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Effect of taurocholate on electrical potential difference across rat small intestine
Pergamon Press
Life Sciences Vol . 11, Part I, pp . 375-386, 1972 . Printed in Great Britain
EFFECT OF TAUROCHOLATE ON ELECTRICAL POTENTIAL DIFFERENCE ACROSS RAT SMALL INTESTINE* Malcolm J . Wall** and R . David Baker Department of Physiology University of Texas Medical Branch Galveston, Texas 77550 (Received 13 January 1972; in final form 6 March 1972)
SUMMARY Taurocholate stimulated the transmural electrical potential difference across rat small intestine incubated in vitro . Stimulation was much greater in terminal ileum w ere taurocholate is actively transported) than in upper jejunum . Fasting the rats for 24 hr decreased the electrical response in the terminal ileum, but biledepletion increased it . Fasting also decreased taurocholate transport by terminal ileum, and bile-depletion is already known to increase transport . The correlation between electrogenicity and transport at different positions along the intestine and when the animals are either fasted or depleted of bile makes us suspect a functional relationship between the electrical effect of taurocholate and its transport . ACTIVELY transported sugars and amino acids increase the transmural potential difference (PD) across small intestine when placed on the mucosal side .
This effect has been repeatedly confirmed
and is generally believed to be closely associated with the active transport of sugars and amino acids since nonactively transported analogs are ineffective (1) .
Bile salts are actively absorbed by
lower ileum (2) and it might be anticipated that bile salts would also increase transmural PD, assuming this effect to be common to * Supported by USPHS Grant No . AM-05778 ** Present address :
Department of Physiology Medical College of Wisconsin 561 North 15th Street Milwaukee, Wisconsin 53233
375
Vol . 11, No . 8
Effect of Taurocholate
37 6
all actively transported organic solutes .
However, conflicting
results have been reported (3,4), and we felt that a new study of intestinal electrical responses to bile salt was necessary .
We
have found a definite effect of taurocholate on transmural PD in rat ileum, thus confirming the report by Lyon (3) ; and, in addition, we have noted certain correlations between this effect and the active transport of taurocholate . Materials and Methods We used male rats derived from the Sprague-Dawley strain (obtained from Cheek-Jones Co ., Houston, Texas) ; they weighed between 150 and 250 g before fasting .
Each rat was killed by a
blow on the head and 6 cm segments of upper jejunum (beginning just below ligament of Treitz) and terminal ileum were excised and everted on a stainless steel rod .
Each segment was tied onto
a glass cannula of the type described by Crane and Wilson (5), and filled with 0 .80 ml of Krebs-Henseleit bicarbonate solution (6) at pH 7 .4, and then immersed in 8 .0 ml of the same solution in a test tube . 5$ C0 2 in 0 2 . (7) .
The mucosal solution was gassed with humidified Transmural PD was recorded, as previously described
The tips of the KC1-agar bridges were soaked in Krebs-
Henseleit solution for at least an hour preceding each experiment to prevent leaching of KC1 during the experiment .
All preparatory
steps and the actual experiment were performed in Krebs-Henseleit solution at 37°C .
Preparation time was about 7 min .
The potential difference was recorded for 1-2 min, then the entire assembly (cannula with attached intestine, KC1-agar bridges, and gassing tube) was quickly transferred to a test tube containing 8 .0 ml of Krebs-Henseleit solution in which sodium taurocholate (from CalBiochem) had been dissolved at a concentration of
Effect of Taurocholate
Vol. il, No . 8
10 mM .
377
The purity of the taurocholate was confirmed by silica
gel thin-layer chromatography (8) .
Both segments, jejunal and
ileal, were transferred simultaneously .
Recording was continuous
throughout the transfer procedure . 1) Unfasted rats maintained
Three groups of rats were used : on Purina Laboratory Chow ; 2)
rats fasted for 24 hr preceding the
experiment with access to water ; and 3) rats from which bile had been drained for 72 hr via a polyethylene tube tied into the bile duct just above the entrances of the pancreatic ducts .
Cannula-
tion of the bile duct was performed under pentobarbital anesthesia . During bile drainage the animals were restrained in the cage described by Baker et al . (9) and were given Purina Laboratory Chow and 0 .38 NaCl solution ad' libitum.
An average of 18 g body
weight was lost during the 72-hr bile-dépletion period . Taurocholate transport was studied in terminal ileal segments from fasted and Unfasted rats, using the cannulated everted sac technique of Crane and Wilson (5) .
In these experiments, the
initial taurocholate concentration was 0 .2 mM in both mucosal and serosal Krebs-Henseleit solutions .
Both solutions contained 0 .2
uc/ml of 14 C-labeled taurocholate (Tracerlab) . for 1 hr at 37°C .
Incubation was
Radioactivity of mucosal and serosal solutions
was determined by liquid scintillation counting .
Water absorption
from the mucosal solution was determined by weighing the intestinal segment with cannula before and after incubation . Results An example of the type of record obtained is shown in Figure 1 .
In the terminal ileum an increase in PD (serosal side
Vol . 11 . No . 8
Effect of Taucocholate
37 8
PD (mv)
PD (mv)
i-T-T'.{"- .j~---~ 6 4 S O 1 2 3 TIME (min)
FIG .
1
A sample polygraph record from one of the experiments using an unfasted rat . Both segments were transferred simultaneously (at the arrows) to mucosal solutions containing 10 mM taurocho late . Calibration signals of 2 mV appear near the beginning and ending of each trace . The frequently observed second phase of the deal response was only slightly evident in this particular experiment . positive) always occurred in response to taurocholate .
There
was no detectable delay in onset and maximum PD was usually reached within about 30 sec .
With unfasted or bile-depleted
animals the response was transitory ; PD usually declined to the pre-transfer level roughly 3 to 8 min after transfer, and often fell below this level .
With fasted rats the response in the
ileum was of less magnitude than with unfasted or bile-depleted rats, but was usually more stable .
Frequently the deal response
included a second stimulatory phase -- a small hump in the curve beginning 1-2 min after transfer .
The record in Figure 1 shows
379
Effect of Taurocholate
Vol. 11, No. 8
a relatively small second phase .
The jejunal segments usually
responded to taurocholate, but only very weakly . PD values recorded just before transferring to taurocholate are summarized in Figure .2 .
Ileal PD averaged 0 .74 mV with the
d 0 ôz .o r v O L 0
O r ,~ 1 .0 o"
O
m h 0
.° c
-v m m â d 0
ILEUM
o. m 0
-n 0 â c
m d e. m
JEJUNUM
FIG . 2 Pre-transfer PD values . These measurements were made just before transfer to taurocholate . Each bar represents the mean for the number of rats shown within the bar . Standard errors of the means are depicted by vertical lines . fasted rats, but was significantly higher with unfasted rats, and even greater with bile-depleted animals .
Jejunal PD followed a
similar pattern . The mean increases in PD in response to 10 mM taurocholate are shown in Figure 3 .
In the terminal ileum of unfasted rats the
Vol. 11, No . 8
Effect of Taurocholate
38 0
ILEUM FIG .
JEJUNUM 3
The electrical response to taurocholate . These measurements were made by subtracting the pre-transfer PD from the maximum PD attained following transfer to 10 mM taurocholate . Standard errors are depicted by vertical lines . The number of animals is shown for each bar . response (BPD) averaged 2 .27 mV, was much less with fasted rats (0 .64 mV), and considerably greater with bile-depleted rats (3 .51 mV) .
In the jejunum, BPD was not detectably influenced by
bile depletion ; fasting caused an apparent reduction in BPD, but there was a 158 chance for the null hypothesis (by t test) . Data obtained related to transport of water and taurocholate by terminal ileal segments are shown in Table 1 .
Fasting the
animals for 24 hr signifi^~ntly reduced the amount of taurocho late absorbed from the mucosal solution and also reduced the final
Vol. 11, No . 8
38 1
Effect of Taurocholate
serosal :mucosal concentration ratio .
Fasting also reduced the
amount of water absorbed . TABLE 1 Effect of Fasting on Water and Taurocholate Transport by Rat Terminal Ileuma Water Absorbed (ml)
Taurocholate Absorbed (moles/g)b
Serosal :Mucosal Concentration Ratio at 1 hr
Unfasted
0 .47 ± 0 .036
0 .52 ± 0 .026
1 .97 ± 0 .085
Fasted
0 .32 ± 0 .016
0 .31 ± 0 .028
1 .30 ± 0 .086
pc
0 .0054
0 .0005
0 .0004
a) The values shown are means ± standard errors of the means for five unfasted rats and six fasted rats . b) Taurocholate absorption is the amount of taurocholate removed from the mucosal solution during a 1-hr incubation per gram of gut wet weight . c) The probability of the null hypothesis being correct as determined by Student's t test . Discussion This investigation demonstrates that taurocholate is electrogenic in the terminal ileum.
Lyon previously observed this effect
of taurocholate (3), but Jackson and Smyth failed to detect it (4) . The promptness of the response resembles that found when sugars are actively transported and implies an immediate effect on ion movements at the apical boundary of the epithelial cells . The response was transitory (except in some of the experiments using fasted rats) .
We believe that the return to and be-
yond control PD may result from an effect of taurocholate that has accumulated intracellularly .
In several preliminary experi-
ments in vivo we have found the response to be stable, presumably
382
Effect of Taurocholate
Vol . 11, No . 8
because with vascular flow intracellular accumulation of taurocholate is less,
The time course of the response to taurocholate in
vitro resembles the response to phenylalanine shown by Kohn et al . (10) .
Lyon did not report the temporal course of the response
and did not note its transitory nature, probably because he used fasted rats .
ATP also causes a transitory stimulation of PD in
rat intestine, but its effect differs from that of taurocholate in that a latent period of about 15 sec occurs (11), Although we usually obtained weak electrical responses in jejunum, they were always far less than the responses consistently obtained in terminal ileum,
A similar observation was made by
Lyon, although his jejunal responses were somewhat larger than ours .
In a recent abstract, Feldman et al . (12) also report
somewhat larger jejunal responses to taurocholate than we obtained . It is well known that the ability of terminal ileum to absorb conjugated bile salts is far greater than that of the jejunum (13-16) and the active transport of bile salts seems to be re stricted to the ileum (2) .
Since the electrogenic effect of
taurocholate is also much greater in terminal ileum than~in jejunum, there is reason to suspect that the electrical effect of taurocholate has something to do with its active transport .
Curi-
ously, this kind of correlation is not observed with sugars or amino acids,
The glucose-induced pPD is as great in terminal
ileum as in upper jejunum (10,17) in spite of a marked difference in ability of these regions to transport glucose (18,19) . glycine-induced pPD is greatest in terminal ileum (10)
The
even though
this is a relatively poor site for glycine transport (20) .
In searching for other correlations between electrogenicity of taurocholate and its transport, we have tested the effects of fasting for 24 hr and depletion of bile for 72 hr,
Holt demon-
Vol . 11, No . $
Effect of Taumcholate
383
strafed that bile depletion greatly increases the, ability of rat terminal ileum to transport 14C-labeled taurocholate (21), and the data shown-here establish that bile depletion also increases the electrical response to taurocholate in the ileum .
Further-
more, as demonstrated here, fasting for 24 hr decreases both bile salt transport and the electrical response to bile salt .
Thus,
the interpretation that electrogenicity of taurocholate is closely related to its active transport rather than to some passive consequence of its physical chemistry is supported by the three correlations demonstrated here :
1) Site along intestine, 2)
effects of fasting, and 3) effects of bile depletion . The effects of fasting and of bile depletion may relate to changes in the endogenous amounts of bile salt in the mucosa . Holt found that after 24 hr of bile drainage there was essentially no bile salt present in small intestinal tissue, whereas the ileum of intact rats contained roughly 1 umole of bile salt per gram of tissue water (21) .
Norman and Sjövall demonstrated that fasting
rats for 24 hr increased the percentage of previously injected 14 C_labeled cholic acid that could be recovered from small intestinal tissue (22) .
Also, Holt showed that when taurocholate was
measured chemically, its final concentration in the serosal solution, after incubating everted sacs of rat ileum for 30 min, was higher when fasted rats were used than when unfasted rats were used (21) .
This result implies that fasting increased the endog-
enous level of bile salt in the tissue .
Thus, it seems that if
the amount of endogenous bile salt in ileal mucosa is decreased by feeding or by bile depletion, the ability of the ileum to transport taurocholate and to respond electrically to taurocholate increases .
Although the mechanism of this relationship is obscure
384
Effect of Taumcholate
Vol . 11, No . 8
and its significance is not clear, there may be an advantage with regard to efficiency of the enterohepatic circulation of bile salts in augmented ileal bile salt transport during periods of digestion . It should be noted that feeding and especially bile depletion increased the pre-taurocholate PD values, a result which may imply a depressant effect of endogenous bile salt on ion transport or on transmural resistance .
This effect could be related to the en
hancement of sugar transport by bile depletion and inhibition of sugar transport by taurocholate shown by Roy et al . (23), and to the inhibition of sugar and amino acid transport by an overnight fast demonstrated by McManus and Isselbacher (24) . Active transport of taurocholate is known to be a Na +dependent process (21)
and is inhibited by ouabain (25) .
These
features suggest a coupling between Na+ and taurocholate influxes across the brush border, as with other actively transported organic solutes (26) .
The presence of an electrogenic effect of
taurocholate supports this suggestion .
Although it has not yet
been demonstrated that increased Na+ transport underlies the taurocholate-induced BPD, such a result may be anticipated and would strengthen the notion of a coupled Na+ and taurocholate influx mechanism .
Vol. 11, No . 8
Effect of Taomcholate
385
References 1.
R .J,C, BARRY, Br . Med . Bull, 23, 266 (1967),
2.
L . LACK and I,M . WEINER, Am . J . Physiol . 2'00, 313 (1961) .
3.
I . LYON, Biochim, 8iophys . Acta 163, 75 (1968),
4.
M .J . JACKSON and D,H, SMYTH, Nature 2'19,, 388 (1968) .
5.
R.K . CRANE and T,H, WILSON,' J . Appl . Ph siol . 12, 145 (1958) .
6.
H .A, KREBS and K, HENSELEIT, ' Z, Physiol, Chem . 210, 33 (1932),
7.
R.D, BAKER, M.J . WALL, S . WATSON and J .L . LONG, Biochim , Sio h s,' Acta' 150, 649 (1968) .
8.
A,F, HOFMANN, J . Li~id Res . 3, 127 (1962) .
9.
R,D, BARER, G.G . GUILLET and C .C . MAYNES, J . A~p1 . Physiol . 17, 1020 (1962) .
10 .
P .G . ROHN, D .H . SMYTH and E .M . WRIGHT, J . Ph siol, 196, 723 (1968) ,
11 .
P .G, ROHN, H, NEWEY and D,H, SMYTH, J. Physiol . 208, 203 (1970) .
12 .
E,B, FELDMAN, J .W . MARX, H . HEYMAN, H . GRUNDMEYER and D .S, FELDMAN, Am . J, - Clin, Nutr . 23, 662 (1970) .
13 .
H .E . TAPPEINER, Ser, Akad . Wiss . Wien . Math .-naturen K1 ., Abt . 3, 77, 281 ZI'g78j--
14 .
E . FRÖLICHER, Siochem . Z . 283, 273 (1936) .
15 .
J, MELLANBY and S,F . SUFFOLK, Proc , R . Soc, Lond . a 126, 287 (1938) .
16 .
R,D, BARER and G,W. SEARLE, Proc . Soc . Exg. Siol , Med 105, 521 (1960) . r -
17 .
R,J,C . BARRY, S . DIKSTEIN, J, MATTHEWS, D .H . SMYTH and E .M . WRIGHT, J, Physiol . 171, 316 (1964) .
18,
B,A, BARRY, J . MATTHEWS and D .H . SMYTH, J . Physiol . 157, 279 (1961) .
19 .
R,D, BARER, G .W . SEARLE and A .S . NUNN, A_m. J_ . Physiol . 200, 301 (1961) .
20 .
R.D . BARER and M,J . GEORGE, Biochim , Siophys . Acta 225, 315 (1971) .
21 .
P .R . HOLT, Am, J, Physiol, 207, 1 (1964) .
22 .
A . NORMAN and J . SJÖVALL, J . Biol . Chem . 233, 872 (1958) .
386
Effect of Taurocholate
Vol . 11, No . 8
23 .
C .C . ROY, R .S, DUBOIS and F . PHILIPPON, Nature 225, 1055 (1970) .
24 .
J .P .A . MC MANUS and K .J . ISSELBACHER, Gastroenterology 5 9 , s 214 (1970),
25 .
T .M . PARKINSON, Biochim. Biophys . Acta 86, 425 (1964) .
26 .
S .G . SCHULTZ and P .F . CURRAN, Phys ol . Rev . 50, 637 (1970) .
Life Sciences Vol . 11, Part I, pp . 375-386, 1972 . Printed in Great Britain
EFFECT OF TAUROCHOLATE ON ELECTRICAL POTENTIAL DIFFERENCE ACROSS RAT SMALL INTESTINE* Malcolm J . Wall** and R . David Baker Department of Physiology University of Texas Medical Branch Galveston, Texas 77550 (Received 13 January 1972; in final form 6 March 1972)
SUMMARY Taurocholate stimulated the transmural electrical potential difference across rat small intestine incubated in vitro . Stimulation was much greater in terminal ileum w ere taurocholate is actively transported) than in upper jejunum . Fasting the rats for 24 hr decreased the electrical response in the terminal ileum, but biledepletion increased it . Fasting also decreased taurocholate transport by terminal ileum, and bile-depletion is already known to increase transport . The correlation between electrogenicity and transport at different positions along the intestine and when the animals are either fasted or depleted of bile makes us suspect a functional relationship between the electrical effect of taurocholate and its transport . ACTIVELY transported sugars and amino acids increase the transmural potential difference (PD) across small intestine when placed on the mucosal side .
This effect has been repeatedly confirmed
and is generally believed to be closely associated with the active transport of sugars and amino acids since nonactively transported analogs are ineffective (1) .
Bile salts are actively absorbed by
lower ileum (2) and it might be anticipated that bile salts would also increase transmural PD, assuming this effect to be common to * Supported by USPHS Grant No . AM-05778 ** Present address :
Department of Physiology Medical College of Wisconsin 561 North 15th Street Milwaukee, Wisconsin 53233
375
Vol . 11, No . 8
Effect of Taurocholate
37 6
all actively transported organic solutes .
However, conflicting
results have been reported (3,4), and we felt that a new study of intestinal electrical responses to bile salt was necessary .
We
have found a definite effect of taurocholate on transmural PD in rat ileum, thus confirming the report by Lyon (3) ; and, in addition, we have noted certain correlations between this effect and the active transport of taurocholate . Materials and Methods We used male rats derived from the Sprague-Dawley strain (obtained from Cheek-Jones Co ., Houston, Texas) ; they weighed between 150 and 250 g before fasting .
Each rat was killed by a
blow on the head and 6 cm segments of upper jejunum (beginning just below ligament of Treitz) and terminal ileum were excised and everted on a stainless steel rod .
Each segment was tied onto
a glass cannula of the type described by Crane and Wilson (5), and filled with 0 .80 ml of Krebs-Henseleit bicarbonate solution (6) at pH 7 .4, and then immersed in 8 .0 ml of the same solution in a test tube . 5$ C0 2 in 0 2 . (7) .
The mucosal solution was gassed with humidified Transmural PD was recorded, as previously described
The tips of the KC1-agar bridges were soaked in Krebs-
Henseleit solution for at least an hour preceding each experiment to prevent leaching of KC1 during the experiment .
All preparatory
steps and the actual experiment were performed in Krebs-Henseleit solution at 37°C .
Preparation time was about 7 min .
The potential difference was recorded for 1-2 min, then the entire assembly (cannula with attached intestine, KC1-agar bridges, and gassing tube) was quickly transferred to a test tube containing 8 .0 ml of Krebs-Henseleit solution in which sodium taurocholate (from CalBiochem) had been dissolved at a concentration of
Effect of Taurocholate
Vol. il, No . 8
10 mM .
377
The purity of the taurocholate was confirmed by silica
gel thin-layer chromatography (8) .
Both segments, jejunal and
ileal, were transferred simultaneously .
Recording was continuous
throughout the transfer procedure . 1) Unfasted rats maintained
Three groups of rats were used : on Purina Laboratory Chow ; 2)
rats fasted for 24 hr preceding the
experiment with access to water ; and 3) rats from which bile had been drained for 72 hr via a polyethylene tube tied into the bile duct just above the entrances of the pancreatic ducts .
Cannula-
tion of the bile duct was performed under pentobarbital anesthesia . During bile drainage the animals were restrained in the cage described by Baker et al . (9) and were given Purina Laboratory Chow and 0 .38 NaCl solution ad' libitum.
An average of 18 g body
weight was lost during the 72-hr bile-dépletion period . Taurocholate transport was studied in terminal ileal segments from fasted and Unfasted rats, using the cannulated everted sac technique of Crane and Wilson (5) .
In these experiments, the
initial taurocholate concentration was 0 .2 mM in both mucosal and serosal Krebs-Henseleit solutions .
Both solutions contained 0 .2
uc/ml of 14 C-labeled taurocholate (Tracerlab) . for 1 hr at 37°C .
Incubation was
Radioactivity of mucosal and serosal solutions
was determined by liquid scintillation counting .
Water absorption
from the mucosal solution was determined by weighing the intestinal segment with cannula before and after incubation . Results An example of the type of record obtained is shown in Figure 1 .
In the terminal ileum an increase in PD (serosal side
Vol . 11 . No . 8
Effect of Taucocholate
37 8
PD (mv)
PD (mv)
i-T-T'.{"- .j~---~ 6 4 S O 1 2 3 TIME (min)
FIG .
1
A sample polygraph record from one of the experiments using an unfasted rat . Both segments were transferred simultaneously (at the arrows) to mucosal solutions containing 10 mM taurocho late . Calibration signals of 2 mV appear near the beginning and ending of each trace . The frequently observed second phase of the deal response was only slightly evident in this particular experiment . positive) always occurred in response to taurocholate .
There
was no detectable delay in onset and maximum PD was usually reached within about 30 sec .
With unfasted or bile-depleted
animals the response was transitory ; PD usually declined to the pre-transfer level roughly 3 to 8 min after transfer, and often fell below this level .
With fasted rats the response in the
ileum was of less magnitude than with unfasted or bile-depleted rats, but was usually more stable .
Frequently the deal response
included a second stimulatory phase -- a small hump in the curve beginning 1-2 min after transfer .
The record in Figure 1 shows
379
Effect of Taurocholate
Vol. 11, No. 8
a relatively small second phase .
The jejunal segments usually
responded to taurocholate, but only very weakly . PD values recorded just before transferring to taurocholate are summarized in Figure .2 .
Ileal PD averaged 0 .74 mV with the
d 0 ôz .o r v O L 0
O r ,~ 1 .0 o"
O
m h 0
.° c
-v m m â d 0
ILEUM
o. m 0
-n 0 â c
m d e. m
JEJUNUM
FIG . 2 Pre-transfer PD values . These measurements were made just before transfer to taurocholate . Each bar represents the mean for the number of rats shown within the bar . Standard errors of the means are depicted by vertical lines . fasted rats, but was significantly higher with unfasted rats, and even greater with bile-depleted animals .
Jejunal PD followed a
similar pattern . The mean increases in PD in response to 10 mM taurocholate are shown in Figure 3 .
In the terminal ileum of unfasted rats the
Vol. 11, No . 8
Effect of Taurocholate
38 0
ILEUM FIG .
JEJUNUM 3
The electrical response to taurocholate . These measurements were made by subtracting the pre-transfer PD from the maximum PD attained following transfer to 10 mM taurocholate . Standard errors are depicted by vertical lines . The number of animals is shown for each bar . response (BPD) averaged 2 .27 mV, was much less with fasted rats (0 .64 mV), and considerably greater with bile-depleted rats (3 .51 mV) .
In the jejunum, BPD was not detectably influenced by
bile depletion ; fasting caused an apparent reduction in BPD, but there was a 158 chance for the null hypothesis (by t test) . Data obtained related to transport of water and taurocholate by terminal ileal segments are shown in Table 1 .
Fasting the
animals for 24 hr signifi^~ntly reduced the amount of taurocho late absorbed from the mucosal solution and also reduced the final
Vol. 11, No . 8
38 1
Effect of Taurocholate
serosal :mucosal concentration ratio .
Fasting also reduced the
amount of water absorbed . TABLE 1 Effect of Fasting on Water and Taurocholate Transport by Rat Terminal Ileuma Water Absorbed (ml)
Taurocholate Absorbed (moles/g)b
Serosal :Mucosal Concentration Ratio at 1 hr
Unfasted
0 .47 ± 0 .036
0 .52 ± 0 .026
1 .97 ± 0 .085
Fasted
0 .32 ± 0 .016
0 .31 ± 0 .028
1 .30 ± 0 .086
pc
0 .0054
0 .0005
0 .0004
a) The values shown are means ± standard errors of the means for five unfasted rats and six fasted rats . b) Taurocholate absorption is the amount of taurocholate removed from the mucosal solution during a 1-hr incubation per gram of gut wet weight . c) The probability of the null hypothesis being correct as determined by Student's t test . Discussion This investigation demonstrates that taurocholate is electrogenic in the terminal ileum.
Lyon previously observed this effect
of taurocholate (3), but Jackson and Smyth failed to detect it (4) . The promptness of the response resembles that found when sugars are actively transported and implies an immediate effect on ion movements at the apical boundary of the epithelial cells . The response was transitory (except in some of the experiments using fasted rats) .
We believe that the return to and be-
yond control PD may result from an effect of taurocholate that has accumulated intracellularly .
In several preliminary experi-
ments in vivo we have found the response to be stable, presumably
382
Effect of Taurocholate
Vol . 11, No . 8
because with vascular flow intracellular accumulation of taurocholate is less,
The time course of the response to taurocholate in
vitro resembles the response to phenylalanine shown by Kohn et al . (10) .
Lyon did not report the temporal course of the response
and did not note its transitory nature, probably because he used fasted rats .
ATP also causes a transitory stimulation of PD in
rat intestine, but its effect differs from that of taurocholate in that a latent period of about 15 sec occurs (11), Although we usually obtained weak electrical responses in jejunum, they were always far less than the responses consistently obtained in terminal ileum,
A similar observation was made by
Lyon, although his jejunal responses were somewhat larger than ours .
In a recent abstract, Feldman et al . (12) also report
somewhat larger jejunal responses to taurocholate than we obtained . It is well known that the ability of terminal ileum to absorb conjugated bile salts is far greater than that of the jejunum (13-16) and the active transport of bile salts seems to be re stricted to the ileum (2) .
Since the electrogenic effect of
taurocholate is also much greater in terminal ileum than~in jejunum, there is reason to suspect that the electrical effect of taurocholate has something to do with its active transport .
Curi-
ously, this kind of correlation is not observed with sugars or amino acids,
The glucose-induced pPD is as great in terminal
ileum as in upper jejunum (10,17) in spite of a marked difference in ability of these regions to transport glucose (18,19) . glycine-induced pPD is greatest in terminal ileum (10)
The
even though
this is a relatively poor site for glycine transport (20) .
In searching for other correlations between electrogenicity of taurocholate and its transport, we have tested the effects of fasting for 24 hr and depletion of bile for 72 hr,
Holt demon-
Vol . 11, No . $
Effect of Taumcholate
383
strafed that bile depletion greatly increases the, ability of rat terminal ileum to transport 14C-labeled taurocholate (21), and the data shown-here establish that bile depletion also increases the electrical response to taurocholate in the ileum .
Further-
more, as demonstrated here, fasting for 24 hr decreases both bile salt transport and the electrical response to bile salt .
Thus,
the interpretation that electrogenicity of taurocholate is closely related to its active transport rather than to some passive consequence of its physical chemistry is supported by the three correlations demonstrated here :
1) Site along intestine, 2)
effects of fasting, and 3) effects of bile depletion . The effects of fasting and of bile depletion may relate to changes in the endogenous amounts of bile salt in the mucosa . Holt found that after 24 hr of bile drainage there was essentially no bile salt present in small intestinal tissue, whereas the ileum of intact rats contained roughly 1 umole of bile salt per gram of tissue water (21) .
Norman and Sjövall demonstrated that fasting
rats for 24 hr increased the percentage of previously injected 14 C_labeled cholic acid that could be recovered from small intestinal tissue (22) .
Also, Holt showed that when taurocholate was
measured chemically, its final concentration in the serosal solution, after incubating everted sacs of rat ileum for 30 min, was higher when fasted rats were used than when unfasted rats were used (21) .
This result implies that fasting increased the endog-
enous level of bile salt in the tissue .
Thus, it seems that if
the amount of endogenous bile salt in ileal mucosa is decreased by feeding or by bile depletion, the ability of the ileum to transport taurocholate and to respond electrically to taurocholate increases .
Although the mechanism of this relationship is obscure
384
Effect of Taumcholate
Vol . 11, No . 8
and its significance is not clear, there may be an advantage with regard to efficiency of the enterohepatic circulation of bile salts in augmented ileal bile salt transport during periods of digestion . It should be noted that feeding and especially bile depletion increased the pre-taurocholate PD values, a result which may imply a depressant effect of endogenous bile salt on ion transport or on transmural resistance .
This effect could be related to the en
hancement of sugar transport by bile depletion and inhibition of sugar transport by taurocholate shown by Roy et al . (23), and to the inhibition of sugar and amino acid transport by an overnight fast demonstrated by McManus and Isselbacher (24) . Active transport of taurocholate is known to be a Na +dependent process (21)
and is inhibited by ouabain (25) .
These
features suggest a coupling between Na+ and taurocholate influxes across the brush border, as with other actively transported organic solutes (26) .
The presence of an electrogenic effect of
taurocholate supports this suggestion .
Although it has not yet
been demonstrated that increased Na+ transport underlies the taurocholate-induced BPD, such a result may be anticipated and would strengthen the notion of a coupled Na+ and taurocholate influx mechanism .
Vol. 11, No . 8
Effect of Taomcholate
385
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