusion refutes the Daxboeck (1982) study with rainbow trout, where chronic coronary ablation had no effect on maximum aerobic swimming speed. Two explanations may be put forward to resolve these different observations. Foremost, coronary vessel regrowth around the ablation site in the chronic study with rainbow trout may have restored some or all of the coronary blood flow. Our acute experiments avoided overt vessel regrowth. In studies with mammals, collateral vessels can develop around an occlusion site in 24 hr and fully restore coronary flow (Schapcr, 1971). The message here is that, as in mammals, coronary flow is important in fish and rapid adjustments are made to restore coronary flow. Secondly, Daxboeck (1982) worked with 300g rainbow trout and small fish have a thinner layer of compact myocardium. It is possible that O2 diffusion from the lumen of the ventricle to the compact layer was significant in the rainbow trout. We reduced this confounding factor by using larger fish in which the compact myocardium is more developed. Santer and Greer Walker (1980) have measured the relative proportion of compact and spongy myocardium in salmonids. The compact epicardium rep-
Coronary
ligation reduces salmon’s maximum speed
resents 30-40% of the ventricular mass. The compact myocardium represented about 30% of the ventricle in the present study with 0. tshawytscha, and is, therefore, consistent with previous observations (Santer and Greer Walker, 1980). Loss of 30.4% compact myocardium was associated with 35.5% reduction in UC,,. Too little is known about cardiac and coronary physiology in fish at this stage to explain why cardiac performance, per cent compact myocardium and Ucri,were so closely related. In summary, the present experiments support the hypothesis that blood flow to the compact myocardium in Chinook salmon is important for maximum sustained aerobic swimming. Acknowledxemenrs-We would like to thank Dr Craig Clarke, Pacific Biological Station, Nanaimo, British Columbia, for supplying fish. The technical expertise of Anna-Marie Hammons was appreciated in determining the tissue partitioning of the ventricles. The cooperation of the Director and staff of the Bamfield Marine Station was most appreciated. This work was supported by a grant to A.P.F. from the British Columbia Health Care Foundation. J.F.S. was supported by a N.A.T.O. Science Fellowship. the Danish Natural Science Research Council and the Carlsberg Foundation. REFERENCES Bell W. H. and Terhune
L. D. B. (1970) Water tunnel design for fisheries research. J. Fish. Res. Bd Can. Tech. Rep. No 195, p. 69.
37
Brett J. R. (1964) The respiratory metabolism and swimming performance of young sockeye salmon. J. Fish. Res. Bd Can. 21, 1183-1226. Daxboeck C. R. (1982) Effect of coronary artery ablation on exercise performance in Salmo gairdneri. Can. J. 2001.60, 375-381. Driedzic W. R., Scott, D. L. and Farrell A. P. (1983) Aerobic and anaerobic contributions to energy metabolism in perfused isolated sea raven (Hemifripferus americanus) hearts. Can. J. Zool. 61, 188&1883. Farrell A. P. (I 984) A review of cardiac performance in the tcleost heart: intrinsic and humoral regulation. Gun. J. 2001. 62, 523 -536. Farrell A. P., Wood S., Hart T. and Driedzic W. R. (1985) Myocardial oxygen consumption in the sea raven, Hemifriprerus americanus: The effects of volume loading, pressure loading, and progressive hypoxia. J. exp. Biol. 117, 237.-250. Glova G. J. and Mclnerney J. E. (1977) Critical swimming speeds of Coho salmon (Oncorhynchus kisutch) fry to smolt stages in relation to salinity and temperature. J. Fish. Res. Bd Can. 34, 151-154. Randall D. J., Mense D., Boutillier B. (in preparation). Santer R. M. (1985) Morphology and Innemotion of the Fish Hearr. Springer-Verlag, Berlin. Santer R. M. and Greer Walker M. (1980) Morphological studies on the ventricle of tcleost and elasmobranch hearts. J. Zool. 190, 259-272. Schaper W. (197 I) The Collateral Circulation of the Heart. North-Holland, Amsterdam. Turner J. D. and Driedzic W. R. (1980) Mechanical and metabolic response of the perfused isolated fish heart to anoxia and acidosis. Con. J. Zoo/. 58, 886-889.
Vol.87A, No. I, pp.
35-37,
0300-9629187 $3.00 + 0.00 Q 1987 Pcrgamon Journals Ltd
1987
CORONARY LIGATION REDUCES MAXIMUM SUSTAINED SWIMMING SPEED IN CHINOOK SALMON, ONCORHYNCHUS
TSHA WYTSCHA
A. P. FARRELL and J. F. SmrrENsEh’*t Biological Sciences, Simon Fraser University, Bumaby, British Columbia, Canada, V5A lS6. Telephone: (604) 291475; *Zoology Department, University of British Columbia, Vancouver, B.C., Canada, V6T 2A9; iPresent address, Dept. of Zoophysiology, University of Aarhus, DK-8000 Aarhus, Denmark (Received 3 July 1986)
Abstract-l. The maximum aerobic swimming speed of Chinook salmon (Oncorhynchus rshuwyrsha) was measured before and after ligation of the coronary artery. 2. Coronary artery ligation prevented blood flow to the compact layer of the ventricular myocardium, which represents 30% of the ventricular mass, and produced a statistically significant 35.5% reduction in maximum swimming speed. 3. We conclude that the coronary circulation is important for maximum aerobic swimming and implicit in this conclusion is that maximum cardiac performance is probably necessary for maximum aerobic swimming performance.
INTRODUCTION
MATERIALS AND METHODS
A coronary circulation is not found in all fish, but when present it supplies arterial blood to the outer compact myocardium of the ventricle (Santer, 1985). The remainder of the ventricle, or the entire ventricle in fish lacking a coronary circulation, is comprised of spongy myocardium which derives oxygen from the venous blood being pumped through the ventricle. The relative proportion of compact myocardium is directly related to the activity level of the fish species. Compact myocardium accounts for 6@-70% of the ventricle mass in active fish such as the tuna, 30-40% in salmonids, and is absent in sedentary fish (Santer and Greer Walker, 1980). Thus, it appears that the coronary circulation provides an essential myocardial oxygen supply for the high level of cardiac performance that is required during sustained swimming, given that fish hearts generally operate aerobically and myocardial oxygen consumption is linearly related to myocardial power output (Turner and Driedzic, 1980; Driedzic et al., 1983; Farrell, 1984; Farrell et al., 1985). Daxboeck (1982) tested this hypothesis by measuring the maximum sustained swimming speed, Lice,, before and after coronary ablation in S. gairdneri. Coronary ablation had no effect on Um,. However, Daxboeck (1982) noted coronary regrowth around the ablation site after the 7 day post-surgery recovery, which may have provided an adequate oxygen supply to the compact myocardium during swimming. In view of the theoretical argument which suggests that the coronary circulation should be essential for maximum aerobic performance of the heart, the present investigation essentially repeated the swimming experiments of Daxboeck (1982). However, the coronary artery was occluded acutely in an attempt to circumvent the problems of coronary vessel regrowth.
Oncorhynchuy rshawytscha (0.9-2.2 kg, N = 9) were obtained from sea pens at the Pacific Biological Station, Nanaimo, British Columbia. They were transported to Bamfield Marine Station, Bamtield, British Columbia, where they were held indoors in large tibreglass tanks supplied with flowing, aerated sea-water. The fish were acclimated to these conditions for l-2 weeks, but were not fed during this period. The experiments were conducted at the ambient water temperature, 10.5-l 1°C.
35
Swim tunnel and measurement of exercise performance The experiments were performed in the original Brett swimming respirometer (Brett, 1964) which has a volume of 1201. Vertical stripes of tape on the swimming section provided visual cues for orientation of the swimming fish, and an electrified screen encouraged the fish to swim continuously. Water oxygen tension in the respirometer was kept above 90% saturation throughout the experiment by a continuous flow (1 S-20 l/min) of aerated sea-water through the respirometer. Swimming speed was calculated by correcting the water velocity for the solid blocking effect of the fish (Bell and Terhune, 1970). Swimming performance was determined by increasing the swimming velocity by 0.25 bl/sec every 20 min until fatigue occurred (bl = body length). Critical swimming speed, C.Jti,, was calculated as described by Brett (1964) using:
UC,,,= u, + (&, x ri,) where fI, = the highest velocity maintained for the 20-min period (cm,&ec); L’,,= velocity increment (cm/s); li = time fish swam at the “fatigue” velocity (min); I,, = 20-min period of swimming. This formula interpolates for those fish that did not fatigue exactly at the beginning or the end of the 20-min swimming period. Experiment protocols
Each fish was anaesthetized (1:5,000 solution of ethyl m-amino benzoate, Sigma Chemicals) and placed supine on an operating sling with the gills irrigated (1: 15,000 ethyl m-amino benzoate). A small incision (< 1 cm) was made in the side of the isthmus to reveal the most cephalic portion
36
A. P. FAKKELL and J. F. STEFFFNSEN
of the ventral aorta inside the pericardial cavity. Here the coronary artery joins the surface of the ventral aorta. A #3-O silk thread was looped around the coronary artery and the two ends were Icd out of the incision. The incision was closed with #3-O silk. The fish, which was usually reviving, was quickly transferred to the swim tunnel, where it recovered its righting reflex and swimming ability within l-2 min. The operation usually took 5 min and bleeding was usually negligible. The fish recovered overnight (> 8 hr) at a swimming speed of 0.8 I blisec. After fatigue was established, the fish was quickly removed from the respirometer, lightly anacsthetized (I : 15,000 ethyl m-amino benzoatc), and placed on the operating sling. The silk loop was tightened to occlude the coronary artery and the fish was returned to the swim tunnel. This procedure took about 6 min and the fish quickly recovered its righting reflex in the swim tunnel. The swimming speed was progressively increased from 0.5 blisec to 1.0 bl/sec during the recovery period. G:,+ was remeasured after a minimum recovery time of 4 hr. Long recovery periods were not used for three reasons. First, WC wished to avoid coronary regrowth around the ablation site. Second, electrical power failures in Bamfield made shortterm experiments more desirable. Third, Brett (1964) found that the 0, debt associated with swimming to fatigue is largely paid off 4 hr after swimming. One Bsh was swum twice with the coronary artery ligated, once after a 4-hr recovery and then again after a further overnight recovery. CJti, was reduced by a similar amount after both recovery periods (51 and 42%, respectively), but small vessels were already forming around the ligature site. fish were killed by a Following the LJm, measurements, sharp blow to the head. The patency and placement of the coronary ligature were checked by visual inspection. The ventricle was removed, blotted, weighed and placed in 10% buffered formaldehyde. The relative proportion of compact and spongy myocardium was determined by dissecting the
compact layer away and comparing the weights of the two tissues afte; drying to a consiant weight at-60°C. Statistical differences (P < 0.05) were determined the Student’s paired f-test.
using
RESULTS
ligation was The mean U,,,, before coronary 1.90bl!sec (SD = 0.253, N = 5). U,, was significantly reduced after acute coronary ligation (I .23 blisec, SD = 0.345) (Table I). This 35.5% reduction in the maximum sustained swimming speed provides support for the hypothesis that the coronary circulation is necessary for maximum aerobic swimming performance. The compact myocardium represented 30.4% (SD = 4.0, N = 5) of the ventricles mass. In an additional fish, only the left branch of the coronary artery was occluded because the ligature was inadvertently placed downstream of the coronary branch and thus, a portion of its 30.5% compact myocardium continued to receive coronary flow. In this fish, coronary ligation reduced Uci,,,,by 12%. DISCUSSION u Crl, was determined following anaesthesia, a brief operation and overnight recovery. Glova and McInerney (1977) who were unable to demonstrate a significant difference in swimming performance between young coho salmon (Oncorhynchus kisurch), allowed 1 and I2 hr recovery in the swimming chamber. As well, Randall et al. (in preparation) measured Uai, in unoperated. but smaller Chinook salmon at 10°C. Their Ucri, values of 59.6cm/sec
Table I. Body length (W, cm --45.5(5.lcn
of
coronary artery ligation on formance in Chinook Salmon
Rody WI kg I .3 I (0.542)
Control
per-
c’c,,,. bl;sL*: Alicr ligation
I .90 (0.253)
Values presented as mean (SD) for five fish. ‘Denotes significant difference in CC,,, from (P < 0.05).
* I .23 (0.345) c~n~ml
value
(SEM = 9.9) and 83.5 cm/see (SEM = 13.2) for fish weighing 76 g (SEM = 12) and 319 g (SEM = 39) compare well with our CJcn,value of 86.5 cmjsec for a 1.31-kg fish. Furthermore, the U,,, was similar in four additional fish (86.0cm/sec), one of which had only the left branch of the coronary ligated later and three of which died as a result of either a power failure or surgical mishaps. Consequently, we believe that the anaesthesia and surgery had a minimal effect on Ucricafter the overnight recovery. There was at least a 4-hr recovery between Ua, determinations before and after coronary artery ligation. The 4-hr recovery time was a compromise between adequate recovery from the exercise and avoidance of coronary vessel regrowth. It is unlikely that the initial exercise regime affected the UC,, dctcrmination after coronary ligation because the 0, debt would have been largely paid off (Brett, 1964). The 35.5% reduction in UCi,,,after coronary artery ligation occurred because coronary flow was eliminated and rendered the compact myocardium hypoxic. With about a third of the ventricle hypoxic, it is unlikely that the heart can generate its maximum power output (Turner and Driedzic, 1980; Farrell et ul., 1985) and maximum aerobic swimming pcrformance is precluded by the inability of the heart to perform maximally. Therefore, by implication, maximum cardiac performance is required for swimming at UC,,,.This is also supported by our observation that when only half of the coronary circulation was tied off in one fish, U,,, was better maintained (only a 12% reduction). Our conclusion refutes the Daxboeck (1982) study with rainbow trout, where chronic coronary ablation had no effect on maximum aerobic swimming speed. Two explanations may be put forward to resolve these different observations. Foremost, coronary vessel regrowth around the ablation site in the chronic study with rainbow trout may have restored some or all of the coronary blood flow. Our acute experiments avoided overt vessel regrowth. In studies with mammals, collateral vessels can develop around an occlusion site in 24 hr and fully restore coronary flow (Schapcr, 1971). The message here is that, as in mammals, coronary flow is important in fish and rapid adjustments are made to restore coronary flow. Secondly, Daxboeck (1982) worked with 300g rainbow trout and small fish have a thinner layer of compact myocardium. It is possible that O2 diffusion from the lumen of the ventricle to the compact layer was significant in the rainbow trout. We reduced this confounding factor by using larger fish in which the compact myocardium is more developed. Santer and Greer Walker (1980) have measured the relative proportion of compact and spongy myocardium in salmonids. The compact epicardium rep-
Coronary
ligation reduces salmon’s maximum speed
resents 30-40% of the ventricular mass. The compact myocardium represented about 30% of the ventricle in the present study with 0. tshawytscha, and is, therefore, consistent with previous observations (Santer and Greer Walker, 1980). Loss of 30.4% compact myocardium was associated with 35.5% reduction in UC,,. Too little is known about cardiac and coronary physiology in fish at this stage to explain why cardiac performance, per cent compact myocardium and Ucri,were so closely related. In summary, the present experiments support the hypothesis that blood flow to the compact myocardium in Chinook salmon is important for maximum sustained aerobic swimming. Acknowledxemenrs-We would like to thank Dr Craig Clarke, Pacific Biological Station, Nanaimo, British Columbia, for supplying fish. The technical expertise of Anna-Marie Hammons was appreciated in determining the tissue partitioning of the ventricles. The cooperation of the Director and staff of the Bamfield Marine Station was most appreciated. This work was supported by a grant to A.P.F. from the British Columbia Health Care Foundation. J.F.S. was supported by a N.A.T.O. Science Fellowship. the Danish Natural Science Research Council and the Carlsberg Foundation. REFERENCES Bell W. H. and Terhune
L. D. B. (1970) Water tunnel design for fisheries research. J. Fish. Res. Bd Can. Tech. Rep. No 195, p. 69.
37
Brett J. R. (1964) The respiratory metabolism and swimming performance of young sockeye salmon. J. Fish. Res. Bd Can. 21, 1183-1226. Daxboeck C. R. (1982) Effect of coronary artery ablation on exercise performance in Salmo gairdneri. Can. J. 2001.60, 375-381. Driedzic W. R., Scott, D. L. and Farrell A. P. (1983) Aerobic and anaerobic contributions to energy metabolism in perfused isolated sea raven (Hemifripferus americanus) hearts. Can. J. Zool. 61, 188&1883. Farrell A. P. (I 984) A review of cardiac performance in the tcleost heart: intrinsic and humoral regulation. Gun. J. 2001. 62, 523 -536. Farrell A. P., Wood S., Hart T. and Driedzic W. R. (1985) Myocardial oxygen consumption in the sea raven, Hemifriprerus americanus: The effects of volume loading, pressure loading, and progressive hypoxia. J. exp. Biol. 117, 237.-250. Glova G. J. and Mclnerney J. E. (1977) Critical swimming speeds of Coho salmon (Oncorhynchus kisutch) fry to smolt stages in relation to salinity and temperature. J. Fish. Res. Bd Can. 34, 151-154. Randall D. J., Mense D., Boutillier B. (in preparation). Santer R. M. (1985) Morphology and Innemotion of the Fish Hearr. Springer-Verlag, Berlin. Santer R. M. and Greer Walker M. (1980) Morphological studies on the ventricle of tcleost and elasmobranch hearts. J. Zool. 190, 259-272. Schaper W. (197 I) The Collateral Circulation of the Heart. North-Holland, Amsterdam. Turner J. D. and Driedzic W. R. (1980) Mechanical and metabolic response of the perfused isolated fish heart to anoxia and acidosis. Con. J. Zoo/. 58, 886-889.