Ryanodine receptors and the mediation of Ca2+-dependent anion secretion across rat colon.

The properties of the Na(+)-Ca(2+) exchanger in isolated crypts from rat colon were studied using the Fura-2 imaging technique. The transport mode of the exchanger was reversed by replacing extracellular Na(+) by the impermeable cation, N-methyl-D-glucamine (NMDG(+)), so that the transporter mediated a Ca(2+) influx into the cells. Depletion of intracellular Ca(2+) stores by inhibitors of sarcoplasmatic endoplasmatic calcium ATPases (SERCA), i.e., cyclopiazonic acid (10(-5) mol l(-1)) or thapsigargin (10(-6) mol l(-1)), reduced the increase in [Ca(2+)](i) evoked by superfusion with NMDG(+), suggesting a cross-talk between the exchanger and the Ca(2+) stores. However, measurement of Ca(2+) influx with the Mn(2+) quench technique revealed that the activity of the exchanger was independent of the filling state of the stores. Instead, the obvious inhibition of the [Ca(2+)](i)response by SERCA blockers was due to a reduction of Ca(2+)-induced Ca(2+) release after inhibition of store-refilling. The functional presence of ryanodine receptors was demonstrated by the increase in [Ca(2+)](i)evoked by ryanodine (10(-7) to 3x10(-4) mol l(-1)) in a concentration-dependent manner. This effect was mimicked by cADP ribose (10(-5) mol l(-1)) in crypts permeabilized with saponin (10 mg l(-1)). Ruthenium red (5x10(-5) mol l(-1)) or high concentrations of ryanodine (3x10(-4) mol l(-1)) inhibited this response. In Ussing chamber experiments ruthenium red (5x10(-4) mol l(-1)) or a high concentration of ryanodine (10(-3) mol l(-1)) inhibited the increase in short-circuit current evoked by the cholinergic agonist, carbachol (5x10(-5) mol l(-1)). Consequently, Ca(2+)-induced Ca(2+) release may act as kind of amplifier during Ca(2+)-dependent Cl(-) secretion in order to maintain a long-lasting increase in the intracellular Ca(2+) concentration.