Ca(2+) influx through the basolateral- and luminal membranes of colonic epithelium in neonatal rats

In this study, the short-circuit currents ( I(sc)) of electrolyte absorption and secretion in neonatal and adult rat colonic mucosa were compared and the role of Ca(2+) influx through luminal membranes examined in relation to the replenishment of intracellular Ca(2+) stores in colonic crypt cells. Neonatal tissues displayed higher amiloride-sensitive I(sc) and larger increases of electrogenic Cl(-) secretion in response to an increase in cytosolic [Ca(2+)] ([Ca(2+)](c)) or cAMP than found in adult colonic epithelium. Ca(2+)-mediated Cl(-) secretion as reflected in the I(sc) responses to carbachol ( I(sc,CCh)) showed milder "run-down" in neonates than in adult rats. We then employed the relatively stable I(sc,CCh) of the neonatal colon to investigate the polarity of Ca(2+) entry pathway after muscarinic stimulation. Repetitive stimulation with CCh under Ca(2+)-free conditions emptied the intracellular Ca(2+) stores and abolished the I(sc,CCh). Re-adding Ca(2+) to the basolateral perfusate rapidly restored I(sc,CCh) (about 71% of control in 10 min). In contrast, after re-adding Ca(2+) to the luminal perfusate only, the recovery of I(sc,CCh) took much longer and was incomplete, recovering to only 28% of control after 30 min. Recovery was accelerated by increasing [Ca(2+)] in the luminal perfusate (5 mM) and blocked by the presence of Gd(3+) (100 microM) in the luminal perfusate. The above results suggest that, in addition to the predominant role of Ca(2+) entry through the basolateral membrane, the influx of Ca(2+) through luminal membranes might also play a role in the Ca(2+) homeostasis of colonic epithelial cells.