Interaction between store-operated non-selective cation channels and the Na+-Ca2+ exchanger during secretion in the rat colon

Abstract

The properties of capacitative Ca2+ influx were studied using the whole-cell patch-clamp technique in crypts isolated from rat distal colon. Store-operated cation influx was evoked by increasing the intracellular buffering capacity for Ca2+ in the pipette solution; contamination by Cl- currents was reduced by the use of NMDG gluconate as the main electrolyte in the pipette solution. The permeability of the non-selective cation conductance stimulated by store depletion had the following sequence for monovalent cations: Cs+ > Na+ ≥ Li+. The store-operated conductance is permeable to Na+ and Ca2+, but in contrast to Na+, Ca2+ also exerts a (feedback) inhibition on its own influx. Other divalent cations shared this inhibitory action with the sequence: Ca2+ ≥ Mg2+ ≥ Ba2+ ≥ Sr2+. Fura-2 experiments revealed that replacement of extracellular Na+ by NMDG+ induced an increase in the intracellular Ca2+ concentration, which was suppressed by the Na+-Ca2+ exchange inhibitor, dichlorobenzamil, indicating the presence of a Na+-Ca2+ exchanger within the colonic crypt cells. In Ussing chamber experiments dichlorobenzamil induced an increase in short-circuit current (Isc) in the majority of tissues tested indicating that this exchanger acts as a Ca2+-extruding transporter under physiological conditions. When Ca2+-dependent anion secretion was stimulated by the acetylcholine analogue carbachol, dichlorobenzamil no longer evoked an increase in Isc, indicating that after stimulation of the store-operated cation conductance the Na+-Ca2+ exchanger is turned off. Therefore, it is concluded that the influx of Na+ across the non-selective store-operated cation conductance serves to reduce the driving force for Ca2+ extrusion via the Na+-Ca2+ exchanger and thereby maintains the increase in the intracellular Ca2+ concentration during induction of secretion.