RESPIRATION IN THE AFRICAN LUNGFISH PROTOPTERUS AETHIOPICUS

Transcription

1 J. Exp. Bol. (968), 49, Wth text-fgures Prnted tn Great Brtan RESPIRATION IN THE AFRICAN LUNGFISH PROTOPTERUS AETHIOPICUS I. RESPIRATORY PROPERTIES OF BLOOD AND NORMAL PATTERNS OF BREATHING AND GAS EXCHANGE* BY CLAUDE LENFANT AND KJELL JOHANSENf Insttute of Respratory PhysologyFrland Sanatorum and Departments of Medcne, Physology and Zoology, Unversty of Washngton, Seattle, Washngton, 9805, U.S.A. (Receved 6 March 968) INTRODUCTION Members of the three exstng genera of lungfshes represent dfferent stages n the transton from aquatc to aeral breathng. Neoceratodus, the Australan lungfsh, s prmarly aquatc and utlzes gll breathng, whle ts lung offers an accessory means for O absorpton (Lenfant, Johansen & Grgg, 966; Johansen, Lenfant & Grgg, 967). Conversely, the South Amercan lungfsh, Lepdosren paradoxa, s a typcal ar breather and the much reduced glls are of lttle or no mportance for O absorpton but may have some functonal sgnfcance for elmnaton of CO (Johansen & Lenfant, 967). The Afrcan lungfsh, Protopterus, resemble L. paradoxa n beng prmarly an ar breather. The purpose of the present paper s to evaluate the status of P. aethopcus n the transton from aquatc to aeral breathng by analyss of the respratory propertes of blood and of the normal pattern of breathng and gas exchange. MATERIAL Ten large specmens of Protopterus were transported by arcraft from Lake Vctora, Uganda, East Afrca to Seattle, Washngton. The fshes arrved n good condton and were kept n aquara wth water temperatures of about 0 0 C. for several days before the experments were started. In addton, experments were performed n the Department of Physology at the Makerere Unversty College n Kampala, Uganda, on materal freshly caught from Lake Vctora. J Ths work not only permtted verfcaton of the data obtaned n the U.S.A., but made t possble to study addtonal problems precluded n the Seattle experments because of the lmted number of fsh. All the fshes were fastng durng the perods of study and observaton. Ths work was supported by grant GB 4038 from the Natonal Scence Foundaton and grant HE from the Natonal Insttute of Health. t Establshed Investgator, Amercan Heart Assocaton. Work supported by Northeastern Chapter, Washngton State Heart Assocaton. t We are ndebted to Professor P. G. Wrght, n the Department of Physology at Makerere Unversty College, for laboratory facltes and other support durng the course of these nvestgatons.

2 438 CLAUDE LENFANT AND KJELL JOHANSEN METHODS AND EXPERIMENTAL PROCEDURES Blood gases and ph were measured by means of a Beckman 60 gas analyser usng an oxygen macro-electrode and a Severnghaus-type CO electrode mounted n specal mcro-cuvettes. Blood ph was measured on a Beckman mcro-assembly. Gas contents were measured usng gas chromatography accordng to the method descrbed by Lenfant & Aucutt (967). The n vtro work on the respratory propertes of blood was done accordng to descrptons by Lenfant & Johansen (965). The methods used to measure gas exchange nvolved a closed temperaturecontrolled chamber. Ths chamber, partally flled wth water, was used to measure gas exchange n both the water and ar phase. Fsh placed n the chamber contnued ther normal aeral and branchal breathng pattern. When wthout water, the chamber was used to measure gas exchange durng ar exposure. In preparaton for an experment the fsh was anaesthetzed by mmerson n MS (Trcane methane sulphonate, Sandoz). An ncson was made along the ventral mdlne slghtly posteror to the heart and by careful dssecton the large vena cava, the common pulmonary ven, the left pulmonary artery and the coelac artery were all dssected free. All four vessels were cannulated n the drecton of the heart usng polyethylene catheters flled wth heparnzed salne. The catheters entered the man vessels through small sde branches or alternatvely through holes made n the vessel walls. All cannulatons were non-obstructve, allowng contnued free passage of blood n the vessels to be recorded or sampled from. Afferent branchal blood samples and blood pressures were obtaned by cannulatng the st or nd afferent branchal artery (aortc arches 3 or 4). It s recalled that n Protopterus these are drect thoroughfare arteres havng no connexon wth respratory exchange crculaton. Followng cannulaton all catheters were anchored n place and the ncson was carefully closed. The ndwellng catheters were kept n place for varable perods of tme extendng to more than 0 days. Most fshes were studed over perods lastng several days. A catheter was also nserted behnd an operculum close to the posteror gll arches to permt samplng of expred water, and another catheter was nserted va the mouth through the pneumatc sphncter n the pharyngeal floor nto the anteror porton of the lungs to allow samplng of lung gas. Both of these catheters were carefully anchored by means of sutures, takng care not to cause undue mechancal nterference wth normal branchal breathng movements and the passage of ar nto and out of the lung. Followng the surgcal procedures the fsh were permtted to recover from anaesthesa n shallow, well-oxygenated water. Ths recovery was usually quck and was often aded by artfcal nflatons va the lung catheter. The fsh were allowed to rest n water for several hours before any samplng or recordng was done. Blood for n vtro work was obtaned early after recovery or from specal fsh used as blood donors only. RESULTS Respratory propertes of blood The followng mean values were determned for three fsh used as blood donors: Haematocrt, 5 % Hb concentraton, 6- g%; mean corpuscular haemoglobn concentraton, 4-8 %. The mean O capacty was 6-8 vol %. Fgure shows a set of

3 Respraton n lungfsh. I 439 oxyhaemoglobn dssocaton curves. The curves are typcally sgmod and attest to a relatvely hgh affnty of haemoglobn for O. Fgure A compares the oxyhaemoglobn dssocaton n Protopterus wth earler publshed curves for Neoceratodus and Lepdosren. At smlar P COj the O affnty s seen to be slghtly hgher for Protopterus and Lepdosren than for Neoceratodus. Fgure B allows comparson wth earler 7 r- Protopterus (temp. = 5 C ) O 0 D Po, (mm. Hg) Fg. Oxyhaemoglobn dssocaton curves for Protopterus aetfuoptcus / /experments 70 I 60 / / *"' 50 - I 40 <S /' V «'''' o,= 6 mm Hg 7 Temp O cap o//o / >/ P co, (mm Hg) / (vol %) Protopterus 5 68 // / /' Temp =5 C Lepdosren 3 57 Neoceratodus 8 77 /-'' I I Po, (mm. Hg) Fg.. (A) A comparson of the oxyhaemoglobn dssocaton curves n varous speces of lungfshes. (B) A comparson of the oxyhaemoglobn dssocaton curve of Protopterus determned by varous authors.

4 440 CLAUDE LENFANT AND KJELL JOHANSEN publshed curves for Protopterus. The curve reported by Fsh (956) shows a hgher affnty at a smlar P COt - Data from Swan & Hall (966) dsclose a much lower affnty than we determned. Our curve at P CO 40 mm. Hg s extrapolated, snce the hghest ^co we u 8^ was 7 mm - Hg. Fgure 3 compares the Bohr effect of Protopterus blood wth our earler publshed data from Neoceratodus and Lepdosren and wth data from Swan & Hall (966) on Protopterus, The present results show a much hgher Bohr JS 30 E E PH Fg. 3 A comparson of the Bohr effect n lungfshes. The numbers n parentheses ndcate the magntude of the Bohr effect. 30 p /absolute temperature Fg. 4. The effect of temperature on oxygen affnty n blood from Protoplerus and Neoceratodut as compared to man.

5 Respraton n lungfsh. I 44 effect than reported by these authors. Fgure 4 shows the nfluence of temperature on the oxyhaemoglobn affnty n Protopterus and Neoceratodus n comparson wth human blood. Protopterus has a much larger temperature shft than Neoceratodus. The annual range of water temperature n Lake Vctora where our anmals were obtaned s very small (5-8 C). Reduced o Oxygenated Poo, (mm. Hg) Fg. 5. A comparson of CO, dssocaton curves n the lungfshes. The curves of Lepdosren and Neoceratodus for oxygenated blood. The numbers n parentheses ndcate the magntude of the Haldane effect n these two last speces.. In Fg. 5 the CO dssocaton curve of Protopterus s compared wth CO dssocaton curves for Neoceratodus and lepdosren. The CO combnng power of Protopterus blood s consderably hgher than for the other lungfshes. Protopterus, moreover, showed no Haldane effect (Fg. 5), whereas the other speces, especally Neoceratodus, dd. In Fg. 6 the bufferng capactes of the blood n the three speces are compared. The bufferng capacty of Protopterus blood exceeds that of the other lungfshes. Pattern of branchal and aeral breathng When Protopterus were observed n aquara t became apparent that the anmals normally utlzed both branchal and pulmonary breathng. Both means of breathng, however, were qute rregular, although steady rhythms sometmes prevaled for long perods when the fsh were kept n well-aerated water n whch they were free to swm and surface for ar. The actvtes of the two modes of breathng were clearly nterrelated. In partcular, the branchal breathng rate ncreased pror to each ar-breath (Johansen & Lenfant, 968). Table lsts branchal breathng rates recorded for 5 mn. ntervals several tmes n

6 44 CLAUDE LENFANT AND KJELL JOHANSEN 5 0 o Oxygenated Reduced E 5 O o X ph Fg. 6. Buffenng capacty of oxygenated and reduced blood and separated plasma from Protopterus n comparson wth buffenng capactes of oxygenated blood from the other lungfshes. The numbers n parentheses ndcate the magntude of the bufferng capacty n mm./l./ph. Table. Branchal respratory rate and oxygen extracton n glls No. of anmals 4 6 No. of experments I BRR per mn. (5mn.) I Z IOO 0 I- 4 4'5 37 f\o, (mm. Hg) 88 IO "5 5 "7 8o 8 oo 04 P*. (mm. Hg) o. extracton (%) each of the three specmens. The P o of external water, exhaled water and the extracton of O t from the branchal water current are also tabulated. Ideally the percentage extracton of O from the water should have been correlated wth actual ventlaton of water, but such ventlaton measurements were not techncally feasble. Four measurements on two dfferent specmens showed CO g tensons n exhaled water rangng from 54 to 06 mm. Hg, when the fsh was n aerated water

7 Respraton n hmgfsh. I 443 Fgure 7 shows the branchal breathng rate for one lungfsh plotted aganst tme durng contnuous recordng for 5 mn. A great varablty s evdent. Exhaled water samples taken at 5 mn. ntervals for calculaton of percentage extracton of O from the water also showed a large varablty. O extracton % 3% 35% 36% 35% g 5 s S 00 - c e I"ffl u C _ ; -. / a. 0 5 Tme (mn). " \ * 0 u 40 g 5 m Fg. 7. Frequency of branchal breathng and percentage extracton of O, from the water durng a 5 mn. perod of normal undsturbed breathng. Table. Interval between breaths (mn.) No. of anmals No. of experments 3 4 Mean 4-8 s-s Normal water A S.D. I-O 5-0 IO O 6 4 I 6 N Mean Ar A S.D. o-o 3 6 N 7 '3 4 Table shows average ntervals between ar breaths recorded several tmes for each ndvdual specmen lsted. The table ncludes fsh n whch a farly regular arbreathng rhythm prevaled. Ar exposure of the fsh by slowly dranng the tank water elcted a drastc ncrease n the rate of ar breathng. Ths ncrease could clearly be dssocated from exctement or arousal as the fsh rarely responded by volent behavour to ar exposure. When ar-exposed the fsh usually started slow searchng movements and after varable tmes attempted to dg wth ther noses, seemngly

8 444 CLAUDE LENFANT AND KJELL JOHANSEN seekng to bury n the substratum. Fsh ar-exposed on a muddy surface where burrowng was possble dd actually bury themselves, slowly but methodcally. Not only dd ar exposure elct an ncreased rate of ar-breathng, t also changed the pattern of breathng nto groups of several breaths at shortened ntervals. Table 3. Rate of gas exchange n glls and n lung Vo, (ml./mn./kg.) Kco, (ml./mn./kg.) Condtons Water Gas Water Gas Total Water Gas Water Gas Total R a [n water wth access 004 to ar [n ar O O-I OI5 O O-IO O-O9 O-O5I o o K M _ A A A A m # A # A A A (mm # M 5-0 * Tme from breath (mm.) * Fg. 8. Tme course of change n Pot and.p00 n the pulmonary ar of Protopterus between successve ar breaths. The gas samples were collected durng fve breath ntervals of varous duraton. 60 The relatve contrbuton of glls and lungs r O absorpton and CO elmnaton are ndcated n Table 3. Gas exchange ratos have been calculated for aquatc versus aeral gas exchange as well as for over-all gas exchange when the fsh s n water. The The lower horzontal column lsts Oj absorpton, CO producton and over-all gasexchange ratos durng ar exposure. The data dsclose that aquatc absorpton of O consttutes only between 0 and % of the entre O uptake. Conversely, aquatc

9 Respraton n lungfsh. I 445 gas exchange s on the average -5 tmes more effectve than the pulmonary gas exchange n elmnatng CO when the fsh s n water. The over-all gas exchange rato s qute varable. All data tabulated on gas exchange represent average values from readngs every hour for 6-0 hr. The relablty of the absolute values suffers from the nevtable exchange between the gas phase and the water phase n the Gas tensons n envronment (mm. Hg) Oxygen I 00 r Carbon doxde Fg. 9. Relatonshp between oxygen uptake n Protopterus and gas composton n the ambent envronment durng ar exposure n a closed respraton chamber. In water wth access to ar Table 4. Blood gas tensons In ar.po, (mm. Hg) Poo, (mm. Hg) Po, (mm. Hg) Poo, (mm. Hg) lo. I 3 4 DAo PV PA S 33 DAo PV PA DAo 367 O-O 35 PV PA 9'O I'O 4-5 DAo PV Each value s the mean of at least eght consecutve measurements for P o% and at least four for Pc DAo = dorsal aorta, PV = pulmonary ven and PA = pulmonary artery. PA metabolsm chambers. However, ths exchange would tend to mnmze the role of aquatc exchange for CO a elmnaton and the aeral exchange for O absorpton. It s notable that ar exposure dd not promptly reduce the over-all oxygen uptake. The apparent reducton n the gas-exchange rato represented the only real change durng the relatvely short perods of observaton. Fgure 8 llustrates the tme course of changes n the partal pressures of pulmonary gases n the nterval between ar breaths. The tme course of the gas exchange rato s

10 446 CLAUDE LENFANT AND KJELL JOHANSEN also plotted. Ths specmen and other fsh used for these measurements were free to swm n large tanks and had not been subjected to any surgcal treatment. The expred gas was collected by trappng the released gas nto water-flled funnels suspended above the heads of the fshes. Water (Pl o,= 5 mm Hg) H Breaths I I II I III II II c o Tme (mn.) Fg. 0. (A) Repettve blood gas analyss durng undsturbed breathng n Protopterus. To the rght are plotted blood gas values durng ar exposure. Arrows mark the tme of ar breaths. PV, Pulmonary ven; PA, pulmonary artery; AO, dorsal aorta. (B) Blood gas values expressed n partal pressure and O t saturaton durng undsturbed breathng n aerated water. Arrows mark the tme of ar breaths. 0 Fgure 9 shows the relatonshp between oxygen uptake (V o ) and the gas composton n the ambent envronment. In ths typcal experment the fsh s seen to mantan hs V Ot down to a P/ O of approxmately 85 mm. Hg wth a correspondng P co ncrease n the metabolsm chamber of about 35 mm. Hg. A further reducton n P 0 was accompaned by an abrupt fall n V Ot. The expermental arrangement dd not make t possble to decde whether the Pr O or the P C o, or both are nstrumental n brngng about ths change. The experments underlyng Fg. 9 lasted for varable perods up to 8 hr. All fsh survved the experments. Consstently, prolonged ar exposure was accompaned by a progressve vasodlaton of the skn. Table 4 shows the blood gas tensons n undsturbed fsh restng n well aerated

11 Respraton n lungfsh. I 447 water as well as durng ar exposure. Fgure 0 A, B emphaszes the dfference n blood gas tensons borne out n the table and shows the tme course of blood gas changes occurrng between consecutve breaths. Note that blood from the dorsal aorta shows a consstently hgher oxygen tenson than pulmonary arteral blood. Expressed n percentage oxygen saturaton (Fg. 0B), the pulmonary arteral blood s seen to be about % saturated aganst more than 75 % average saturaton n systemc arteral blood from the dorsal aorta. Fnally, pulmonary venous blood s about 85 % saturated wth oxygen. To the rght n Fg. 0 A are plotted the blood oxygen tensons followng ar exposure. The arrows ndcate the number of ar breaths and draw attenton to a conspcuous stmulaton of ar breathng assocated wth ar exposure. All blood gas values are ncreased and the gradent between systemc arteral blood and pulmonary arteral blood has become larger. DISCUSSION The lungfshes occupy habtats smlar to those of many other ar-breathng fshes, yet the O capacty of ther blood seems to be generally much lower. Thus, the blood of Protopterus compares well wth values reported for the South Amercan Lepdosren and the Australan Neoceratodts (Johansen & Lenfant, 967; Lenfant et al. 966). In contrast, three South Amercan ar-breathng fshesthe electrc eel, Electrophorus electrcus (Johansen, Lenfant, Schmdt-Nelsen & Petersen, 968); the hassa, Hoplosternum tttorale (Wllmer, 934) and Symbranchts marmoratus (Johansen, 966) showed average O capactes of -3, 8- and 4-7 vol. % respectvely. Snce tropcal ar-breathng fshes largely occupy smlar habtats, the marked dfference n O capacty may be related to the effcency n O transport. Many ar-breathng fshes have a permanently low arteral O saturaton because the blood dranng the aeral gas-exchange organ s mxed wth the general systemc venous blood before t perfuses the arteral crculaton. In the lungfshes such extensve shuntng of oxygenated blood to the venous crculaton s prevented by a separate pulmonary vascular crcut and a partal dvson of the heart and ts outflow channels (Johansen, Lenfant & Hanson, 968). The oxyhaemoglobn dssocaton curves (Fgs., ) conform n shape to those reported for lungfshes earler but the actual affnty for O shows defnte varatons and was hgher n our experments. Fsh (956), workng on haemoglobn solutons, whch n general have hgher affnty than whole blood, reported a P M value of about mm. Hg at P c0 6 mm. Hg for P. aethopcus. Swan & Hall (966), workng wth whole blood from Protopterus, found an O affnty much lower than our experments revealed (Fg. B). It has been suggested that the O affnty of haemoglobn generally decreases wth ncreasng dependence on aeral gas exchange (McCutcheon & Hall, 937). Ths tendency should reflect an adjustment to the hgher O avalablty n ar than water. More recent data from transtonal forms among amphbans substantate ths hypothess (Lenfant & Johansen, 967). In regard to fshes, most workers endorse the early suggeston by Krogh & Letch (99) that hgher O affnty of haemoglobn correlates wth the ablty of a speces to survve n an O poor medum. Among ar-breathng fshes, those showng accessory ar breathng wth a domnance of aquatc gas exchange 9 Exp. B0L 49,

12 448 CLAUDE LENFANT AND KJELL JOHANSEN may show adaptaton to condtons n the water whereas oblgatory ar-breathng fshes, lke Lepdosren and Protopterus, should show adjustments towards atmospherc condtons. Wllmer (934) offered convncng data demonstratng that blood from freshwater fsh nhabtng well-aerated water of low CO content dsplays a marked senstvty to CO changes (Bohr and Root's shfts), whereas blood from fsh n stagnant swamps and muddy creeks wth a hgh CO content was practcally nsenstve to CO. Later workers, notably Carter (95, 96) and Fsh (956), used such data to generalze that a transton from aquatc to aeral gas exchange s accompaned by a reducton n the COj, senstvty of the blood. Carter's generalzaton on the evoluton of the Bohr shft receves support from a comparson of blood from Neoceratodus (Lenfant et al. 966) and Lepdosren (Johansen & Lenfant, 967). Lepdosren, whch s decdedly more of an ar breather, shows a much smaller Bohr shft than Neoceratodus. An extenson of ths comparson to Protopterus, based on the present results or those of Swan & Hall (966), does not support such a trend. A comparson of Bohr shfts n selected amphbans showng ncreasng dependence on ar breathng smlarly does not provde support for Carter's hypothess (Lenfant & Johansen, 967). The latter comparson, however, s complcated by neoteny of the speces of urodeles studed. Further work s obvously desrable to resolve the role of adaptve changes n the CO senstvty of the blood n the evoluton of ar breathng. The effect of a temperature change on the O s affnty of haemoglobn s dfferent n blood from the Australan and Afrcan lungfshes. Neoceratodus, whch occupes waters that may show large annual and durnal changes n temperature (up to 0 C), has blood that s notably nsenstve to temperature changes. Conversely, Protopterus, wth very slght temperature varatons n ts habtat, possesses blood whch s much more senstve to temperature changes. The CO a combnng power and bufferng capacty of blood from the lungfshes and other forms n the transton to ar breathng reveals some consstent trends. Fgure 5 shows an ncreasng CO combnng power from Neoceratodus through Lepdosren to Protopterus. Ths tendency substantates predctons made by Lenfant et al. (966) that a greater dependence on ar breathng s correlated wth an ncreasng CO a combnng power. The role of the glls and skn n O uptake n Protopterus was found to be about 0 % of the total uptake when the fsh rests n aerated water (Table 3). In Lepdosren the mportance of the glls for O absorpton s even less and reported to be only % of the total O uptake (Sawaya, 946). The relatvely hgh extracton of O from the water found presently (Table ) was of lttle mportance to oxygen uptake because of extremely low values of ventlaton. On the other hand, aquatc exchange by glls and skn was -5 tmes as effcent as the pulmonary exchange for CO elmnaton. The role of the skn could be mportant n ths comparson snce Cunnngham (934) reports that the skn n Lepdosren has a gas-exchange rato of more than 0 when the fsh s n water. The progressve vasodlaton of the skn n Protopterus durng ar exposure attests to an actve role n gas exchange. The present results for Protopterus are n accord wth the tendency for anmals havng bmodal, aquatc and aeral, gas exchange to show a low gas-exchange rato for the ar-breathng organ, and a gas-

13 Respraton n kngfsh. I 449 exchange rato greater than unty for the water-breathng organ (Lenfant & Johansen, 967). Fgure 9 demonstrates that Protopterus whle n ar retans respratory ndependence down to envronmental O tensons of about 85 mm. Hg correlated wth an external ^co, f 3 ~35 mxn - Hg. Most aquatc vertebrates show crtcal O g tensons as low as 30 mm. Hg. Hall (99) compared factors lmtng oxygen uptake n actve and" sluggsh speces of fshes. Hs data suggest that actve fshes have a greater tolerance to varatons n external oxygen tenson. Smlarly, fshes wth a hgher haemoglobn content were more capable of mantanng a steady oxygen uptake n spte of reduced external oxygen avalablty. If Protopterus was prmarly a water breather, Hall's generalzatons would be applcable to explan the narrow range of O g ndependence, but beng 45 - O, saturaton (%) X Protopterus *» Neoceratodus Po, (mm Hg) Fg. II. Nomogram comparng blood gas values n Ncoceratodus and Protopterus durng undsturbed breathng n water (low Poo,) an d followng ar exposure (duraton of ar exposure s ndcated by the numbers n parentheses). PA, pulmonary artery; PV, pulmonary ven; DAO, dorsal aorta. prmarly an ar breather, Protopterus should be more tolerant to external P o changes than water breathers. The profound metabolc adjustments occurrng durng fastng and aestvaton (Smth, 935) may be related to the unusual metabolc responses of Protopterus to changes n ambent gas composton. The nomogram (Fg. ) provdes a bass for dscussng dfferences n gas exchange between Neoceratodus, n whch the lung functons as an auxlary gas exchanger and Protopterus whch depends predomnantly on pulmonary breathng. When restng n well-aerated water Neoceratodus employs ar breathng very nfrequently. Durng such condtons the lung s of no consequence to gas exchange, and smlar blood gas values preval n systemc arteral, pulmonary arteral and pulmonary venous blood. Effcent branchal gas exchange mantans the arteral blood more than 90 % oxygen saturated and arteral P COj values are extremely low, a character shared wth typcal aquatc 9-

14 45 CLAUDE LENFANT AND KJELL JOHANSEN breathers. Protopterus, on the other hand, practses actve, frequent ar breathng and the lungs play a major role n gas exchange. The mportance of the lungs s apparent n the dfferences n oxygen saturaton as well as P c0 values of pulmonary arteral and pulmonary venous blood. The ntermedate saturaton value of systemc arteral blood sampled from the dorsal aorta testfes that pulmonary venous blood s channelled o Protopterus a * Neoceratodus - ^PV X / er' ( ts^^* PV -.cdao ± P A PA-DAO I I Duraton of ar exposure (mm). (A) 40 o Protopterus A A Neoceratodus PA. ~ * DAO 30 PV 0 PA-DAO - ^r.c-' Duraton of ar exposure (mm.) Fg. a. Tme course of changes n blood gas composton durng ar exposure of Neoceratodu and Protopteru. (A) Changes n blood oxygen tensons; (B) changes n blood carbon doxde tensons. PA, pulmonary artery; PV, pulmonary ven; DAO dorsal aorta. (B)

15 Respraton n kngfsh. I 45 selectvely nto the cardac outflow channels gvng rse to the systemc crculaton. The emphass on ar breathng n Protopterus s also borne out by the much hgher blood Pco, values, beng on the average more than 0 mm. Hg above the values n Neoceratodus. The lmtatons of pulmonary ar breathng n the bmodal gas exchange of lungfshes were tested by exposng the fsh to the ar and thus nterruptng aquatc gas exchange altogether. Unlke Neoceratodus, Protopterus never reacted volently or became restless and excted when removed from water. Ar-exposure accelerated ar breathng n both speces. In Neoceratodus ths became manfest n a conspcuous ncrease n oxygen tenson of pulmonary venous blood. However, systemc arteral oxygen tenson dropped sharply and CO tensons ncreased markedly n all blood samples. In Protopterus ar exposure ncreased the oxygen saturaton n both pulmonary venous and systemc arteral blood, whle pulmonary arteral blood mantaned the same saturaton level. The ncrease n blood CO tensons was much less marked than n Neoceratodus. Fgure A and B show the tme course of blood gas tensons followng ar exposure. The nablty of the lung of Neoceratodus to keep up the oxygen saturaton of arteral blood seems attrbutable to a low pulmonary blood flow rather than to the effcency of gas exchange n the lung. In contrast, the pulmonary blood flow n Protopterus s hgh enough to mantan the arteral oxygen tenson when gll breathng s suspended (Fg. A). In Neoceratodus the arteral CO tensons were ncreased more than 4 tmes durng 30 mn. exposure to ar. Protopterus blood showed a correspondng ncrease of about 30%. In Protopterus, however, CO elmnaton represents the most severe lmtaton of pulmonary breathng. The ablty of Protopterus to survve several months of terrestral exstence whle aestvatng durng perodc droughts s most probably related to the most condtons n the cocoon allowng some CO to escape through the skn. Also the markedly reduced over-all metabolsm durng aestvaton wll reduce the requrements for gas exchange. SUMMARY. Respratory propertes of blood and pattern of aeral and aquatc breathng and gas exchange have been studed n the Afrcan lungfsh, Protopterus aethopcus.. The mean value for haematocrt was 5 %. Haemoglobn concentraton was 6- g % and O capacty 6-8 vol. %. 3. The affnty of haemoglobn for O was hgh. P w was 0 mm. Hg at JPCO, 6 mm. Hg and 5 C. The Bohr effect was smaller than for the Australan lungfsh, Neoceratodus, but exceeded that for the South Amercan lungfsh, Lepdosren. The O affnty showed a larger temperature shft n Protopterus than Neoceratodus. 4. The CO combnng power and the over-all bufferng capacty of the blood exceeded values for the other lungfshes. 5. Both aeral and aquatc breathng showed a lable frequency. Ar exposure elcted a marked ncrease n the rate of ar breathng. 6. When restng n aerated water, ar breathng accounted for about 90% of the OJJ absorpton. Aquatc gas exchange wth glls and skn was 5 tmes more effectve than pulmonary gas exchange n removng CO. The low gas-exchange rato for the lung dmnshed further n the nterval between breaths.