Endogenous Biologically Active Human Parathyroid Hormone: Measurement by a Guanyl Nucleotide-Amplified Renal Adenylate Cyclase Assay*

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Abstract
The purpose of the present study was to establish and validate a routine procedure for the measurement of endogenous biologically active human parathyroid hormome (PTH). To accomplish this, we measured the activation of canine renal cortical plasma membrane adenylate cyclase activity produced by PTH standards or test sera in the presence of the hydrolysis-resistant GTP analog, 5′-guanylimidodiphosphate (100 [M). In this system, as little as 3 pg (=0 3 fmol) human PTH-(1–U≃84)′ assay tube produced significant adenylate cyclase activation. Glucagon, human calcitonin, ACTH-(l–), arginine vasopressin, epinephrine, and prostaglandin E1 had little or no activity. A competitive PTH antagonist, [8Nle, 18Nle, 34Tyr]bovine PTH(3–34) amide, completely inhibited enzyme activation produced by exogenous PTH, parathyroid venous effluent serum, or peripheral chronic renal failure serum. Thus, the activity in serum was due specifically to biologically active PTH.The concentrations of biologically active PTH and immunoreactive PTH [using both intact and carboxyl (C)-region PTH RIAs] in parathyroid venous effluent sera from eight patients with primary hyperparathyroidism were highly correlated (r== 0.97; P < 0.01). Absolute concentrations of biologically active PTH were generally greater than those of intact immunoreactive PTH, but were consistently less than C-region immunoreactive PTH concentrations. In peripheral sera from patients with hyperparathyroidism secondary to chronic renal failure, most (generally >90%) of the C-region immunoreactive PTH was found to be biologically inert. In contrast, PTH bioassay values and intact RIA values obtained with these sera agreed much more closely. The data demonstrate that the guanyl nucleotide-amplified renal adenylate cyclase assay is a highly sensitive, specific, and convenient tool for the evaluation of endogenous, biologically active human PTH. Our results also suggest that, at least in chronic renal failure, values derived with intact PTH RIAs more accurately reflect levels of circulating, biologically active hormone than those obtained with C-region PTH antisera. (J Clin Endocrinol Metab52:840, 1981)