Humoral hypercalcemia caused by a rat Leydig-cell tumor is associated with suppressed parathyroid hormone secretion and increased urinary cAMP excretion.

We studied the effect of a transplantable Leydig-cell tumor (Rice H-500) on serum calcium, parathyroid hormone (PTH), and urinary cAMP in intact Fischer-344 rats. The tumor caused rapid and severe hypercalcemia (control = 10.5 +/- 0.1 mg/dl [mean +/- S.E.] vs. 14.6 +/- 0.9 at day 12 post tumor inoculation) without evidence of metastasis. Progressive renal impairment and death generally occurred within 15 days of tumor inoculation. Serum PTH declined from control values before hypercalcemia occurred and was significantly reduced in tumor-bearing hypercalcemic rats (mean = 60 +/- 8% of control values). Urinary cAMP excretion was increased in tumor-bearing rats (mean at day 12 post inoculation = 12.2 +/- 1.4 nmol/dl creatinine clearance vs. control = 6.2 +/- 0.2) and correlated positively with serum calcium. The Rice H-500 Leydig-cell tumor appears to secrete a humoral factor capable of causing hypercalcemia. This factor may also increase urinary cAMP excretion in a manner analogous to PTH, but it is not detected by PTH radioimmunoassay.