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Heart rate in resting seals on land and in water
Camp Biochem. Physiol., Vol. 61A. pp 77 to 83 0 Pergamon Press Ltd 1980. Printed in Great Bntain
03W-9629/80/0901-0077SO2
00/O
HEART RATE IN RESTING SEALS ON LAND AND IN WATER ARVID PASCHEand JOHN KROG Institute
of Zoophysiology,
University
of Oslo, Blindern,
(Received 4 December
Oslo 3, Norway
1979)
Abstract-l. Heart rate of unrestrained resting seals was recorded under three different conditions: (a) on land, (b) in water, and (c) with only the head above the water. 2. A comparison of the heart rates obtained showed that there was no significant difference in the bradycardia during diving and apneic periods on land. 3. In both cases the heart rate decreased to 30% of the rate during breathing. Nor was a difference found in the time course of the bradycardia in water and on land. 4. The observed bradycardia seemed to be correlated to a Valsalva-like maneuver, which is known to cause a change in intrathoracic pressure. 5. Changes in the intrathoracic pressure, caused by muscular contraction, might occur during the dives. 6. Such changes could then explain the variation in heart rate seen during some of the dives and apneic periods. 7. A correspondence between ventilation and variation in heart rate was found, but only in the young seals while they had an unusually low ventilation rate.
the bradycardia of immersion (Jones et al., 1973; Dykes, 1974a). It has been suggested that afferent information from peripheral receptors in the oral cavity of ducks (Andersen, 1963) and muskrats (Drummond & Jones, 1972) and in the face of seals plays an important role in the initiation of the reflex. In the case of diving, wetting of receptors in the region of respiratory orifices has even been mentioned as the adequate stimulus for eliciting the characteristic changes of the circulation (Andersen, 1963). When there is no information from these peripheral receptors, which are mediated through the trigeminal nerves, the time course of the diving bradycardia is similar to the slow bradycardia with simple apnea in air (Dykes, 1974b). Because most experiments have been conducted on restrained animals which were forcibly submerged or subjected to forced apneic periods (Dykes, 1974b), many of the results may be misleading and of limited value in determining the natural response of the animals. For instance, the far greater bradycardia seen in forced compared to unrestrained dives may be a reflection of the general stress experienced during such experimental procedures. Butler & Woakes (1975) showed that there is a significant difference in bradycardia during forced laboratory submergences compared to natural unrestrained dives. On the basis of previous reports the following question was therefore asked: do unrestrained seals show the same change in heart rate during voluntary apneic periods on land and during dives as those reported from experiments with forced submergence? The purpose of this study was therefore to record heart rates of unrestrained resting seals in water and on land, and to compare the observed bradycardia in the animals during apneic periods in these two different environmental conditions. On the basis of the
INTRODUCTION Virtually all marine animals that have been studied display some form of bradycardia in response to submergence (Irving, 1966). Several investigators have studied the physiological changes that occur during immersion in a number of diving animals and have contributed to a growing understanding of the ability of diving animals to remain underwater for extended periods of time on limited oxygen stores. A change in the heart rate frequency is used as the indicator of the onset of the sequence of cardiovascular events known as the dive reflex (Scholander, 1940; Andersen, 1963; Harrison & Kooyman, 1968). In diving animals the changes associated with the bradycardia observed during immersion include a decreased cardiac output, peripheral vasoconstriction, and in seals a constriction of a sphincter in the inferior vena cava. The efferent pathways of the dive reflex include the vagus nerve, phrenic nerve, and autonomic nerves to the peripheral vasculature (Elsner, 1969). The afferent pathways and the stimuli sufficient to elicit the bradycardia of submersion are not well defined. The respiratory pattern of the diving animals has been demonstrated to be one of the major variables influencing their heart rate (Irving et al., 1935; Irving, 1939; Hubbard, 1968; Lin et al., 1972; Jones et al., 1973; Dykes, 1974a; Casson & Ronald, 1975). In a study of the California sea lion, Lin et al. (1972) concluded that the diving bradycardia involves only the elimination of the periodic tachycardia during a respiratory cycle. Other studies comparing apneic bradycardia and diving bradycardia in seals have stated that the bradycardia which develops in reponse to apnea alone is slower in onset than that seen after submergence. Thus the lack of the respiratory movements could not be the only variable responsible for 11
ARVID P&HE
78
results it was assumed to be possible to judge the relative importance of water for eliciting the bradycardia in seals during natural dives.
MATERIALS
AND METHODS
One hooded seal (Cystophora cristata) and six harbour seals (Phoca Gtulina) ranging in age from 1 week to 4 years were used for experimentation. The animals were maintained at the Institute of Zoophysiology. Oslo, where they were fed a diet of frozen herring with added vitamins and seasalt. The seals appeared to be in good health. Records were obtained under three resting conditions: (I) on land, (2) in water, submerged, and (3) with only the head out of water. All of the seals used were studied in at least two of the conditions mentioned. The oldest animal for the experiments. a 4-year-old hooded seal. preferred to rest in a 10001. pool, with the head at the bottom of the pool and the hind flippers above the water (Fig. 1). Four electrodes were connected to the walls of this pool (mean distance from the animal = 40 cm). Heart rate was recorded from these electrodes on a two channel recorder (Beckman RS Dynograph) using an ECG- and a heart-rate coupler. The other seals did not have the same preference for the 10001. pool, and the set-up showin in Fig. 1 could not be used for them. The animals, however, were often seen resting in the water in a 30,000 1. pool fully submerged or with only the head out of water. To obtain the necessary recordings from these seals while they were resting in the water, a special heart beat transmitter (sound telemetery) was made (Holand, 1975). The transmitter was connected to the seal by two simple rubber straps. These straps pressed the electrodes tightly to the skin of the animals without interfering with ventilation. Within a few minutes they ignored the straps and the transmitter. The receiving equipment included a hydrophone which transferred the heart beat signals to a detector and finally to a recorder. In the experiments with the animals resting on land, the heart rate was recorded using ordinary electrocardiographic leads and plate electrodes. The electrodes were connected to the animal by the use of rubber straps or simply placed beneath the sleeping animal. In the experiments using the heart beat transmitter and the electrocardiographic leads the heart rate in beats.min-’ was calculated from the time Interval between beats (cardiac interval).
and JOHN KROG
The operator was hidden from the seals but was able to observe the behavior and ventilatory movements of the animals during the recording period. lnspiratory and cxpiratory activity was noted by observing the time of opcning and closing of the nostrils as well as thoracic movcmerits. Diving was indicated when the nostrils went undcrwater and emergence when the nostrils broke the surface. Opening of the nostrils indicated the onset of breathing No attempts were made to forcibly submerge or rcbtram the seals during these experiments. Recordings wcrc made only when the animals seemed to be “asleep”. a condition where they were resting with closed eyes. and whcrc low noise or speaking did not induce behavioral arousal (Ridgeway et al., 1975). The results were treated statistically using Student t-test. limit of significancs and Y;, was used as the acceptable (Sverdrup, 1964).
RESULTS
Heart rate of a restirlg hooded seal In watrr, submerged. The results for the 4-year-old hooded seal with the rather unusual resting postion, shown in Fig. 1, were presented as mean values in Table 1. Mean diving time was found to be 226 set but dives lasting as long as 413 xc also occurred. A dive was accompanied by a sudden drop in heart rate to 349, of the rate during breathing. This change in heart rate started I- 2 set before diving and was usually finished within 5 sec. Initially the heart rate sometimes would drop to rather low values and then increase to a level which was held throughout the dive. Such a heart rate change is shown in Fig. 2a, where the heart rate initially drops to a value as low as 7.7 beats/min and then slowly increases to a level of 30 beats/min. Considerable changes in heart rate could occur while the animal was resting, apparently asleep, at the bottom of the pool. Even though such changes were not a common phenomenon, Fig. 2b shows a sudden fall in heart rate from 35 to 4 beats/ min during a dive. At the time this occurred the dive had lasted for 2.5 min. The heart rate increased, but was held at an unusually low rate for the rest of the
Fig. 1. The figure shows the resting position of the adult hooded seal in water, and recording heart rate using electrodes connected to the walls of the pool.
the set-up
for
79
Heart rate in seals Table I. Results of heart rate and duration of apneic and breathing periods for one hooded seal and six harbour seals during resting conditions on land and in water (submerged) IN WATER
HOODED SEAL
Heart Rate (beats/min)
(4 years old)
(4 months - 2 years old)
Breathing
Apnea
Breathing
34.8r4.1
101.6?6.1
36.1t3.7
101.5i6.3
fn=70)
226.0i91.6
(set) HARBOUR SEALS (6)
Diving
in=70)
Duration
ON LAND
in=70!
46.3t12.5 in=701
Heart Rate
51.124.3
15O.Of7.6
(beats/min)
in=BZl
!n=Bl'i
Duration (set)
72.4219.4
23.7f2.9
in=B2/
!n=821
fn=XOI
in=ZO)
43.9t16.3
13.2i4.1
(n=201
fr,=%Oj
52.7i4.5
144.6t8.2
fn=,lji
in=231
56.1t16.0
19.024.2
cy1=2.3/
1q=23)
The results are presented as mean values + SE. diving period (1.5 min). The end of the dives was accompanied by a rapid increase in heart rate. This increase was as rapid as the decrease accompanying the initiating diving (Fig. 3). The changes in heart rate did not correspond exactly with the diving period, as it decreased before the dive and also increased just before the emergence. End of dive
Diving
On land. While the hooded seal was resting on land the initial stages of the resting periods were characterized by short breathing periods (2-3 breaths) interrupted by apneic periods lasting a few seconds. The heart rate varied between 40 and 75 beats/min (Fig. 4a). When the breathing- and apneic-periods became longer and more regular, the difference between End of dwe
I
ECG
t
(a)
End of dive t
(b)
30s
Fig. 2. ECG of the hooded seal during resting dives showing the occasionally occurring variations in heart rate. In (a), the heart rate initially dropped to a low value, then increased to a plateau which was held throughout the rest of the dive. In (b) the heart rate dropped suddenly in the middle of the dive.
ARVIII P&HE
and
JOHN KKOG
Fig. 3. Heart rate recording of the hooded seal resting submerged in water, showing rapid changes in heart rate in the beginning as weli as at the end of the dive. Heart rate during the dive shows no tendency to change.
ECG
F‘ig 4. Heart rate of the hooded sea1 during the initial part of a resting period on land. (a) is a recording during a period characterized by apneic periods lasting only a few seconds. In (b) the recording shows larger variations in the heart rate than in (a). This period also had apneic periods of longer duralion.
Heart
heart rate in breathing- and apneic-periods increased (Fig 4b). In a condition where the animal seemed to be asleep, the ventilatory pattern was characterized by a respiratory frequency (33-36 breaths/min) and a duration of the breathing periods rather similar to that of the animal while it was resting in water. Mean values from 20 registered periods, including both apneic and breathing periods, are presented in Table 1. As seen from the table, apnea was accompanied by a drop in heart rate to 36% of the rate during breathing, which is not significantly different from the changes in heart rate while the animal was diving. The rapidity of the changes as well as the levels of the heart rate seemed similar on land and in water (Fig. 5). Heart rate in resting harhour seals On land. The studies of the harbour seals commenced when they were 47 days old and with negligible diving experience. Resting on land they showed the same breathing pattern as the older animals, with apneic periods interrupting a continuous breathing. During such apneic periods their heart rates dropped to a mean value of 64.9 (SE = 7.0) beats/min (n = 64) from a mean value of 118.4 (SE = 6.2) (n = 64) during breathing periods. While the older harbour seals were found to have a fairly constant ventilatory rate throughout the breathing period (52.8 breaths/min), these seal pups started the breathing periods with a
[al z E 5 z
ventilatory rate of 60 breaths/min, decreasing to about 20 breaths/min at the end of the period. The decrease in ventilatory rate was accompanied by deeper breathing. The heart rate was not affected by the change between inspiration and expiration while the animals were breathing at a high rate. However, when the ventilatory rate decreased and the tidal volume increased at the end of the breathing period, the heart rate varied between 120 and 70 beats/min. The lowest heart rate coincided with the expirations. In water, submerged. The results of the experiments with the harbour seals ranging in age from 3 months to 2 years are presented as mean values in Table 1. Compared to the hooded seal, the harbour seals used had shorter diving and breathing periods during rest. The harbour seals had a ventilatory rate much higher than that of the hooded seal: 52.9 breaths/min (SD = 11.7 breaths/min) compared to 34.3 breaths/ min (SD = 5.1 breaths/min). As seen in the experiments with the hooded seal, the change in heart rate at the start and at the end of the dive was very rapid. In spite of certain differences in heart rate values, the relative change in rate when the animal dived was approximately the same in the harbour seals and the hooded seal: a mean reduction of 66%. Head out ofwater. The relation between the length of apneic periods and breathing periods was the same as that between diving and breathing periods of the seals resting in the pool. Apneic periods as long as End of dive
In water
150
81
rate in seals
1
100
d
,,,1’-
_*
_/,’
r
ECG
[bl
On land 150. 2 E ii?
i
‘I:_
;‘-
I” OL
ECG
11
I
I
I
Fig. 5. Heart rate recordings of the hooded seal resting submerged in water recordings show similar heart rate levels during apneic- and breathing-periods
(a), and on land (b). The in these two conditions.
82
ARVID PASCHE and JOHN KROG
3 min did occur. The heart rates presented in Table 1 show a decrease in rate during apnea to 36% of the values during breathing. The decrease is not significantly different from the variation in heart rate seen in the animals while resting in water. For two of the animals used in this experiment the heart rate was recorded while they were resting on land. There was no significant difference in heart rate during apneic and breathing periods while the animals were resting immersed to the neck or on land, respectively. DISCLSSlON In studies of parameters such as heart rate it is of utmost importance to establish the most satisfactory condition in which to measure the extent and nature of variation. Most previous experiments on divingand apneic-bradycardia have involved force used on the animals. Moreover, comparison of bradycardia in situations of varying degrees of restraint has frequently occurred. It is not to be denied that diving bradycardia exists and that is is an important adaptation to diving. What exactly triggers the bradycardia in marine mammals is not clear and has been the cause of much discussion and controversy. Many stimuli have been found to elicit the bradycardia in seals. A volitional component has been suggested (Jones et al., 1973) and stimuli involving water in the face and cessation of respiratory movements (Dykes, 1974b). The presence of chemoreceptor-mediated effects upon heart rate in seals has been suggested to exist, and to contribute to the bradycardia observed in seals during diving (Tanji (‘r al.. 1975). These effects, however, appear to be less important than they are in diving birds (Jones & Purves, 1970; Holm & Serrensen, 1972: Butler & Taylor. 1973). A triggering of the rapid onset of bradycardia by cold receptors in the face, as in man. is not the case in the harbour seals (Dykes, 1974b). Among the stimuli mentioned which are found to elicit the bradycardia in seals, the water and its effect on special receptors in the face have been considered of greatest importance. According to the results presented in this paper, however, the importance of water needed for the elicitation of this cardiovascular reflex may have been overestimated in the literature. Even though the results showed a more pronounced bradycardia when the animals dived compared to the bradycardia during corresponding apneic periods on land, the differences were small and not significant on a 5”,, level. Moreover, no difference was found in the time course of the bradycardia in water and on land. The apneic periods on land as well as during the dives were initiated by closure of the nostrils. The bradycardia observed seemed to be correlated with this closure or a succeeding movement of the chest, which may be the result of a relaxation of the diaphragm or a contraction of expiratory muscles. This movement could be related to a change in muscular activity in the body wall similar to those engaged in Valsalva’s maneuver. which is known to produce bradycardia in man (Craig, 1963). Ridgeway (1972) also often noticed a Valsalva-like maneuver in the bottlenosed porpoises just after inspiration. A change in the activity of the respiratory muscles with closed nostrils would cause a change in intrathoracic press-
ure. In other animals (Angelone & Coulter, 1964), changes in intrathoracic pressure are known to affect the heart rate. It is reasonable to believe that this might be one of the causal factors of the bradycardia. Changes in intrathoracic pressure can also be caused by muscular contractions during the dives. Such changes might then explain a sudden decrease in heart rate in the middle of a dive, like that seen in the resting animal with heart rate recordings shown in Fig. 2b. Heart rate and respiration are intimately associated in marine mammals, and the effect of ventilation on heart rate has been described in seals as well as in other diving animals. Blix et al. (1976) concluded that the initial bradycardia in a diving duck is closely coupled to, and dependent upon, an enforced apnea response. This response is evoked by stimulation of receptors which are not specific to water. According to our present investigation a respiratory arrest alone cannot be responsible for the bradycardia in seals, a finding which is supported by the observations of Murdaugh et al. (1961). They found that the heart rate often returned to normal when the seal was approaching the surface, before its nostrils reached above water. If the seal decided to dive again before surfacing, the bradycardia returned. A correspondence between ventilatory maneuvers and variation in heart rate was also found in the present study. However, this was seen only in the young animals while they had an unusually low ventilation rate. In the older animals the ventilation rate seemed too high to show any effect on the heart rate at the chart speed used for our recordings. The intrathoracic pressure varied during ventilation, and the observed changes of heart rate during breathing might be the result of mechanisms similar to those responsible for the bradycardia during diving and apneic periods on land. AcknowlPdgements-The authors would like to thank MS E. A. Sommers for her invaluable help with linguistic editing of the manuscript, and Mr M. Bronndal for his assistance during the experiments and for taking care of the animals. This work was supported by grants from the Norwegian Council for Science and Humanities.
REFERENCES ANDERSEN H. T. (1963) Factors determining the circulatory adjustments to diving--I. Water immersion. ilcra Physiol. Stand. 58, 173-175. ANGELONE A. & COIJL~EK N. A. JR. (1964) Respiratory sinus arrhythmia: a frequency dependent phenomenon. J. appl. PhJ,siol. 19, 479-482. BLIX A. S., RETTEDAL A. & S~OKKAN K.-A. (1976) On the elicitation of the diving responses in ducks. Acta Physiol. Stand. 98, 478.-483. BUTLER P. J. & TAYLOR E. W. (1973) The effect of hypercapnic hypoxia, accompanied by different levels of lung ventilation on heart rate in the duck. Resp. Physiol. 19, 17G187. BUTLER P. J. & WOAKES A. J. (1975) Changes in heart rate and respiratory frequency associated with natural submersion of ducks. Proc. Physiol. Sot.. December 1975. D. 16, 1617.
Heart rate in seals CASSOND. M. & RONALDK. .(1975) The harp seal, Pagophilus groenlandicus (Erxleben, 1777).-XIV. Cardiac arrhythmias Camp. Biochem. Physiol. SOA, 307-314. CRAIG, A. B. JR. (1963) Heart rate responses to apneic
underwater diving and to breath holding in man. J. app!. Physiol. 18, 854. DRUMMOND P. C. & JONESD. R. (1972) Initiation of diving bradycardia in muskrats. J. Ph.vsiol. Land. 222, 165-166. DYKESR. W. (1974a) Factors related to the dive reflex in harbor seals: respiration, immersion bradycardia. and lability of the heart rate. Can. J. Physiol. Pharmacol. 52, 248-258. DUKESW. R. (1974b) Factors related to the dive reflex in harbor seals: Sensory contributions from the trigeminal region. Can. J. Phpsiol. Phurmaco~. 52, 259-265. ELSNER R. (1969) Cardiovascular adjustment to diving. In The Biology of Marine hinds (Edited by ANDERSEN H. T.), pp. 117-162. Academic Press, New York. HARRISONR. J. & KCIOYMAN G. L. (1968) General physiology of the pinnipedia. In The Beha~iour and Physiology of Pi~~jpeds (Edited by HARRISONR. J., HUBBARDR. C., PETERSON R. S., RICEC. E. and SCHUSTERMA~ R. J.), pp. 2122296. Appleton-Century-Crofts, New York. HOLANDB. (1975) Fish telemetrv. renort 6devices and results 1975. &port for No&s ‘Fiskeriforskningsrid, SINTEF report STF 48 A75065 HOLM B. & SQRENSEN S. C. (1972) The role of the carotic body in the diving reflex in the duck. R&r. Physiol. 15, 302-309.
Hr BBARI~R. C. (1965) ~~.~ba~dr~ and Laboratory Care qf Pinniprds (Edited by HARRISONR. J., HUBBARDR. C., PETERSONR. S., RICE C. E. & SCHUSTERMAN R. J.), pp. 299-358. Appleton-Century-Crofts, New York. IRVINGL. (1939) Respiration in diving mammals. Physiol. Rev. 19, 112-134. IRVINGL. (1966) Elective regulations of the circulation in
83
diving animals. In Whales, Dolphins di Porpoises (Edited by NORRISK. S.), pp. 381-396. University of California Press, Los Angeles. IRVINGL., SOLANDT0. M., SOLANDTD. Y. & FISHERK. C. (1935) The respiratory metabolism of the seal and its adjustment to diving. J. cell. camp. Physiof. 7, 137-l 51. JONESD. R. & PURVESM. J. (1970) The carotid body in the duck and the consequences of its denervation upon the cardiac responses to immersion. J. Physiol. Lorid. 211, 279-294. JONESD. R., FISHERH. D., MCTAGGERTS. & WEST N. H. (1973) Heart rate during breath-holding and diving in the unrestrained harbor seal (Phocn rituIina richardi). Can. J. Zoo!. 51, 671-680. Lrw Y.-C., MAT~UURAD. T. & WHITTOWG. C. (1972) Respiratory variation of heart rate in the California sea lion. Am. J. Physiol. 222, 26&264.
MURDAUGHH. V. JR., SEABURYJ. C. & MITCHELLW. L. (1961) Electrocardiogram of the diving seal. Circulation Res. 9, 358-361.
RIDGEWAYS. H. (1972) Homeostasis in the aquatic environment. In ~~~~1s of the Sea. Bjo~og~land Medicine (Edited by RIDGEWAYS. H.), pp. 590-747. Charles C. Thomas, Springfield, Illinois. RIDGEWAYS. H., HARRISONR. J. & JOYCE P. L. (1975) Sleep and cardiac rhythm in the gray seal. Science 187, 553-555.
SCHOLANDER P. F. (1940) Experimental investigation on the respiratory function in diving mammals and birds. ~vair~d, Skr. 22, f-131.
SVERDRUPE. (1964) Lov og tilfeldighet I, Oslo, Universitetsforlaget, 57-51 and 196200. TANI]D. G., WESTEJ. & DYKESR. W. (1975) Interactions of respiration and the submersion in harbor seals. Can. J. Physiol. Pharmacol. 53, 555-559.
03W-9629/80/0901-0077SO2
00/O
HEART RATE IN RESTING SEALS ON LAND AND IN WATER ARVID PASCHEand JOHN KROG Institute
of Zoophysiology,
University
of Oslo, Blindern,
(Received 4 December
Oslo 3, Norway
1979)
Abstract-l. Heart rate of unrestrained resting seals was recorded under three different conditions: (a) on land, (b) in water, and (c) with only the head above the water. 2. A comparison of the heart rates obtained showed that there was no significant difference in the bradycardia during diving and apneic periods on land. 3. In both cases the heart rate decreased to 30% of the rate during breathing. Nor was a difference found in the time course of the bradycardia in water and on land. 4. The observed bradycardia seemed to be correlated to a Valsalva-like maneuver, which is known to cause a change in intrathoracic pressure. 5. Changes in the intrathoracic pressure, caused by muscular contraction, might occur during the dives. 6. Such changes could then explain the variation in heart rate seen during some of the dives and apneic periods. 7. A correspondence between ventilation and variation in heart rate was found, but only in the young seals while they had an unusually low ventilation rate.
the bradycardia of immersion (Jones et al., 1973; Dykes, 1974a). It has been suggested that afferent information from peripheral receptors in the oral cavity of ducks (Andersen, 1963) and muskrats (Drummond & Jones, 1972) and in the face of seals plays an important role in the initiation of the reflex. In the case of diving, wetting of receptors in the region of respiratory orifices has even been mentioned as the adequate stimulus for eliciting the characteristic changes of the circulation (Andersen, 1963). When there is no information from these peripheral receptors, which are mediated through the trigeminal nerves, the time course of the diving bradycardia is similar to the slow bradycardia with simple apnea in air (Dykes, 1974b). Because most experiments have been conducted on restrained animals which were forcibly submerged or subjected to forced apneic periods (Dykes, 1974b), many of the results may be misleading and of limited value in determining the natural response of the animals. For instance, the far greater bradycardia seen in forced compared to unrestrained dives may be a reflection of the general stress experienced during such experimental procedures. Butler & Woakes (1975) showed that there is a significant difference in bradycardia during forced laboratory submergences compared to natural unrestrained dives. On the basis of previous reports the following question was therefore asked: do unrestrained seals show the same change in heart rate during voluntary apneic periods on land and during dives as those reported from experiments with forced submergence? The purpose of this study was therefore to record heart rates of unrestrained resting seals in water and on land, and to compare the observed bradycardia in the animals during apneic periods in these two different environmental conditions. On the basis of the
INTRODUCTION Virtually all marine animals that have been studied display some form of bradycardia in response to submergence (Irving, 1966). Several investigators have studied the physiological changes that occur during immersion in a number of diving animals and have contributed to a growing understanding of the ability of diving animals to remain underwater for extended periods of time on limited oxygen stores. A change in the heart rate frequency is used as the indicator of the onset of the sequence of cardiovascular events known as the dive reflex (Scholander, 1940; Andersen, 1963; Harrison & Kooyman, 1968). In diving animals the changes associated with the bradycardia observed during immersion include a decreased cardiac output, peripheral vasoconstriction, and in seals a constriction of a sphincter in the inferior vena cava. The efferent pathways of the dive reflex include the vagus nerve, phrenic nerve, and autonomic nerves to the peripheral vasculature (Elsner, 1969). The afferent pathways and the stimuli sufficient to elicit the bradycardia of submersion are not well defined. The respiratory pattern of the diving animals has been demonstrated to be one of the major variables influencing their heart rate (Irving et al., 1935; Irving, 1939; Hubbard, 1968; Lin et al., 1972; Jones et al., 1973; Dykes, 1974a; Casson & Ronald, 1975). In a study of the California sea lion, Lin et al. (1972) concluded that the diving bradycardia involves only the elimination of the periodic tachycardia during a respiratory cycle. Other studies comparing apneic bradycardia and diving bradycardia in seals have stated that the bradycardia which develops in reponse to apnea alone is slower in onset than that seen after submergence. Thus the lack of the respiratory movements could not be the only variable responsible for 11
ARVID P&HE
78
results it was assumed to be possible to judge the relative importance of water for eliciting the bradycardia in seals during natural dives.
MATERIALS
AND METHODS
One hooded seal (Cystophora cristata) and six harbour seals (Phoca Gtulina) ranging in age from 1 week to 4 years were used for experimentation. The animals were maintained at the Institute of Zoophysiology. Oslo, where they were fed a diet of frozen herring with added vitamins and seasalt. The seals appeared to be in good health. Records were obtained under three resting conditions: (I) on land, (2) in water, submerged, and (3) with only the head out of water. All of the seals used were studied in at least two of the conditions mentioned. The oldest animal for the experiments. a 4-year-old hooded seal. preferred to rest in a 10001. pool, with the head at the bottom of the pool and the hind flippers above the water (Fig. 1). Four electrodes were connected to the walls of this pool (mean distance from the animal = 40 cm). Heart rate was recorded from these electrodes on a two channel recorder (Beckman RS Dynograph) using an ECG- and a heart-rate coupler. The other seals did not have the same preference for the 10001. pool, and the set-up showin in Fig. 1 could not be used for them. The animals, however, were often seen resting in the water in a 30,000 1. pool fully submerged or with only the head out of water. To obtain the necessary recordings from these seals while they were resting in the water, a special heart beat transmitter (sound telemetery) was made (Holand, 1975). The transmitter was connected to the seal by two simple rubber straps. These straps pressed the electrodes tightly to the skin of the animals without interfering with ventilation. Within a few minutes they ignored the straps and the transmitter. The receiving equipment included a hydrophone which transferred the heart beat signals to a detector and finally to a recorder. In the experiments with the animals resting on land, the heart rate was recorded using ordinary electrocardiographic leads and plate electrodes. The electrodes were connected to the animal by the use of rubber straps or simply placed beneath the sleeping animal. In the experiments using the heart beat transmitter and the electrocardiographic leads the heart rate in beats.min-’ was calculated from the time Interval between beats (cardiac interval).
and JOHN KROG
The operator was hidden from the seals but was able to observe the behavior and ventilatory movements of the animals during the recording period. lnspiratory and cxpiratory activity was noted by observing the time of opcning and closing of the nostrils as well as thoracic movcmerits. Diving was indicated when the nostrils went undcrwater and emergence when the nostrils broke the surface. Opening of the nostrils indicated the onset of breathing No attempts were made to forcibly submerge or rcbtram the seals during these experiments. Recordings wcrc made only when the animals seemed to be “asleep”. a condition where they were resting with closed eyes. and whcrc low noise or speaking did not induce behavioral arousal (Ridgeway et al., 1975). The results were treated statistically using Student t-test. limit of significancs and Y;, was used as the acceptable (Sverdrup, 1964).
RESULTS
Heart rate of a restirlg hooded seal In watrr, submerged. The results for the 4-year-old hooded seal with the rather unusual resting postion, shown in Fig. 1, were presented as mean values in Table 1. Mean diving time was found to be 226 set but dives lasting as long as 413 xc also occurred. A dive was accompanied by a sudden drop in heart rate to 349, of the rate during breathing. This change in heart rate started I- 2 set before diving and was usually finished within 5 sec. Initially the heart rate sometimes would drop to rather low values and then increase to a level which was held throughout the dive. Such a heart rate change is shown in Fig. 2a, where the heart rate initially drops to a value as low as 7.7 beats/min and then slowly increases to a level of 30 beats/min. Considerable changes in heart rate could occur while the animal was resting, apparently asleep, at the bottom of the pool. Even though such changes were not a common phenomenon, Fig. 2b shows a sudden fall in heart rate from 35 to 4 beats/ min during a dive. At the time this occurred the dive had lasted for 2.5 min. The heart rate increased, but was held at an unusually low rate for the rest of the
Fig. 1. The figure shows the resting position of the adult hooded seal in water, and recording heart rate using electrodes connected to the walls of the pool.
the set-up
for
79
Heart rate in seals Table I. Results of heart rate and duration of apneic and breathing periods for one hooded seal and six harbour seals during resting conditions on land and in water (submerged) IN WATER
HOODED SEAL
Heart Rate (beats/min)
(4 years old)
(4 months - 2 years old)
Breathing
Apnea
Breathing
34.8r4.1
101.6?6.1
36.1t3.7
101.5i6.3
fn=70)
226.0i91.6
(set) HARBOUR SEALS (6)
Diving
in=70)
Duration
ON LAND
in=70!
46.3t12.5 in=701
Heart Rate
51.124.3
15O.Of7.6
(beats/min)
in=BZl
!n=Bl'i
Duration (set)
72.4219.4
23.7f2.9
in=B2/
!n=821
fn=XOI
in=ZO)
43.9t16.3
13.2i4.1
(n=201
fr,=%Oj
52.7i4.5
144.6t8.2
fn=,lji
in=231
56.1t16.0
19.024.2
cy1=2.3/
1q=23)
The results are presented as mean values + SE. diving period (1.5 min). The end of the dives was accompanied by a rapid increase in heart rate. This increase was as rapid as the decrease accompanying the initiating diving (Fig. 3). The changes in heart rate did not correspond exactly with the diving period, as it decreased before the dive and also increased just before the emergence. End of dive
Diving
On land. While the hooded seal was resting on land the initial stages of the resting periods were characterized by short breathing periods (2-3 breaths) interrupted by apneic periods lasting a few seconds. The heart rate varied between 40 and 75 beats/min (Fig. 4a). When the breathing- and apneic-periods became longer and more regular, the difference between End of dwe
I
ECG
t
(a)
End of dive t
(b)
30s
Fig. 2. ECG of the hooded seal during resting dives showing the occasionally occurring variations in heart rate. In (a), the heart rate initially dropped to a low value, then increased to a plateau which was held throughout the rest of the dive. In (b) the heart rate dropped suddenly in the middle of the dive.
ARVIII P&HE
and
JOHN KKOG
Fig. 3. Heart rate recording of the hooded seal resting submerged in water, showing rapid changes in heart rate in the beginning as weli as at the end of the dive. Heart rate during the dive shows no tendency to change.
ECG
F‘ig 4. Heart rate of the hooded sea1 during the initial part of a resting period on land. (a) is a recording during a period characterized by apneic periods lasting only a few seconds. In (b) the recording shows larger variations in the heart rate than in (a). This period also had apneic periods of longer duralion.
Heart
heart rate in breathing- and apneic-periods increased (Fig 4b). In a condition where the animal seemed to be asleep, the ventilatory pattern was characterized by a respiratory frequency (33-36 breaths/min) and a duration of the breathing periods rather similar to that of the animal while it was resting in water. Mean values from 20 registered periods, including both apneic and breathing periods, are presented in Table 1. As seen from the table, apnea was accompanied by a drop in heart rate to 36% of the rate during breathing, which is not significantly different from the changes in heart rate while the animal was diving. The rapidity of the changes as well as the levels of the heart rate seemed similar on land and in water (Fig. 5). Heart rate in resting harhour seals On land. The studies of the harbour seals commenced when they were 47 days old and with negligible diving experience. Resting on land they showed the same breathing pattern as the older animals, with apneic periods interrupting a continuous breathing. During such apneic periods their heart rates dropped to a mean value of 64.9 (SE = 7.0) beats/min (n = 64) from a mean value of 118.4 (SE = 6.2) (n = 64) during breathing periods. While the older harbour seals were found to have a fairly constant ventilatory rate throughout the breathing period (52.8 breaths/min), these seal pups started the breathing periods with a
[al z E 5 z
ventilatory rate of 60 breaths/min, decreasing to about 20 breaths/min at the end of the period. The decrease in ventilatory rate was accompanied by deeper breathing. The heart rate was not affected by the change between inspiration and expiration while the animals were breathing at a high rate. However, when the ventilatory rate decreased and the tidal volume increased at the end of the breathing period, the heart rate varied between 120 and 70 beats/min. The lowest heart rate coincided with the expirations. In water, submerged. The results of the experiments with the harbour seals ranging in age from 3 months to 2 years are presented as mean values in Table 1. Compared to the hooded seal, the harbour seals used had shorter diving and breathing periods during rest. The harbour seals had a ventilatory rate much higher than that of the hooded seal: 52.9 breaths/min (SD = 11.7 breaths/min) compared to 34.3 breaths/ min (SD = 5.1 breaths/min). As seen in the experiments with the hooded seal, the change in heart rate at the start and at the end of the dive was very rapid. In spite of certain differences in heart rate values, the relative change in rate when the animal dived was approximately the same in the harbour seals and the hooded seal: a mean reduction of 66%. Head out ofwater. The relation between the length of apneic periods and breathing periods was the same as that between diving and breathing periods of the seals resting in the pool. Apneic periods as long as End of dive
In water
150
81
rate in seals
1
100
d
,,,1’-
_*
_/,’
r
ECG
[bl
On land 150. 2 E ii?
i
‘I:_
;‘-
I” OL
ECG
11
I
I
I
Fig. 5. Heart rate recordings of the hooded seal resting submerged in water recordings show similar heart rate levels during apneic- and breathing-periods
(a), and on land (b). The in these two conditions.
82
ARVID PASCHE and JOHN KROG
3 min did occur. The heart rates presented in Table 1 show a decrease in rate during apnea to 36% of the values during breathing. The decrease is not significantly different from the variation in heart rate seen in the animals while resting in water. For two of the animals used in this experiment the heart rate was recorded while they were resting on land. There was no significant difference in heart rate during apneic and breathing periods while the animals were resting immersed to the neck or on land, respectively. DISCLSSlON In studies of parameters such as heart rate it is of utmost importance to establish the most satisfactory condition in which to measure the extent and nature of variation. Most previous experiments on divingand apneic-bradycardia have involved force used on the animals. Moreover, comparison of bradycardia in situations of varying degrees of restraint has frequently occurred. It is not to be denied that diving bradycardia exists and that is is an important adaptation to diving. What exactly triggers the bradycardia in marine mammals is not clear and has been the cause of much discussion and controversy. Many stimuli have been found to elicit the bradycardia in seals. A volitional component has been suggested (Jones et al., 1973) and stimuli involving water in the face and cessation of respiratory movements (Dykes, 1974b). The presence of chemoreceptor-mediated effects upon heart rate in seals has been suggested to exist, and to contribute to the bradycardia observed in seals during diving (Tanji (‘r al.. 1975). These effects, however, appear to be less important than they are in diving birds (Jones & Purves, 1970; Holm & Serrensen, 1972: Butler & Taylor. 1973). A triggering of the rapid onset of bradycardia by cold receptors in the face, as in man. is not the case in the harbour seals (Dykes, 1974b). Among the stimuli mentioned which are found to elicit the bradycardia in seals, the water and its effect on special receptors in the face have been considered of greatest importance. According to the results presented in this paper, however, the importance of water needed for the elicitation of this cardiovascular reflex may have been overestimated in the literature. Even though the results showed a more pronounced bradycardia when the animals dived compared to the bradycardia during corresponding apneic periods on land, the differences were small and not significant on a 5”,, level. Moreover, no difference was found in the time course of the bradycardia in water and on land. The apneic periods on land as well as during the dives were initiated by closure of the nostrils. The bradycardia observed seemed to be correlated with this closure or a succeeding movement of the chest, which may be the result of a relaxation of the diaphragm or a contraction of expiratory muscles. This movement could be related to a change in muscular activity in the body wall similar to those engaged in Valsalva’s maneuver. which is known to produce bradycardia in man (Craig, 1963). Ridgeway (1972) also often noticed a Valsalva-like maneuver in the bottlenosed porpoises just after inspiration. A change in the activity of the respiratory muscles with closed nostrils would cause a change in intrathoracic press-
ure. In other animals (Angelone & Coulter, 1964), changes in intrathoracic pressure are known to affect the heart rate. It is reasonable to believe that this might be one of the causal factors of the bradycardia. Changes in intrathoracic pressure can also be caused by muscular contractions during the dives. Such changes might then explain a sudden decrease in heart rate in the middle of a dive, like that seen in the resting animal with heart rate recordings shown in Fig. 2b. Heart rate and respiration are intimately associated in marine mammals, and the effect of ventilation on heart rate has been described in seals as well as in other diving animals. Blix et al. (1976) concluded that the initial bradycardia in a diving duck is closely coupled to, and dependent upon, an enforced apnea response. This response is evoked by stimulation of receptors which are not specific to water. According to our present investigation a respiratory arrest alone cannot be responsible for the bradycardia in seals, a finding which is supported by the observations of Murdaugh et al. (1961). They found that the heart rate often returned to normal when the seal was approaching the surface, before its nostrils reached above water. If the seal decided to dive again before surfacing, the bradycardia returned. A correspondence between ventilatory maneuvers and variation in heart rate was also found in the present study. However, this was seen only in the young animals while they had an unusually low ventilation rate. In the older animals the ventilation rate seemed too high to show any effect on the heart rate at the chart speed used for our recordings. The intrathoracic pressure varied during ventilation, and the observed changes of heart rate during breathing might be the result of mechanisms similar to those responsible for the bradycardia during diving and apneic periods on land. AcknowlPdgements-The authors would like to thank MS E. A. Sommers for her invaluable help with linguistic editing of the manuscript, and Mr M. Bronndal for his assistance during the experiments and for taking care of the animals. This work was supported by grants from the Norwegian Council for Science and Humanities.
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