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

The properties of capacitative Ca2+ influx were studied using the whole-cel l patch-clamp technique in crypts isolated from rat distal colon. Store-ope rated cation influx was evoked by increasing the intracellular buffering ca pacity 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 stimula ted by store depletion had the following sequence for monovalent cations: C s+ > Na+ greater than or equal to Li+. The store-operated conductance is pe rmeable to Na+ and Ca2+, but in contrast to Na, Ca2+ also exerts a (feedbac k) inhibition on its own influx. Other divalent cations action with the seq uence: Ca2+ greater than or equal to Mg2+ greater than or equal to Ba2+ gre ater than or equal to Sr-2. 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, di chlorobenzamil, 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 (I-sc) in the majority of tissues tes ted indicating that this exchanger acts as a Ca2+-extruding transporter und er physiological conditions. When Ca2+-dependent anion secretion was stimul ated by the acetylcholine analogue carbachol, dichlorobenzamil no longer ev oked an increase in I-sc, indicating that after stimulation of the store-op erated cation conductance the Na+-Ca2+ exchanger is turned off. Therefore, it is concluded that the influx of Na+ across the nonselective store-operat ed cation conductance serves to reduce the driving force for Ca2+ extrusion via the Na+-Ca2+ exchanger and thereby maintains the increase in the intra cellular Ca2+ concentration during induction of secretion.