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Time (min) Fig. 2. Rate of secretion of Nu+ ions into serosal (vascular)effruentbefore and a f e r introduction of iodoacetate in the luminal perfusate Sodium was present (145mequivJitre) in the luminal perfusate but absent from the arterial infusate. After three 5min control periods sodium iodoacetate (0.1 mM) was introduced into the luminal perfusate. Values are means ~ s . E . M .of eight experiments.
(Fig. 2). The Na+ concentration in the intestinal lumen was 145mequivJitre and the mean concentration in the serosal effluent was 19mequiv./litre.Therefore Na+ movement was definitely down a concentration gradient. It also appeared to be down an electrochemical gradient since the equilibrium (Nernst) potential calculated for this distribution of Na+ activity (55mV) is much greater than the normal maximum potential difference across the intestine in the presence of glucose of about 12mV (Barry et al., 1964). The latter would itself be decreased by the iodoacetate. Preliminary experiments with an alternative metabolic inhibitor, 2,4dinitrophenol, have shown the same inhibition of 'downhill' Na+ movement. This is in contrast with the effects of ouabain which only inhibited Na+ movement when there was a relatively high Na+ concentration in the arterial infusate (Fisher & Gardner, 1974). Separate control experiments without any inhibitor showed that the rate of Na+ movement remained constant after a rise during the initial 5-l0min. Thus 'downhill' net movement of Na+ ions appears to require metabolicenergy. Either this requirement is at a stage other than the ouabain-sensitiveNa+ pump, or else the Na+ pump is inhibited by ouabain only in the presence of a relatively high concentration of Na+ (>72mequiv./litre) in the serosal extracellular fluid. Barry, R. J. C.. Dickstein, S., Matthews, J., Smyth, D. H. & Wright, E. M. (1964)J. Physiol. (London)171,316338 Fisher, R. B. & Gardner, M. L. G. (1974)J. Physiol. (London) 241,235-260 Schultz, S. G.& Zalusky, R. (1964)J. Gen. Physiol. 47,567-584 Schultz, S.G., Fuisz, R. E. & Curran, P.F. (1966)J. Gen. Physiol. 49,849-866
Characteristics of Intact Lactose Transport in the Rat Jejunum in v i m J. E. JOSfi OYESIKU, D. P. R. MULLER and J. T. HARRIES Institute of Child Health, 30 Guilford Street, London WC1N 1 EH, U.K.
The jejunal transport of glucose and galactose is carrier-mediated, sodium- and energydependent. The characteristics of intact disaccharide transport, however, have not been defined, This paper reports on the magnitude and kinetics of transmural transport of 1975
554th MEETING, LONDON
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intact lactose in the everted sac preparation of the rat jejunum. The results were as follows. (1) Approx. 1 % of lactose was absorbed as the intact disaccharide when various concentrations(5-22Om)of lactose were presented to the mucosa. (2) There was a direct linear relationship between lactose concentration and transport rates up to 1 5 0 m lactose. (3) At 2 2 0 m the rate of lactose transport was greater than predicted from the lower concentrations. (4) Neither ouabain nor phlorrhizin inhibited lactose transport. ( 5 ) The ratio between serosal and mucosal concentrations of lactose never exceeded unity. These results indicate that transmural transport of intact lactose occurs by a process of simple passive diffusion. The higher-than-predictedtransport rates observed at high concentrations of lactose may be due to substrate saturation of brush-border lactase leading to a build-up of concentration in the microvillus region.
CarbohydrateMetabolism, Placental Lactogen and the Control of Uhe hitiation of Lactation DAVID P. LEADER Department of Biochemistry, University of Glasgow, Glasgow G12 BQQ, U.K. Experiments in which mammary tissue has been cultured in vitro (Stockdale et al., 1966) or in which hormones have been injected into animals in viuo (Cowie, 1966) have clearly established that prolactin and a corticosteroid are required to promote differentiation of the mammary gland into a state where it is competent to Secrete milk, and also to maintain its secretory ability. Many workers also believe that increased secretion (Amenomori et al., 1970; Gala & Westphal, 1965) of one or both of these hormones might be the stimulus for the greatly increased synthesis of milk at parturition. Implicit in this idea is the assumption that the concentrationsof these hormones during pregnancy are so low as to limit milk synthesis. However, there are strong grounds for doubting this. In many (if not all) mammalian speciesthere are present during the latter half of pregnancy high concentrations of the potent lactogenichormone, placental lactogen, which is thought to play a major role in the mammary differentiation occurring then. Thus there is in fact no net increase in lactogenic-hormone activity in blood at parturition, merely a change-over from placental to pituitary-derived lactogen (Buttle et al., 1972). Moreover, although the concentration of free corticosteroids does increase at parturition in some species, the elevated concentrationof the hormone falls within the range of circadian variation during pregnancy (Kuhn, 1971). Hence some restraint other than limitation of lactogenic hormones must exist to restrain milk synthesis during pregnancy, and some other stimulus must be responsiblefor expression of secretory potential at parturition. I suggest that the initiation of milk synthesis might be controlled through hormonal effects on carbohydrate metabolism. It is well established that the change from pregnancy to lactation is characterized by a change in the general direction of carbohydratemetabolism in extra-uterine tissues. Thus pregnancy is typified by lipaemia (Scow et al., 1964),characteristicof what I shall call an ‘anti-insulin’ direction of carbohydrate metabolism (i.e. showing a diminished capacity for glucose utilization). In lactation the lipaemia disappears (Otway & Robinson, 1968) and there is a characteristic ‘insulin-promoted’direction of carbohydrate metabolism, e.g. increased fatty acid synthesis and glucose oxidation (Hansen & Carlson, 1961). The most likely explanation for the lipaemia of pregnancy is the action of placental lactogen (Grumbach et al., 1968) which, in addition to having lactogenic properties, has an ‘antiinsulin’ effect on carbohydrate metabolism (Turtle & Kipnis, 1967) similar to that of its evolutionary cousin (Niall et af., 1971), growth hormone (Hales, 1967). What I propose, then, is that during late pregnancy, when the mammary gland is already differentiated,it is prevented from synthesizingmilk constituents(e.g. fatty acids, proteins) by the ‘anti-insulin’effect of placental lactogen on carbohydrate metabolism.
Vol. 3
BIOCHEMICAL SOCIETY TRANSACTIONS
5
10
IS
20
25
30
35
40
45
Time (min) Fig. 2. Rate of secretion of Nu+ ions into serosal (vascular)effruentbefore and a f e r introduction of iodoacetate in the luminal perfusate Sodium was present (145mequivJitre) in the luminal perfusate but absent from the arterial infusate. After three 5min control periods sodium iodoacetate (0.1 mM) was introduced into the luminal perfusate. Values are means ~ s . E . M .of eight experiments.
(Fig. 2). The Na+ concentration in the intestinal lumen was 145mequivJitre and the mean concentration in the serosal effluent was 19mequiv./litre.Therefore Na+ movement was definitely down a concentration gradient. It also appeared to be down an electrochemical gradient since the equilibrium (Nernst) potential calculated for this distribution of Na+ activity (55mV) is much greater than the normal maximum potential difference across the intestine in the presence of glucose of about 12mV (Barry et al., 1964). The latter would itself be decreased by the iodoacetate. Preliminary experiments with an alternative metabolic inhibitor, 2,4dinitrophenol, have shown the same inhibition of 'downhill' Na+ movement. This is in contrast with the effects of ouabain which only inhibited Na+ movement when there was a relatively high Na+ concentration in the arterial infusate (Fisher & Gardner, 1974). Separate control experiments without any inhibitor showed that the rate of Na+ movement remained constant after a rise during the initial 5-l0min. Thus 'downhill' net movement of Na+ ions appears to require metabolicenergy. Either this requirement is at a stage other than the ouabain-sensitiveNa+ pump, or else the Na+ pump is inhibited by ouabain only in the presence of a relatively high concentration of Na+ (>72mequiv./litre) in the serosal extracellular fluid. Barry, R. J. C.. Dickstein, S., Matthews, J., Smyth, D. H. & Wright, E. M. (1964)J. Physiol. (London)171,316338 Fisher, R. B. & Gardner, M. L. G. (1974)J. Physiol. (London) 241,235-260 Schultz, S. G.& Zalusky, R. (1964)J. Gen. Physiol. 47,567-584 Schultz, S.G., Fuisz, R. E. & Curran, P.F. (1966)J. Gen. Physiol. 49,849-866
Characteristics of Intact Lactose Transport in the Rat Jejunum in v i m J. E. JOSfi OYESIKU, D. P. R. MULLER and J. T. HARRIES Institute of Child Health, 30 Guilford Street, London WC1N 1 EH, U.K.
The jejunal transport of glucose and galactose is carrier-mediated, sodium- and energydependent. The characteristics of intact disaccharide transport, however, have not been defined, This paper reports on the magnitude and kinetics of transmural transport of 1975
554th MEETING, LONDON
257
intact lactose in the everted sac preparation of the rat jejunum. The results were as follows. (1) Approx. 1 % of lactose was absorbed as the intact disaccharide when various concentrations(5-22Om)of lactose were presented to the mucosa. (2) There was a direct linear relationship between lactose concentration and transport rates up to 1 5 0 m lactose. (3) At 2 2 0 m the rate of lactose transport was greater than predicted from the lower concentrations. (4) Neither ouabain nor phlorrhizin inhibited lactose transport. ( 5 ) The ratio between serosal and mucosal concentrations of lactose never exceeded unity. These results indicate that transmural transport of intact lactose occurs by a process of simple passive diffusion. The higher-than-predictedtransport rates observed at high concentrations of lactose may be due to substrate saturation of brush-border lactase leading to a build-up of concentration in the microvillus region.
CarbohydrateMetabolism, Placental Lactogen and the Control of Uhe hitiation of Lactation DAVID P. LEADER Department of Biochemistry, University of Glasgow, Glasgow G12 BQQ, U.K. Experiments in which mammary tissue has been cultured in vitro (Stockdale et al., 1966) or in which hormones have been injected into animals in viuo (Cowie, 1966) have clearly established that prolactin and a corticosteroid are required to promote differentiation of the mammary gland into a state where it is competent to Secrete milk, and also to maintain its secretory ability. Many workers also believe that increased secretion (Amenomori et al., 1970; Gala & Westphal, 1965) of one or both of these hormones might be the stimulus for the greatly increased synthesis of milk at parturition. Implicit in this idea is the assumption that the concentrationsof these hormones during pregnancy are so low as to limit milk synthesis. However, there are strong grounds for doubting this. In many (if not all) mammalian speciesthere are present during the latter half of pregnancy high concentrations of the potent lactogenichormone, placental lactogen, which is thought to play a major role in the mammary differentiation occurring then. Thus there is in fact no net increase in lactogenic-hormone activity in blood at parturition, merely a change-over from placental to pituitary-derived lactogen (Buttle et al., 1972). Moreover, although the concentration of free corticosteroids does increase at parturition in some species, the elevated concentrationof the hormone falls within the range of circadian variation during pregnancy (Kuhn, 1971). Hence some restraint other than limitation of lactogenic hormones must exist to restrain milk synthesis during pregnancy, and some other stimulus must be responsiblefor expression of secretory potential at parturition. I suggest that the initiation of milk synthesis might be controlled through hormonal effects on carbohydrate metabolism. It is well established that the change from pregnancy to lactation is characterized by a change in the general direction of carbohydratemetabolism in extra-uterine tissues. Thus pregnancy is typified by lipaemia (Scow et al., 1964),characteristicof what I shall call an ‘anti-insulin’ direction of carbohydrate metabolism (i.e. showing a diminished capacity for glucose utilization). In lactation the lipaemia disappears (Otway & Robinson, 1968) and there is a characteristic ‘insulin-promoted’direction of carbohydrate metabolism, e.g. increased fatty acid synthesis and glucose oxidation (Hansen & Carlson, 1961). The most likely explanation for the lipaemia of pregnancy is the action of placental lactogen (Grumbach et al., 1968) which, in addition to having lactogenic properties, has an ‘antiinsulin’ effect on carbohydrate metabolism (Turtle & Kipnis, 1967) similar to that of its evolutionary cousin (Niall et af., 1971), growth hormone (Hales, 1967). What I propose, then, is that during late pregnancy, when the mammary gland is already differentiated,it is prevented from synthesizingmilk constituents(e.g. fatty acids, proteins) by the ‘anti-insulin’effect of placental lactogen on carbohydrate metabolism.
Vol. 3
