The use of salivary cortisol measurements for the non-invasive assessment of adrenal cortical function in guinea pigs.

Abstract

For collection of saliva, cotton buds (Q-tips) were inserted into the guinea pig's cheek pouch, parallel with the cheek teeth. The recovery of saliva from the buds increased with the volume applied: whereas the recovery was 63% when 20 microliters were applied, it increased to 85% when 120 microliters were applied. The recovery of cortisol was closely related to the volume of saliva obtained by centrifugation (r = 0.99, p < 0.001). The mean value of salivary cortisol concentrations in untreated animals was 6.6 ng/ml, with relatively large variations across minutes and days within and between animals. Salivary cortisol was significantly increased if animals were singly caged, either in their familiar housing room or in an unfamiliar empty test room. In comparison to these changes, a much more pronounced increase of salivary cortisol occurred after the intramuscular (i.m.) administration of 20 IU ACTH: while the pretreatment value was 2.2 ng/ml, cortisol concentrations increased to 47 ng/ml (1 h), 72 ng/ml (2 h), 137 ng/ml (3 h) and 170 ng/ml (4 h), respectively. Similarly, i.m. administration of 2 IU insulin resulted in a significant increase of salivary cortisol (2 h: 37 ng/ml, 3 h: 24 ng/ml, 4 h: 25 ng/ml). The present study shows that the cortisol concentrations in the saliva of guinea pigs can be used as an index of adrenal cortical function in preference to the more commonly measured concentrations in the plasma. The advantages of the saliva method are: Firstly, cortisol values reflect the biologically active, unbound fraction in the plasma and are thus less affected by concentration changes of the corticosteroid-binding globulin. Secondly, saliva is easy to collect and the collection method is non-invasive; thus, no handling- or stress-induced changes of the adrenal gland occur. Thirdly, the ease of collection facilitates investigations which require frequent sampling.