Colon epithelial electrical responses to acute hypoxia and reoxygenation in vitro

Electrogenic epithelial transport depends on oxidative metabolism. Acute hypoxia and subsequent reoxygenation effects on short-circuit current (Isc), transepithelial potential difference (PD) and tissue resistivity (TR) of rat distal colon were assessed. The tissue was mounted in an Ussing chamber filled with Ringer-HCO3-solution at 37 degrees C and bubbled with 95% O2- 5% CO2 which was switched to 95% N2- 5% CO2 for inducing hypoxia; afterwards normal oxygenation was resumed. The effect of 5, 10, 15 and 20 min-hypoxic periods was assessed in isolated mucosa preparations. Recovery was complete after 10- and 15-min hypoxia, but not after 20-min hypoxia. After 5-min hypoxia, an overshoot of Isc and PD was seen on reoxygenation. This effect was further characterized comparatively in mucosa-submucosa and isolated mucosa preparations. In the former (n = 10), control values were Isc = 71.7 +/- 8.6 microA. cm-2, PD = 9.7 +/- 1.6 mV and TR = 134.9 +/- 13.6 omega cm2. A 5-min hypoxia reduced Isc by 47.2 +/- 7.3% and PD by 61.5 +/- 4.9%. Peak values on reoxygenation were 28.1 +/- 4.1% for Isc and 16.8 +/- 5.4% for PD, over controls values. In the isolated mucosa (n = 9), control values were Isc = 52.04 +/- 5.5 microA. cm-2, PD = 5.0 +/- 0.8 mV and TR = 101.04 +/- 10.5 omega. cm2. In hypoxia, Isc decreased by 64.5 +/- 7.6% and PD by 57.2 +/- 7.8%. On reoxygenation peak values of 78.0 +/- 19.0% and 87.5 +/- 17.1%, respectively, were seen. The response to a 5 min-hypoxia was comparable, but that to reoxygenation was weaker and slower, in mucosa-submucosa than in isolated mucosa preparations. This may be explained by a hindrance to oxygen diffusion caused by the submucosal tissue. TR did not change with any period of hypoxia tested, but decreased slightly (8.9 +/- 1.3%) upon reoxygenation in the mucosa-submucosa preparations. Ouabain (10(-3) M) markedly blunted the response to reoxygenation. We conclude that hypoxic periods of 20 min lead to irreversible functional deterioration. Hypoxia decreases electrogenic transepithelial pumping, which may allow sodium to accumulate intracellularly and, if the hypoxia is short enough to prevent damage to the epithelium, increase sodium pump activity when oxygenation is resumed.