Physiological and metabolic changes associated with weaning in the tammar wallaby,Macropus eugenii

Summary

The tammar wallaby (Macropus eugenii) is a small macropodid marsupial in which the major part of weaning occupies the period between 28 and 36 weeks of pouch life. Before weaning the diet of the tammar is high in carbohydrate and low in lipid/volatile fatty acid whereas the reverse applies after weaning. The adult tammar is a forestomach fermenter. The aim of this study was to elucidate some of the physiological and metabolic changes associated with this major change in the diet.

Hepatic glycogen content increased gradually early in development to a maximum of 7% of liver weight at 28–30 weeks of pouch life. It then fell precipitously to less than 1% of liver weight at 36 weeks before recovering to the adult level of about 3% liver weight. Plasma glucose levels were maintained at about 10 mM until 36 weeks, after which they fell gradually to adult values of about 4 mM. Hepatic hexokinase activity increased several-fold between 18 and 30 weeks of pouch life, remained high until 42 weeks, and then fell to the adult level. The hepatic activities of fructose-bisphosphatase and particulate phosphoenolpyruvate carboxykinase (PEPCK) were unchanged during development but soluble hepatic PEPCK activity, which was low until 28 weeks of pouch life, increased 3–4 fold between 30 and 36 weeks and then fell slightly to the adult level. Hepatic pyruvate kinase increased in activity up to 28 weeks and then fell to about half peak values at 36 weeks and 20% of peak activity in the adult. There was a greater than ten-fold increase in the ratio of soluble PEPCK activity to pyruvate kinase activity between 28 and 36 weeks of development. It has previously been reported that hepatic gluconeogenesis is inducible in pouch young but constitutive in adults. We conclude that the change in regulation of hepatic gluconeogenesis at the PEPCK/pyruvate kinase level is part of the weaning process.

The urea content of the plasma changed little during development but plasma ammonia increased consistently through pouch life. Urine urea content was low until about 28 weeks of age and then increased rapidly to adult levels. Urine ammonia increased from about 20 mM early in pouch life to a maximum of more than 100 mM at 28 weeks. Thereafter, urine ammonia content fell rapidly to the adult value of about 20 mM. For the first 27 weeks of pouch life, urine pH was consistently between 4.4 and 5.7, but subsequently it rose and became more variable. Urine pH in adults was 8.1±0.3. The activities of the five enzymes of the ornithine-urea cycle increased 3–5 fold in activity between 28 and 36 weeks of pouch life.

These findings indicate that there are major changes in metabolic regulation associated with weaning in the tammar. During weaning, glucose becomes essentially unavailable to the young animal and there is an increase in the rate of hepatic gluconeogenesis which is attributable primarily to increased activity of soluble PEPCK. Metabolism, which is acidotic before weaning, becomes alkalotic and there is a decrease in urinary ammonia content as proton excretion decreases. As ammonia excretion falls, the activity of the urea cycle increases and the concentration of urea in the urine rises. Weaning in the tammar is therefore a complex and well-orchestrated process which may be associated with the change in diet.