High [K+] alters the stimulus-hydrosmotic response coupling in toad bladder

Area of Science:

• Physiology
• Cell Biology
• Membrane Transport

Background:

• The toad urinary bladder is a model epithelium for studying water transport.
• Hormonal and osmotic stimuli regulate water permeability in this tissue.
• The role of ions, particularly potassium (K+), in modulating these responses is not fully understood.

Purpose of the Study:

• To investigate the effect of substituting sodium (Na+) with K+ on the hydrosmotic response of the toad urinary bladder.
• To explore the potential involvement of calcium (Ca2+) in mediating K+-induced alterations in water permeability.

Main Methods:

• Experiments were conducted on isolated toad urinary bladders (Bufo marinus).
• Ringer solutions with varying ion compositions (chloride vs. sulfate) and K+ concentrations were used.
• Stimuli included vasopressin, cyclic adenosine monophosphate (cAMP), theophylline, and serosal hypertonicity.
• Calcium's role was assessed using a Ca2+ antagonist (cobalt) and nominally Ca2+-free Ringer solutions.

Main Results:

• K+ substitution did not affect basal water permeability but significantly altered stimulus-induced water flow.
• In chloride-Ringer, high K+ enhanced responses to vasopressin and cAMP but decreased responses to theophylline and serosal hypertonicity.
• In sulfate-Ringer, most responses were enhanced by K+, except for serosal hypertonicity, which was diminished.
• Reducing Ca2+ availability (with cobalt or Ca2+-free Ringer) markedly reduced the hydrosmotic effects of vasopressin and cAMP.

Conclusions:

• Potassium ions modulate the hydrosmotic response of the toad urinary bladder to various stimuli.
• A transient influx of Ca2+ through voltage-sensitive, cobalt-blockable channels may mediate some of the K+-induced effects on water transport.
• These findings suggest a complex interplay between ion substitution, calcium signaling, and water permeability regulation in epithelial tissues.