Bicarbonate-activated adenylyl cyclase in fluid-transporting tissues.

A bicarbonate-stimulated adenylyl cyclase (AC) activity was found in ocular ciliary processes, which secrete the aqueous humor of the eye. Other fluid-transporting tissues also showed HCO3(-)-stimulated AC activity. Relative to basal, the response to 10 mM NaHCO3 was greatest in the particulate fraction of bovine ciliary processes, followed by bovine corneal endothelium, bovine choroid plexus, and rat kidney (medulla and cortex). However, no activity was detectable in bovine retina, cerebral cortex, or cerebellum. The activity in ciliary processes was present only in the particulate fraction, was supported by Mg2+ or Mn2+, was independent of GTP, and was additive to the stimulatory G protein-dependent AC response (via beta-adrenergic receptor stimulation). The activation of AC was dose dependent up to 100 mM bicarbonate with a 50% effective concentration of 2-3 mM. The HCO3- response was unaffected by 1 mM methazolamide, a carbonic anhydrase inhibitor, but HSO3- was a partially selective inhibitor compared with its effect on forskolin-stimulated AC activities. The presence of membrane-bound bicarbonate-sensitive AC in fluid-transporting tissues suggests an autoregulatory mechanism for intracellular HCO3- concentration acting via adenosine 3',5'-cyclic monophosphate in the control of membrane ion transporters.