Effects of dietary protein and calcium levels on renal glucose-6-phosphatase activity of intact and thyroparathyroidectomized rats.

In order to determine the relationship of parathyroid hormone and levels of dietary protein and calcium with the activity of renal glucose-6-phosphatase (G6Pase), effects of two levels of dietary protein, namely, 25 and 75%, on the enzyme activity were compared at three levels of dietary calcium, namely, 0.06, 0.63, and 1.83%, with the use of intact and thyroparathyroidectomized (TPTX) rats. In intact rats, 0.06% dietary calcium caused an increase in renal G6Pase activity in rats fed the high carbohydrate diet, and dietary calcium in excess (1.83%) caused the enzyme activity to decrease. Similar responses in the activity of renal G6Pase to the variation of dietary calcium levels were seen in rats fed the high protein diet, but significant differences were not obtained. In TPTX rats fed the high carbohydrate diet, the activity of renal G6Pase was significantly decreased compared with that of intact rats. When TPTX rats were fed the high protein diet, however, no significant decrease in the enzyme activity was observed. Free access to aqueous 0.1% CaC1(2) solution by TPTX rats tended to restore the activity of renal G6Pase and serum calcium concentrations depressed by thyroparathyroidectomy. In addition, a significant correlation was observed between the total activity of renal G6Pase and serum calcium concentrations. Hypothyroidism produced by oral administration of propylthiouracil (0.05% of diets) did not affect the enzyme activity in the kidneys of rats fed the high carbohydrate and the high protein diets. The results suggest that the activity of renal G6Pase of rats fed the high protein diet might be less susceptible both to dietary calcium levels and to parathyroid function than that of rats fed the high carbohydrate diet.