Bicarbonate-activated adenylyl cyclase in fluid-transporting tissues

Area of Science:

• Biochemistry
• Ophthalmology
• Physiology

Background:

• Adenylyl cyclase (AC) plays a crucial role in cellular signaling pathways.
• Fluid secretion in the eye, particularly aqueous humor production, is a complex physiological process.
• The role of bicarbonate in regulating AC activity in ocular tissues was not well understood.

Purpose of the Study:

• To investigate the presence and characteristics of bicarbonate-stimulated AC activity in ocular fluid-transporting tissues.
• To determine the potential role of this activity in regulating intraocular pressure and fluid balance.

Main Methods:

• Enzyme assays were performed on particulate fractions of bovine ciliary processes, corneal endothelium, choroid plexus, and rat kidney.
• AC activity was measured under varying conditions, including the presence of bicarbonate, Mg2+/Mn2+, GTP, and inhibitors.
• Dose-response studies were conducted to determine the effective concentration of bicarbonate.

Main Results:

• Bicarbonate-stimulated AC activity was detected in bovine ciliary processes, corneal endothelium, choroid plexus, and rat kidney, but not in retinal or brain tissues.
• The activity was primarily membrane-bound, dependent on Mg2+ or Mn2+, independent of GTP, and additive to G protein-dependent AC activity.
• Activation was dose-dependent, with maximal stimulation at 100 mM bicarbonate and an EC50 of 2-3 mM.
• HSO3- partially inhibited the bicarbonate response, while methazolamide had no effect.

Conclusions:

• Fluid-transporting tissues possess membrane-bound, bicarbonate-sensitive AC.
• This enzyme may play a role in autoregulating intracellular bicarbonate levels.
• The bicarbonate-AC pathway could influence the activity of membrane ion transporters involved in fluid secretion.