Acta Ph,ysiol Scand 1991, 143, 295-298
Acute but not chronic gastric sodium ad ministration regulates vasoactive intestinal peptide metabolism by the liver C. M. H A W L E Y , K. A. D U G G A N , G. J. M A c D O N A L D and S. S H E L L E Y Department of Nephrology, Prince Henry Hospital, Sydney, Australia C. M., DUGGAN, K. A., MACDONALD, G. J. 8z SHELLEY, S. 1991. Acute but HAWLEY, not chronic gastric sodium administration regulates vasoactive intestinal peptide metabolism by the liver. Actu Pizyszol Scand 143, 295-298. Received 23 April 1991, accepted 6 June 1991. ISSN 0001-6772. Department of Nephrology, Prince Henry Hospital, Sydney, Australia. We have shown previously that gastric sodium loading releases vasoactive intestinal peptide from the intestine and in rabbits on a low sodium diet it appears to decrease vasoactive intestinal peptide metabolism by the liver. To determine the contributions of the low sodium diet and the acute sodium load to changes in vasoactive intestinal peptide metabolism, metabolic clearance studies of vasoactive intestinal peptide infused intraportally were performed. These studies were performed in male New Zealand white rabbits equilibrated on normal and low sodium diets before and after an acute gastric sodium load of 1.5 mmol kg-’. No difference was detectable in metabolic clearance rates between normal and low salt diets, however, decreases in metabolic clearance rates were observed in response to the sodium load (normal diet P < 0.005, low salt P < 0.0005). Secretion rates also decreased following the gastric sodium load (normal P < 0.005, low salt P < 0.05). We conclude that hepatic VIP metabolism is decreased by acute gastric sodium loading but it is not affected by chronic sodium intake. Key words : peptide, sodium state, vasoactive intestinal peptide.
We and others have demonstrated that vasoactive intestinal peptide (VIP) is natriuretic (Dimaline et al. 1983, Duggan & Macdonald 1987, Rossa et al. 1987) and is released by gastric sodium loading (Ebeid et al. 1977, Duggan et al. 1989), suggesting that it may have a role in sodium metabolism. In addition to releasing VIP from the gut, a gastric sodium load appears to decrease hepatic metabolism of VIP in rabbits maintained on a low sodium diet. When sodium chloride is administered intragastrically to rabbits on low sodium diets the reported difference between portal and systemic blood VIP concentrations disappeared, suggesting that the hepatic metabolism of VIP had decreased (Duggan et al. 1989). This raised the question whether such a decrease Correspondence : Dr K. A. Duggan, Department of Nephrology, Prince Henry Hospital, Anzac Pde, Little Bay, N.S.W. 2036, Sydney, Australia
in metabolism of VIP by the liver occurred as a result of the low sodium intake per se or whether it reflected modulation by both the low sodium diet and the administered gastric sodium load. I n the current study we sought to determine the effects of dietary sodium intake and acute gastric sodium loading on the hepatic metabolism of VIP. To determine the contributions of each, we measured the metabolic clearance rates (MCR) of VIP infused intraportally in rabbits maintained on normal and low sodium diets with and without prior administration of an acute gastric sodium load. MATERIALS AND METHODS A gastrostomy tube and a portal vein catheter were inserted into male New Zealand white rabbits under general anaesthesia as described previously (Duggan et al. 1989). After a l-week recovery period the rabbits
295
296
C. 41. H a m l q et al.
were randoml? assigned to a low sodium (0.1O0) or normal sodium (2.i0,) diet (Glen Forrest Stockfeeders, M-estern Australia) and allowed distilled water ad hbitum. The rabbits were housed in metabolic cages, weighed second daily and their urine collected for determination of 24-h urinary sodium excretions. When the! were in sodium balance, as determined by two equal successive 2-1-h urinary sodium excretions metabolic clearance studies for \-IP were performed. On the day of experiment the rabbits were placed in restraint cages and an ear artery cannula was inserted using lignocaine local anaesthesia. The rabbits \?ere then permitted a 2-h rest period during which the vehicle alone (Haernaccel, 1 ioechst Australia) \ u s infused into the portal catheter at 0.033 ml min I . VIP was then infused at 10 pmol k g ' min-' for 50 min. Arterial blood was sampled for YIP levels prior to commencing the YIP infusion and at 40, -15 and 50 min after commencement of the infusion to determine the basal and steady state levels respectivel!-. After a second 2-h equilibration period during which the vehicle alone was infused again, a sodium load of 1.5 mmol kg-' (as 0.313 .\I saline), was administered into the gastrostomv tube and the vehicle infusion continued a further 30 min. This time period was chosen because the increase in plasma VIP levels seen in response to gastric sodium returns to baseline by this time (Duggan et al. 1989). VIP u-as infused as before, for a period of 50 min and blood sampled for determination of basal and steady state levels. The rabbits were then equilibrated on the alternate diet and the experiments were repeated. YIP levels were measured by a radioimmunoassajas previousl!- described (Duggan et rrl. 1989) and the metabolic clearance rates were calculated by the method of Tait r~ a / . (1962), in tvhich the infusion rate is divided by the steady state concentration. VIP infusion rates w-ere calculated bk- radioimmunoassa>of infusate obtained through the whole system at the end of each experiment. Theoretical secretion rates were calculated as the product of the metabolic clearance rate and the basal VIP concentration. Statistical methods. The achieved steady state plasma levels and metabolic clearance rates for 1'11' for the various experimental groups were compared by analysis of \ariance for multiple groups wirh P values of less than 0.05 being considered significant. When significant differences were obtained individual pairs of data groups were compared using Student's ttest.
plasma VIP concentrations achieved by infusion were greater after gastric sodium compared with those before its administration for rabbits on both normal (pre-sodium 10.3720.81 pmol 1-I; post sodium 17.9 f 1.33 pmol I-l, P < 0.0005) and low sodium diets (pre-sodium 11.32f 0.73 pmol I-' ; post sodium 23.61 1.69 pmol I-', P < 0.0005). However only after the gastric sodium load had been administered was there a significant difference between the normal and lou salt diet groups ( P < 0.01).
MCR a n d secretion rates MCRs were not significantly different for rabbits on the normal or low sodium diet either before or after the sodium load although significant decreases did occur aftcr the gastric sodium for both diet groups (see Fig. 1). For rabbits on a normal sodium diet the MCR decreased from 569.33 5 59.75 rnl kg-' before the sodium load to 320.67 26.44 ml kg-I min-' afterwards (P< 0.005). For rabbits on the low sodium diet it decreased from 525.33 536.70 ml kg-' min-' to 259.50 22.39 ml kg min-' following the gas-
T
v
Low Salt
Normal
Fig. 1. Metabolic clearance rates of VIP before (0) and after (m) administration of the gastric sodium for n = 6 rabbits in each group.
R E SU L T S Basal and mean steady state concentrations Basal plasma VIP levels were similar in all
experimental groups, i.e. they were not influenced by the sodium content of the diet. The
Low Salt
Normal
Fig. 2. Secretion rates calculated from infusion data and after (B)the gastric sodium load for before (0) n = 6 rabbits in each group.
Acute sodium regulates liver V I P metabolism tric sodium ( P < 0.0005). Delivery of the gastric sodium load decreased the theoretical secretion rate from 4.24 & 0.61 to 2.53 0.51 pmol min-’ (P< 0.05) for the low sodium group (see Fig. 2 ) and in the normal sodium group SR decreased from 4.57 k 0 . 3 6 pmol min-’ before the sodium load to 2.73 0.21 pmol min-’ after its administration ( P < 0.005). DISCUSSION This study demonstrates that metabolism of V I P by the liver is independent of the chronic level of dietary sodium intake, as M C R s were similar in rabbits on both normal and low sodium diets before the acute gastric sodium load. Further it shows that administration of an acute sodium load intragastrically profoundly decreases the hepatic metabolism of V I P irrespective of the preceding dietary sodium intake, as M C R decreased by 50% in rabbits on the low sodium diet and 44”/b in rabbits on the normal diet. These results contrast with our previous study which showed that a decrease was only present in rabbits on the low sodium diet. However, the greater sensitivity of the technique employed in the present study to detect changes in M C R shows that the decrease is also present in rabbits previously maintained on normal sodium diets. T h u s the findings of this study lend further support to the hypothesis that VIP is the humoral mediator for the gastric sodium monitor proposed by Lennane et d. (1975a, b) as the metabolism of this peptide is decreased in response to gastric sodium in rabbits maintained on both normal and low sodium diets. Our findings are also consistent with Lennane’s observation that the grater natriuretic response to a gastric sodium load than the same load given intravenously is only present when the rabbits have been maintained on a low sodium diet (Lennane, personal communication). The greater magnitude of the changes in the metabolism of V I P seen in this group makes it more likely that significant changes in sodium excretion would also occur. T h e observed changes in the theoretical secretion rates, with a decrease after intragastric sodium, are at variance with our previous report of an initial increase in portal vein hormone levels following this stimulus. However, in this study secretion rates were measured approximately 2 h after administration of the sodium so
297
that one might postulate that intragastric sodium initially released V I P and decreased its metabolism resulting in high systemic plasma levels. These in turn fed back directly or indirectly through an intermediate mediator(s) to regulate further V I P release. This interpretation is consistent with the prolonged change in metabolism and with the similar levels of circulating V I P before gastric sodium and 30 min afterwards. T h e current view of sodium metabolism involves the concept of the interaction of natriuretic and antinatriuretic systems. How V I P interacts with the known sodium homeostatic mechanisms and the purpose of its role in pathological states involving derangement of salt and water balance is yet to be determined. However from our present data it may be hypothesized that it has a role in avoiding overcompensation when an animal previously maintained on diets low in sodium is exposed to a sudden increase in available dietary sodium. We wish to thank the Department of Clinical Chemistry, Prince of Wales Hospital for performing the urinary electrolyte measurements and Dr R. Finlayson of the Department of Analytical Chemistry, University of New South Wales for performing the dietary analyses. We wish to acknowledge the Australian Kidney Foundation and the National Health and Medical Research Council of Australia for their support of this project. REFERENCES DIMALINE, R., PEART,W.S. & UNWIN,R.J. 1983. Effects of vasoactive intestinal polypeptide (VIP) on renal function in the conscious rabbit. 3 Physiol 344, 379-399. DUGGAN, K.A. & MACDONALD, G.J. 1987. VIP: A direct renal natriuretic substance. Clin Sci 72, 195-200. DUGGAN, K.A., HAWLEY, C.M., MACDONALD, G.J. & SHELLEY,S. 1989. Sodium depletion decreases hepatic metabolism of vasoactive intestinal peptide in the rabbit. 3 Ph,ysio/ 418, 251-259. J., SOETERS, P.B., MURRAY, EBEID,A.M., ESCOURROU, P. & FISHER,J.E. 1977. Release of vasoactive intestinal peptide (VIP) by intraluminal osmotic stimuli. 3 Surg Res 23, 25-30. R.M. & SHAW, LENNANE, J.R., PEART,W.S., CAREY, J. 1975a. A comparison of natriuresis after oral and intravenous sodium loading in sodium-depleted rabbits: evidence for a gastrointestinal or portal monitor of sodium intake. Clin Sci Molec Med 49, 433436.
298
C. M . Hawley et al.
LENNASE,R.J., CAREY-,R.M., GOODWIS,T.J. & PEART, W.S. 1975 b. A comparison of natriuresis after oral and intravenous sodium loading in sodium-depleted man : evidence for a gastrointestinal or portal monitor of sodium intake. Clin Sci Molec Med 49, 437440. ROSSA,R.M., SILVA,P., STOFF, J.S. & EPSTEIS,F.1-I. 1987. Effect of vasoactive intestinal peptide on
isolated perfused rat kidney. Am 3 Physiof 249, E49+E497. TAIT, J.F., LITTLE,B., TAIT,S.A.S. & FLOOD,C. 1962. T h e metabolic clearance rate of aldosterone in pregnant and non-pregnant subjects estimated by both single injection and constant infusion methods. 3 Clin Incest 41, 2093-2100.
Acute but not chronic gastric sodium ad ministration regulates vasoactive intestinal peptide metabolism by the liver C. M. H A W L E Y , K. A. D U G G A N , G. J. M A c D O N A L D and S. S H E L L E Y Department of Nephrology, Prince Henry Hospital, Sydney, Australia C. M., DUGGAN, K. A., MACDONALD, G. J. 8z SHELLEY, S. 1991. Acute but HAWLEY, not chronic gastric sodium administration regulates vasoactive intestinal peptide metabolism by the liver. Actu Pizyszol Scand 143, 295-298. Received 23 April 1991, accepted 6 June 1991. ISSN 0001-6772. Department of Nephrology, Prince Henry Hospital, Sydney, Australia. We have shown previously that gastric sodium loading releases vasoactive intestinal peptide from the intestine and in rabbits on a low sodium diet it appears to decrease vasoactive intestinal peptide metabolism by the liver. To determine the contributions of the low sodium diet and the acute sodium load to changes in vasoactive intestinal peptide metabolism, metabolic clearance studies of vasoactive intestinal peptide infused intraportally were performed. These studies were performed in male New Zealand white rabbits equilibrated on normal and low sodium diets before and after an acute gastric sodium load of 1.5 mmol kg-’. No difference was detectable in metabolic clearance rates between normal and low salt diets, however, decreases in metabolic clearance rates were observed in response to the sodium load (normal diet P < 0.005, low salt P < 0.0005). Secretion rates also decreased following the gastric sodium load (normal P < 0.005, low salt P < 0.05). We conclude that hepatic VIP metabolism is decreased by acute gastric sodium loading but it is not affected by chronic sodium intake. Key words : peptide, sodium state, vasoactive intestinal peptide.
We and others have demonstrated that vasoactive intestinal peptide (VIP) is natriuretic (Dimaline et al. 1983, Duggan & Macdonald 1987, Rossa et al. 1987) and is released by gastric sodium loading (Ebeid et al. 1977, Duggan et al. 1989), suggesting that it may have a role in sodium metabolism. In addition to releasing VIP from the gut, a gastric sodium load appears to decrease hepatic metabolism of VIP in rabbits maintained on a low sodium diet. When sodium chloride is administered intragastrically to rabbits on low sodium diets the reported difference between portal and systemic blood VIP concentrations disappeared, suggesting that the hepatic metabolism of VIP had decreased (Duggan et al. 1989). This raised the question whether such a decrease Correspondence : Dr K. A. Duggan, Department of Nephrology, Prince Henry Hospital, Anzac Pde, Little Bay, N.S.W. 2036, Sydney, Australia
in metabolism of VIP by the liver occurred as a result of the low sodium intake per se or whether it reflected modulation by both the low sodium diet and the administered gastric sodium load. I n the current study we sought to determine the effects of dietary sodium intake and acute gastric sodium loading on the hepatic metabolism of VIP. To determine the contributions of each, we measured the metabolic clearance rates (MCR) of VIP infused intraportally in rabbits maintained on normal and low sodium diets with and without prior administration of an acute gastric sodium load. MATERIALS AND METHODS A gastrostomy tube and a portal vein catheter were inserted into male New Zealand white rabbits under general anaesthesia as described previously (Duggan et al. 1989). After a l-week recovery period the rabbits
295
296
C. 41. H a m l q et al.
were randoml? assigned to a low sodium (0.1O0) or normal sodium (2.i0,) diet (Glen Forrest Stockfeeders, M-estern Australia) and allowed distilled water ad hbitum. The rabbits were housed in metabolic cages, weighed second daily and their urine collected for determination of 24-h urinary sodium excretions. When the! were in sodium balance, as determined by two equal successive 2-1-h urinary sodium excretions metabolic clearance studies for \-IP were performed. On the day of experiment the rabbits were placed in restraint cages and an ear artery cannula was inserted using lignocaine local anaesthesia. The rabbits \?ere then permitted a 2-h rest period during which the vehicle alone (Haernaccel, 1 ioechst Australia) \ u s infused into the portal catheter at 0.033 ml min I . VIP was then infused at 10 pmol k g ' min-' for 50 min. Arterial blood was sampled for YIP levels prior to commencing the YIP infusion and at 40, -15 and 50 min after commencement of the infusion to determine the basal and steady state levels respectivel!-. After a second 2-h equilibration period during which the vehicle alone was infused again, a sodium load of 1.5 mmol kg-' (as 0.313 .\I saline), was administered into the gastrostomv tube and the vehicle infusion continued a further 30 min. This time period was chosen because the increase in plasma VIP levels seen in response to gastric sodium returns to baseline by this time (Duggan et al. 1989). VIP u-as infused as before, for a period of 50 min and blood sampled for determination of basal and steady state levels. The rabbits were then equilibrated on the alternate diet and the experiments were repeated. YIP levels were measured by a radioimmunoassajas previousl!- described (Duggan et rrl. 1989) and the metabolic clearance rates were calculated by the method of Tait r~ a / . (1962), in tvhich the infusion rate is divided by the steady state concentration. VIP infusion rates w-ere calculated bk- radioimmunoassa>of infusate obtained through the whole system at the end of each experiment. Theoretical secretion rates were calculated as the product of the metabolic clearance rate and the basal VIP concentration. Statistical methods. The achieved steady state plasma levels and metabolic clearance rates for 1'11' for the various experimental groups were compared by analysis of \ariance for multiple groups wirh P values of less than 0.05 being considered significant. When significant differences were obtained individual pairs of data groups were compared using Student's ttest.
plasma VIP concentrations achieved by infusion were greater after gastric sodium compared with those before its administration for rabbits on both normal (pre-sodium 10.3720.81 pmol 1-I; post sodium 17.9 f 1.33 pmol I-l, P < 0.0005) and low sodium diets (pre-sodium 11.32f 0.73 pmol I-' ; post sodium 23.61 1.69 pmol I-', P < 0.0005). However only after the gastric sodium load had been administered was there a significant difference between the normal and lou salt diet groups ( P < 0.01).
MCR a n d secretion rates MCRs were not significantly different for rabbits on the normal or low sodium diet either before or after the sodium load although significant decreases did occur aftcr the gastric sodium for both diet groups (see Fig. 1). For rabbits on a normal sodium diet the MCR decreased from 569.33 5 59.75 rnl kg-' before the sodium load to 320.67 26.44 ml kg-I min-' afterwards (P< 0.005). For rabbits on the low sodium diet it decreased from 525.33 536.70 ml kg-' min-' to 259.50 22.39 ml kg min-' following the gas-
T
v
Low Salt
Normal
Fig. 1. Metabolic clearance rates of VIP before (0) and after (m) administration of the gastric sodium for n = 6 rabbits in each group.
R E SU L T S Basal and mean steady state concentrations Basal plasma VIP levels were similar in all
experimental groups, i.e. they were not influenced by the sodium content of the diet. The
Low Salt
Normal
Fig. 2. Secretion rates calculated from infusion data and after (B)the gastric sodium load for before (0) n = 6 rabbits in each group.
Acute sodium regulates liver V I P metabolism tric sodium ( P < 0.0005). Delivery of the gastric sodium load decreased the theoretical secretion rate from 4.24 & 0.61 to 2.53 0.51 pmol min-’ (P< 0.05) for the low sodium group (see Fig. 2 ) and in the normal sodium group SR decreased from 4.57 k 0 . 3 6 pmol min-’ before the sodium load to 2.73 0.21 pmol min-’ after its administration ( P < 0.005). DISCUSSION This study demonstrates that metabolism of V I P by the liver is independent of the chronic level of dietary sodium intake, as M C R s were similar in rabbits on both normal and low sodium diets before the acute gastric sodium load. Further it shows that administration of an acute sodium load intragastrically profoundly decreases the hepatic metabolism of V I P irrespective of the preceding dietary sodium intake, as M C R decreased by 50% in rabbits on the low sodium diet and 44”/b in rabbits on the normal diet. These results contrast with our previous study which showed that a decrease was only present in rabbits on the low sodium diet. However, the greater sensitivity of the technique employed in the present study to detect changes in M C R shows that the decrease is also present in rabbits previously maintained on normal sodium diets. T h u s the findings of this study lend further support to the hypothesis that VIP is the humoral mediator for the gastric sodium monitor proposed by Lennane et d. (1975a, b) as the metabolism of this peptide is decreased in response to gastric sodium in rabbits maintained on both normal and low sodium diets. Our findings are also consistent with Lennane’s observation that the grater natriuretic response to a gastric sodium load than the same load given intravenously is only present when the rabbits have been maintained on a low sodium diet (Lennane, personal communication). The greater magnitude of the changes in the metabolism of V I P seen in this group makes it more likely that significant changes in sodium excretion would also occur. T h e observed changes in the theoretical secretion rates, with a decrease after intragastric sodium, are at variance with our previous report of an initial increase in portal vein hormone levels following this stimulus. However, in this study secretion rates were measured approximately 2 h after administration of the sodium so
297
that one might postulate that intragastric sodium initially released V I P and decreased its metabolism resulting in high systemic plasma levels. These in turn fed back directly or indirectly through an intermediate mediator(s) to regulate further V I P release. This interpretation is consistent with the prolonged change in metabolism and with the similar levels of circulating V I P before gastric sodium and 30 min afterwards. T h e current view of sodium metabolism involves the concept of the interaction of natriuretic and antinatriuretic systems. How V I P interacts with the known sodium homeostatic mechanisms and the purpose of its role in pathological states involving derangement of salt and water balance is yet to be determined. However from our present data it may be hypothesized that it has a role in avoiding overcompensation when an animal previously maintained on diets low in sodium is exposed to a sudden increase in available dietary sodium. We wish to thank the Department of Clinical Chemistry, Prince of Wales Hospital for performing the urinary electrolyte measurements and Dr R. Finlayson of the Department of Analytical Chemistry, University of New South Wales for performing the dietary analyses. We wish to acknowledge the Australian Kidney Foundation and the National Health and Medical Research Council of Australia for their support of this project. REFERENCES DIMALINE, R., PEART,W.S. & UNWIN,R.J. 1983. Effects of vasoactive intestinal polypeptide (VIP) on renal function in the conscious rabbit. 3 Physiol 344, 379-399. DUGGAN, K.A. & MACDONALD, G.J. 1987. VIP: A direct renal natriuretic substance. Clin Sci 72, 195-200. DUGGAN, K.A., HAWLEY, C.M., MACDONALD, G.J. & SHELLEY,S. 1989. Sodium depletion decreases hepatic metabolism of vasoactive intestinal peptide in the rabbit. 3 Ph,ysio/ 418, 251-259. J., SOETERS, P.B., MURRAY, EBEID,A.M., ESCOURROU, P. & FISHER,J.E. 1977. Release of vasoactive intestinal peptide (VIP) by intraluminal osmotic stimuli. 3 Surg Res 23, 25-30. R.M. & SHAW, LENNANE, J.R., PEART,W.S., CAREY, J. 1975a. A comparison of natriuresis after oral and intravenous sodium loading in sodium-depleted rabbits: evidence for a gastrointestinal or portal monitor of sodium intake. Clin Sci Molec Med 49, 433436.
298
C. M . Hawley et al.
LENNASE,R.J., CAREY-,R.M., GOODWIS,T.J. & PEART, W.S. 1975 b. A comparison of natriuresis after oral and intravenous sodium loading in sodium-depleted man : evidence for a gastrointestinal or portal monitor of sodium intake. Clin Sci Molec Med 49, 437440. ROSSA,R.M., SILVA,P., STOFF, J.S. & EPSTEIS,F.1-I. 1987. Effect of vasoactive intestinal peptide on
isolated perfused rat kidney. Am 3 Physiof 249, E49+E497. TAIT, J.F., LITTLE,B., TAIT,S.A.S. & FLOOD,C. 1962. T h e metabolic clearance rate of aldosterone in pregnant and non-pregnant subjects estimated by both single injection and constant infusion methods. 3 Clin Incest 41, 2093-2100.
