Adaptation to metabolic acidosis by turtle urinary bladder.

Area of Science:

• Physiology
• Cell Biology
• Acid-Base Balance

Background:

• Metabolic acidosis triggers adaptive responses in urinary organs.
• The turtle urinary bladder is a model for studying acid secretion mechanisms.
• Previous studies suggested intracellular pH or cell number changes mediate adaptation.

Purpose of the Study:

• Investigate mechanisms of adaptive H+ secretion in turtle bladders during metabolic acidosis.
• Determine the roles of intracellular pH, cell number, and acidic vesicles in this adaptation.
• Assess changes in H+-ATPase activity in response to acid loading.

Main Methods:

• In vitro H+ and HCO3 secretion measurements.
• Fluorescence microscopy using 6-carboxyfluorescein diacetate, acridine orange, and rhodamine 123 to assess intracellular pH and cell characteristics.
• In vivo urinary acidification measurements.
• Enzymatic activity and transport studies of plasma membrane fractions.

Main Results:

• Acidotic turtle bladders showed increased H+ secretion but not HCO3 secretion.
• Urinary acidification increased with acid loading.
• Intracellular pH decreased only in bladders with high-acid load for 48h.
• Increased numbers of acridine orange-positive cells and mitochondria-rich cells were observed.
• The increase in acidic vesicles, not just mitochondria-rich cells, contributed to acridine orange staining.
• No increase in H+-ATPase activity was detected.

Conclusions:

• Adaptive increase in H+ secretion in metabolic acidosis is linked to increased mitochondria-rich cells.
• An increase in acidic vesicles within these cells also plays a significant role.
• Intracellular pH and H+-ATPase activity do not appear to mediate this specific adaptive response.