Renal clearance of phosphate and clacium in the fresh-water turtle: Effects of parathyroid hormone

Summary

The renal clearances of phosphate and calcium were studied in fresh-water turtles,Chrysemys picta andC. scripta elegans, at various times after either parathyroidectomy (PTX) or parathyroid hormone (PTH) administration. The relative clearance of phosphate${{C_{PO_4 } } \mathord{\left/ {\vphantom {{C_{PO_4 } } {C_{In} }}} \right. \kern-\nulldelimiterspace} {C_{In} }}$ fell from a mean value greater than one, to values near zero following PTX. This effect occurred over a period of 5–7 days, and was accompanied by increased values of net tubular phosphate reabsorption (TxPO₄/ml GFR). There were no consistent changes in the relative clearance or tubular transport rates of calcium after PTX. Administration of mammalian PTH to chronic PTX animals had no effect for up to 2 h on any of the measured variables. However, beginning one day after a single intramuscular injection of 100 units of PTH,${{C_{PO_4 } } \mathord{\left/ {\vphantom {{C_{PO_4 } } {C_{In} }}} \right. \kern-\nulldelimiterspace} {C_{In} }}$ increased significantly over vehicle-injected controls. This phosphaturic effect reached a maximal level at 3 days after the hormone injection. PTH administration caused a slight, but not significant increase in the relative clearance of calcium at 5 and 7 days. It was concluded that the fresh-water turtle maintains a bidirectional renal phosphate transport system, which is under the control of the level of circulating parathyroid hormone. These species do not appear to possess a PTH-sensitive renal calcium conserving mechanism, such as is found in mammals.