Antidiuretic hormone stimulation of adenylate cyclase in semicircular canal epithelium

Summary

Basal adenosine 3′,5′-cyclic monophosphate (cAMP) content and the modulation of its production were studied in the frog's semicircular canal epithelium. This epithelium secretes endolymph, a K⁺-rich, positively polarized fluid. The basal cAMP content measured by microradioimmunoassay was 244 ± 14.2 fmol/structure per 5 min (n = 30). This content was increased about 8 times by 10⁻⁵ M forskolin. Vasotocin, the frog antidiuretic hormone, increased the cAMP production by factors of 1.3 and 3.3 at concentrations of 10⁻⁸ M and 10⁻⁷ M, respectively. This stimulatory effect of vasotocin was blunted by the addition of α₂-adrenergic agonists, such as 10⁻⁸ M-10⁻⁵ M norepinephrine, in the presence of 10⁻⁵ M propranolol, or 10⁻⁵ M clonidine. Prostaglandin E₂ at a concentration of 10⁻⁸ M, which did not affect the cAMP production, did not modify the response to vasotocin. Glucagon (10⁻⁶ M), calcitonin (10⁻⁶ M), and parathyroid hormone (10 units/ml) did not affect the cAMP content. Prostaglandin E₂ (10⁻⁷ M) and the β-adrenergic agonist isoproterenol (10⁻⁶ M) stimulated the cAMP production by a factor of 1.6. These results indicate that the frog semicircular canal is a target of both vasotocin and catecholamines and that catecholamines through α₂-receptors modulate vasotocin-induced cAMP generation. Further, this interaction might be of physiological relevance in the modulation of ion transport in this structure.