Parathyroid hormone-related protein is a factor in normal fish pituitary

Using antibodies to the amino-terminal region of human parathyroid hormone-related protein (PTHrP) we have demonstrated PTHrP immunoreactivity in pituitaries and plasma of the sea bream (Sparus aurata). Pituitary cells at two distinct locations contained immunodetectable PTHrP; an anterior group in the rostral pars distalis which also contained immunoreactive thyroid stimulating hormone (TSH), and a posterior group lying at the border of the pars intermedia and proximal pars distalis between cells which stained with antibody to human corticotrophin-like intermediate lobe peptide. By Western blot analysis pituitary extracts contained two immunoreactive isoforms of PTHrP, one of 29 kDa and the other of 26 kDa. Media of pituitaries incubated for up to 14 days in Krebs-Ringer bicarbonate also had several isoforms of immunodetectable PTHrP, two of them corresponding to the 29- and 26-kDa molecular forms but there were in addition both larger and smaller molecules. The concentration of PTHrP in sea bream plasma was comparable with levels observed in human subjects with humoral hypercalcaemia of malignancy. There was no reaction between pituitary cells or pituitary extracts and antibody to human parathyroid hormone. Thus sea bream pituitary contains immunoreactive PTHrP, which appears to be released into medium during in vitro incubation and which may be a significant source of plasma immunoreactive PTHrP in vivo.