Parathyroid hormone-like immunoreactivity in fish plasma and tissues

Abstract

Although fish lack parathyroid glands, parathyroid hormone (PTH)-like substances, reacting with guinea pig antibodies raised against bovine PTH 1–84, were detected in the circulation of trout ( Salmo gairdneri) and goldfish ( Carassius auratus). The immunoreactive (IR) PTH concentration in plasma (0.5–1.5 ng/ml) was less than that measured in heat-treated and dialyzed 0.6% NaCl extracts of brain tissue (4.0–5.0 ng/g wet wt brain) and much less than that present in pituitary extracts (20–40 ng/g pituitary gland). Heat-treated saline extracts of hake ( Urophycis tenuis) and coho salmon ( Oncorhynchus kisutch) pituitary glands also had PTH-like immunoreactivity in radioimmunoassays for C-terminal and N-terminal PTH, in which serial dilutions of the extracts were parallel to the respective standards. The IR PTH material present in the pituitary glands was absorbed on Sep Pak columns and was eluted in the same buffer (90% methanol, 4% acetic acid) and same volume as authentic bovine PTH 1–84. IR PTH was not detected in extracts of muscle, liver, or kidney tissue, but was present in the corpuscles of Stannius (CS), at levels comparable to those in the pituitary glands. While heating the CS extracts had no effect on the IR C-terminal PTH content, it completely destroyed that of the N-terminal PTH. These results suggest that the brain, pituitary, and CS all produce a peptide which shows immunoreactive cross-reactivity with mammalian PTH but that the peptide produced by the CS is not identical to that produced by the brain or pituitary. The brain, pituitary, and CS may be the source of plasma IR PTH in fish.