Immunoreactive glandular kallikrein in rat plasma: a radioimmunoassay for its determination

A radioimmunoassay (RIA) has been developed to measure immunoreactive glandular kallikrein in rat plasma. To prevent the binding of radioactive kallikrein to plasma inhibitors, 125I-kallikrein was inactivated with phenylmethylsulfonyl fluoride (PMSF), a procedure that maintained 125I-kallikrein immunoreactivity. Different volumes of plasma displaced 125I-PMSF-kallikrein in a parallel fashion to the kallikrein standard curve. The sensitivity of the RIA was 200 pg, and the recovery of nonradioactive active kallikrein added to plasma was 58.7%. The concentration of immunoreactive glandular kallikrein in normal rat plasma averaged 47.1 +/- 1.7 (SE) ng/ml. Bilateral nephrectomy caused a threefold increase in circulating glandular kallikrein (50 +/- 2.7 to 167 +/- 7 ng/ml; P less than 0.001). REmoval of the submandibular and sublingual glands significantly decreased its concentration from 52 +/- 2.3 to 34 +/- 1.6 ng/ml (P less than 0.001). Immunoreactive glandular kallikrein was higher in the submandibular gland vein than in arterial blood (venous: 94 +/- 10.5; arterial: 64 +/- 6.3 ng/ml; P less than 0.05) and was lower in the renal venous blood (venous: 44 +/- 2.2; arterial: 53 +/- 2.6 ng/ml; P less than 0.05). In conclusion, this study shows that the use of 125I-PMSF-kallikrein as tracer prevents the interference in the RIA caused by plasma protease inhibitors. It also indicates that the submandibular gland is an important source of the immunoreactive glandular kallikrein in rat plasma and that the kidney probably participates in its metabolism. Glandular kallikrein released by the submandibular gland into the circulation may participate in regulating local blood flow before it is inactivated by plasma inhibitors.