Responsiveness of the Parotid Salivary Gland of Red Kangaroos (macropus Rufus) to Mineralocorticoids

During both acute and chronic mineralocorticoid administration, parotid saliva was obtained by acetylcholine stimulation at rates of 1-0—1 -5 mlmin" from anaesthetized red kangaroos. The Na/K ratio of saliva from chronically Na-replete kangaroos was virtually unaltered by ipsilateral intracarotid infusion of aldosterone at rates of 8, 40 or 80/igh" for 4h, the ratio falling from 20-1 ± 1-09 to 17-5 ± 0-53 (t6 = 2-07; NS) at 80/Jgh~'. Kangaroos given intramuscular injection of the mineralocorticoid, deoxycorticosterone (DOCA), at rates of 0-25 or O-SOmgkg" 12 h" showed a progressive fall in salivary Na/K ratio from 19-1 ±0-47 to l-76± 0-41 (t5 = 27-4; P<0-001) over the 21-day period of injection. The DOCA treatment caused hypertrophy of the ducts, particularly the intralobular ducts of the parotid gland. Aldosterone acetate given intramuscularly at 0-03 mgkg" 12b." for 10 days also reduced the Na/K ratio of the saliva. As soon as the salivary Na/K ratio had returned to replete values, some 3-4 days after cessation of the DOCA injections, the kangaroos were given a 5-h infusion of aldosterone. Intracarotid infusion of aldosterone at 8 /igh" produced a near maximal fall in salivary Na/K ratio after 3 h of infusion, and increasing the infusion to 80/igh" had little additional effect. The minimum Na/K ratio obtained at this time was 5-7 ± 1-04 (t5 = 14-21; P< 0-001), which was equivalent to the ratio obtained at 3-6 days of DOCA injection. Significant regression of the intralobular ducts occurred during the 3 days following cessation of DOCA administration; 24 days after the end of DOCA treatment duct development was approaching that of Na-replete, untreated kangaroos. The results demonstrate that the parotid glands of kangaroos from a sodium-rich environment are almost unable to respond to acute fluctuations in endogenous aldosterone levels; that chronically high levels of mineralocorticoids cause hypertrophy of the sodium-transporting ducts of the parotid gland, which results in an increasing ability to reduce the Na/K ratio of the saliva; and that responsiveness to mineralocorticoids declines rapidly in the absence of high mineralocorticoid levels due to regression of the ducts.