Dietary Na+ Depletion Transcriptionally Up‐regulates BK Channel Mediated K+ Secretion in Proximal Colon
Authors: Narayanan Karthikeyan M. , Andrew Nickerson M. , Coon Steven D. , Hoque Kazi Mirajul , Rajendran Vazhaikkurichi M. 
Published: April 2020

Background

Electrogenic K+ secretion and H+‐K+‐ATPase‐mediated electroneutral K+ absorption occur in normal rat proximal and distal colon, respectively. Although both high‐K+ diet and dietary‐Na+ depletion induce K+ secretion in distal colon, only the dietary‐Na+ depletion has been shown to stimulate K+ secretion in proximal colon. Dietary‐Na+ depletion induced both intermediate conductance K+ (IK) and large conductance K+ (BK) channels mediated K+ secretion, while high‐K+ diet induced only the BK channel expression and activity in distal colon. Although K+ secretions are present, the molecular identity of K+ channels that mediate K+ secretion in normal and dietary‐Na+ depleted rat proximal colon is not known.

Hypothesis

It is hypothesized that dietary‐Na+ depletion may induce K+ secretion by stimulating IK and/or BK channel in proximal colon.

Aim

This study was initiated to identify the molecular identity of the dietary‐Na+ stimulated K+ secretion in rat proximal colon.

Methods

Normal rats (Male Sprague‐Dawly; 200–225g) were fed either normal, high‐K+ or Na+‐depleted diet ad libitum for 6–7 days. Mucosal to serosal (m‐s) and serosal to mucosal (s‐m) 86Rb+ (K+ surrogate) fluxes were measured in stripped proximal colon epithelia mounted under voltage clamp condition. Net K+ fluxes were calculated by subtracting s‐m fluxes from m‐s fluxes. Positive and negative net K+ fluxes represent active absorption and secretion, respectively. IK and BK channel mRNA abundance and protein expression were determined by RT‐qPCR and western blot analyses, respectively. Cell and membrane specific IK and BK channel expressions were determined by immunofluorescence studies.

Results

Similar to earlier demonstration, active K+ secretion is present in normal proximal colon. Dietary‐Na+ depletion, but not high‐K+ diet stimulated K+ secretion in proximal colon (Normal vs Na+‐depletion: 0.42 ± 0.06 vs 2.33 ± 0.16 μEq/cm2.h; p < 0.001). Mucosal iberiotoxin (IbTX; 10 μM; BK channel blocker) completely inhibited the K+ in normal, while it inhibited only 72% in Na+‐depleted rat proximal colon. Mucosal TRAM34 (10 μM; IK channel blocker) did not affect K+ secretion in normal, but it significantly (22%; p < 0.02) inhibited in Na+‐depleted rat colon. RT‐qPCR analyses revealed increased IK (2.2‐fold) and BK (6.2‐fold) specific mRNA abundance, while western blot analyses confirmed increased expression of IK and BK channel expression. Immunofluorescence studies established that in vitro aldosterone (1 μM) incubation increased both IK and BK protein expression in apical membranes of proximal crypt glands.

Conclusion

We conclude that high (~1000 ng/ml; Na+‐depletion), but not low (~300 ng/ml; high‐K+ diet) blood aldosterone level stimulates IK/BK expression to increase K+ secretion in proximal colon.

Support or Funding Information

This study was supported by DK112085 and DK104791 from the National Institute of Health Diabetes and Digestive and Kidney Diseases.